Aerospace Science
&
Technology Dictionary

P mode: A wave mode generated by an acoustic wave (or "sound wave").
P-band: A frequency band used in radar extending approximately from 225 to 390 megacycles per second. See frequency band.
P-display: = plan position indicator.
P-indicator: = plan position indicator (PPI).
package: Any assembly or apparatus, complete in itself or practically so, identifiable as a unit and readily available for use or installation. See power package.
packaging: (1) The technique of preparing goods for distribution; (2) The design criteria, processes, and procedures used to protect materials from deterioration and damage from the time manufacturing is completed until use or disposal; (3) The processses and procedures used to protect an item in a unit package.
packet switching: Switching circuit system for multiple access time division data transmission.
packet transmission: Transmission of bursts of digital data.
packets (communication): Digital data messages which are almost always preceded by headers (containing address information and other control characters) and followed by control characters which signify the end of a message.
pad: = launch pad.
pad deluge: Water sprayed upon certain launch pads during the launch of a rocket so as to reduce the temperatures of critical parts of the pad or the rocket. See underdeck spray.
paddlewheel satellite: A satellite, such as Explorer VI, that has solar vanes or similarly shaped objects attached.
pahoehoe: Pahoehoe is a Hawaiian term for basaltic lava that has a smooth, hummocky, or ropy surface. A pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust. The surface texture of pahoehoe flows varies widely, displaying all kinds of bizarre shapes often referred to as lava sculpture.
pair production: The physical process whereby a gamma-ray photon, usually through an interaction with the electromagnetic field of a nucleus, produces an electron and an anti-electron (positron). The original photon no longer exists, its energy having gone to the two resulting particles. The inverse process, pair annihilation, creates two gamma-ray photons from the mutual destruction of an electron/positron pair.
pair production: An absorption process for X-ray and gamma ray radiation in which the incident photon is annihilated in the vicinity of the nucleus of the absorbing atom, with subsequent production of an electron and positron pair.
This reaction does not occur for incident radiation energies of less than 1.02 million electron volts.
Palapa satellites: Satellites launched by the US for the Indonesian government for their domestic communications network.
paleobiology: The study of life and organisms that existed in the geologic past.
paleoclimatology: The study of climates in the geologic past, involving fossil, glacial, isotropic, or other data.
Paleozoic Era: An era of geologic time, from the end of the Precambrian Period to the beginning of the Mesozoic Era, or about 570 to about 225 million years ago.
palimpsest: A roughly circular albedo spot on icy satellites that is presumed to mark the site of a crater and its rim deposit. Little, if any, of the topographic structure exists, but visual distinction from adjacent crust remains.
PAM: Payload Assist Module upper stage.
PAM (abbr): = pulse amplitude modulation.
PAM/FM (abbr): Frequency modulation of a carrier by pulses which are amplitude modulated by information.
PAM/FM/FM (abbr): Frequency modulation of a carrier by subcarrier(s) which is (are) modulated by pulses which are amplitude modulated by information.
pampas: A vast treeless grassy plain of temperate regions, especially as used in Argentina and adjacent parts of Uruguay. It is comparable to the prairies of North America, the steppes of the Eurasia, and the veld of South Africa.
panchromatic: See color sensitive, note.
Pandora: A natural satellite of Saturn, orbiting at a mean distance of 141,700 kilometers.
panel method (fluid dynamics): Technique for analyzing and predicting the properties and characteristics of fluid flow; sometimes called the finite element method.
panspermia: The theory that holds that reproductive bodies of living organisms exist throughout the universe and develop wherever the environment is favorable.
PANT program: The passive nosetip technology (PANT) program is an investigation of flow phenomena over reentry vehicle nosetips by the Air Force. Used for ablative nosetips and passive nosetip technology.
paper (material): Felted or matted sheets of cellulose fibers, formed on a fine wire screen from a dilute water suspension, and bonded together as the water is removed and the sheet is dried.
parabola: An open curve all points of which are equidistant from a fixed point, called the focus , and a straight line. See conic section.
The limiting case occurs when the point is on the line, in which case the parabola becomes a straight line.
parabolic: Pertaining to, or shaped like, a parabola.
parabolic bodies: Surfaces of revolution generated by revolving sections of parabolas about their major axis. Used for paraboloids.
parabolic orbit: An orbit shaped like a parabola; the orbit representing the least eccentricity (that of 1) for escape from an attracting body.
parabolic reflector: A reflecting surface having the cross section along the axis in the shape of a parabola. See corner reflector, radar reflector, scanner.
Parallel rays striking the reflector are brought to a focus at a point, or if the source of the rays is placed at the focus, the reflected rays are parallel.
paraboloid: A surface of revolution generated by revolving a section of a parabola about its major axis.
paraboloidal: Pertaining to, or shaped like, a paraboloid.
parabrake: = deceleration parachute.
parachutes: A device used, or intended to be used to retard the fall of a body or object through the air.
paracone: A system for recovering men and objects from great distances above the Earth's surface and landing them safely onto the Earth.
parafoveal vision: Vision in which the eye is so oriented toward the pertinent light source as to have the light fall upon some portion of the retina surrounding the fovea. Also called scotopic vision. See foveal vision.
The portion of the retina used in this type of vision contains receptors known as rods. Although these rods do not permit the sort of color-sensing vision possible with the cones in the central or foveal region of the retina, they have the useful property of responding to very low illuminance, particularly after dark adaptation is complete. Nighttime vision is performed primarily with the rods.
parallactic angle: The angle between a body's hour circle and its vertical circle. Also called position angle.
parallactic inequality: A secondary effect in the solar perturbations in the moon's longitude due to the ellipticity of the earth's orbit.
parallax: The difference in the apparent direction or position of an object when viewed from different points expressed as an angle.
For bodies of the solar system, parallax is measured from the surface of the earth and its center and is called geocentric parallax, varying with the body's altitude and distance form the earth. The geocentric parallax when a body is in the horizon is called horizontal parallax and is the angular semidiameter of the earth as seen from the body. Parallax of the moon is called lunar parallex. For stars, parallax is measured from the earth and the sun, and is called annual, heliocentric, or stellar parallax. Compare aberration.
parallax error: The error in measurement between two pairs of antenna caused by the fact that the center of the two base lines do not coincide.
This error is a function of the distance of the target from the baseline, as well as its relative direction.
parallax in altitude: Geocentric parallax of a body at any altitude.
The expression is used to distinguish the parallax at the given altitude from the horizontal parallax.
parallax second: See parsec.
parallel: A circle on the surface of the earth, parallel to the plane of the equator and connecting all points of equal latitude, or a circle parallel to the primary great circle of a sphere or spheroid; also a closed curve approximating such a circle. Also called parallel of latitude, circle of longitude. See coordinate, table.
An astronomical parallel is a line connecting points having the same astronomical latitude. A geodetic parallel is a line connecting points of equal geodetic latitude. Geodetic and sometimes astronomical parallels are also called geographic parallels. Geodetic parallels are shown on charts. A standard parallel is one along which the scale of a chart is as stated. A fictitious, grid, transverse, incerse, or oblique parallel is parallel to a fictitious, grid, transverse, inverse, or oblique equator, respectively. A magnetic parallel is a line connecting points of equal magnetic dip.
parallel of altitude: A circle of the celestial sphere parallel to the horizon connecting all points of equal altitude. Also called altitude circle, almucantar. See circle of equal altitude.
parallel of declination: A circle of the celestial sphere parallel to the celestial equator. Also called circle of equal declination. See diurnal circle.
parallel of latitude: 1. A circle (or approximation of a circle) on the surface of the earth, parallel to the equator, and connecting points of equal latitude. Also called parallel.
2. A circle of the celestial sphere, parallel to the ecliptic, and connecting points of equal celestial latitude. Also called circle of longitude.
parallel processing (computers): The concurrent or simultaneous execution of more than one program, or the handling of input for more than one operation at the same time.
parallel voltage drops: Voltage drops along magnetic field lines.
paramagnetic: Having a magnetic permeability greater than unity.
parameter: 1. In general, any quantity of a problem that is not an independent variable. More specifically, the term is often used to distinguish, from dependent variables, quantities which may be assigned more or less arbitrary values for purposes of the problem at hand.
2. In statistical terminology, any numerical constant derived from a population or a probability distribution. Specifically, it is an arbitrary constant in the mathematical expression of a probability distribution. For example, in the distribution given by

the constant is a parameter.
3. In celestial mechanics , the semi-latus rectum.
parameter identification: The estimation of the unknown parameters of models of physical plants or processes from their dynamic response.
parameterization: The representation, in a mathematical model, of physical effects in terms of admittedly oversimplified parameters, rather than realistically requiring such effects to be consequences of the dynamics of the system.
Parameterization is often used in system analysis to determine the effect on the system of changing one parameter while holding other parameters constant.
parametric amplifiers: Inverting parametric devices used to amplify a signal without frequency translation from input to output. Used for parametric oscillators and reactance amplifiers.
parametric equations: A set of equations in which the independent variables or coordinates are each expressed in terms of a parameter.
For example, instead of investigating y = f(x) or F(x,y) = 0 it is often advantageous to express both x and y in terms of a parameter u: x = g(u); y = G(u). The parameter may or may not have a useful geometric or physical interpretation.
parametric instability: Instability which occurs in a system whose equilibrium is weakly modulated in time or space. The modulation produces a coupling of the linear eigenmodes of the system and can lead to destabilization.
parasitic element: A radiating element, not coupled directly to the feed line of the antenna, which materially affects the pattern of the antenna.
parcel: = fluid parcel.
Pardop (abbr): = passive ranging Doppler system.
parent: A radionuclide that upon disintegration yields a specified nuclide, the daughter, either directly or as a later member of a radioactive series.
Thus, U²³⁸ is the parent of all members of the uranium series, including the end product, Pb²⁰⁶.
parity: A symmetry property of a wave function.
The parity is 1 (or even) if the wave function is unchanged by an inversion (reflection in the origin) of the coordinate system; it is -1 (or odd) if the wave function is changed only in sign.
parity bit: A bit added to a binary code group which is used to indicate whether the number of recorded 1 or 0 is even or odd.
parking orbit: An orbit of a spacecraft around a celestial body, used for assembly of components or to wait for conditions favorable for departure from the orbit.
parse: To break down a sequence of letters or numbers into meaningful parts based on their location in the character sequence. For example, the first three numbers in the GLIS access phone number 6055946888 are the area code numbers that identify the phone number as a South Dakota location.
parsec: (abbr pc)
A unit of length equal to the distance from the sun to a point having a heliocentric parallax of 1 second (1"), used as a measure of stellar distance.
The name parsec is derived from the words parallax second. 1 parsec = pc = 3.084 X 10E13 kilometers = 206,265 astronomical units = 3.262 light years
parsing algorithms: Computer routines for the syntactic and/or semantic analysis and restructuring of natural language instructions or data for internal processing.
part: 1. One of the constituents into which a thing may be divided. Applicable to a major assembly, subassembly, or the smallest individual piece in a given thing.
2. Restrictive. The least subdivision of a thing; a piece that functions in interaction with other elements of a thing, but it itself not ordinarily subject to disassembly.
partial correlation: The correlation between the residuals of two random variables with respect to common regressors. Denoting the regression function of two variates y and z with respect to a common set of regressors x₁, x₂, ...x_n by Y and Z ; the coefficient of partial correlation between y and z is defined as the coefficient of simple, linear correlation between ( y - Y ) and ( z - Z ). See regression.
partial derivative: The ordinary derivative of a function of two or more variables with respect to one of the variables, the others being considered constants. If the variables are x and y , the partial derivatives of f(x, y) are written ,, or D_xf and D_uf or f_x and f_y.
The partial derivative of a variable with respect to time is known as the local derivative.
partial lunar eclipse: See lunar eclipse, note.
partial node: A point, line, or surface in a standing wave system where some characteristic of the wave field has a minimum amplitude differing from zero.
The appropriate modifier should be used with the words partial node to signify the type that is intended; e.g., displacement partial node, velocity partial node, pressure partial node.
partial pressure: The pressure exerted by a designated component or components of a gaseous mixture.
This may be separately measured in some cases by suitable selection of gases, traps, or analytical trains. When the percentage composition of the mixture is known, the partial pressure may be calculated from the total pressure by Dalton law of partial pressures.
partial pressure suit: A skintight suit which does not completely enclose the body but which is capable of exerting pressure on the major portion of the body in order to counteract an increased oxygen pressure in the lungs.
partial solar eclipse: See solar eclipse.
partial-admission turbine: A type of turbine in which the working substance is directed only through part of the annular area swept by the rotating turbine blades.
particle: 1. An elementary subatomic particle such as proton, electron, neutron, etc.
2. A very small piece of matter.
3. In celestial mechanics, a hypothetical entity which responds to gravitational forces but which exerts no appreciable gravitational force on other bodies, thus simplifying orbital computations.
particle accelerator: Specifically a device for imparting large kinetic energy to charged particles, such as electrons, protons, deuterons, and helium ions.
Common types of accelerators are the cyclotron, synchrotron, synchrocyclotron, betatron, linear accelerator, and Van de Graaff electrostatic accelerator.
particle density (concentration): Number of particles present per unit volume(typically a cubic centimeter).
particle laden jets: Fluid, mainly issuing from a nozzle, that are turbulent and contain dispersed particles.
particle precipitation: The precipitation of particles other than electrons and protons.
particles: Elementary subatomic particles such as protons, electrons or neutrons. Very small pieces of matter. In celestial mechanics, hypothetical entities which respond to gravitational forces but which exert no appreciable gravitational force on other bodies, thus simplifying orbital computations.
Pascal (programming language): High order computer programming language developed by Niklaus Wirth originally as an educational tool to foster structured programming.
Paschen law: A theoretical relationship for the direct-current breakdown voltage of two parallel-plane electrodes immersed in a gas as a function of the gas pressure and electrode separation. This relationship predicts the occurrence of a minimum breakdown voltage for a certain product of the pressure times the separation.
The phenomenon is well verified experimentally and is referred to as the Paschen minimum. This minimum voltage is on the order of 300 to 500 volts and, for a gas pressure of 1 millimeter of mercury, occurs at an electrode separation of 0.2 to 1 centimeter depending on the gas.
Paschen minimum: See Paschen law, note.
Pasiphae: A natural satellite of Jupiter orbiting at a mean distance of 23,500,000 kilometers.
pass: 1. A single circuit of the earth by a satellite. Passes start at the time the satellite crosses the equator from the southern hemisphere into the northern hemisphere (the ascending node). See orbit.
2. The period of time the satellite is within telemetry range of a data acquisition station.
passageways: A pass-through between non-adjacent modules or spaces.
passive: Containing no power sources to augment output power, e.g., passive electrical network, passive reflector (as in the Echo satellite). Applied to a device that draws all its power from the input signal. Compare active.
passive homing: The homing of an aircraft or spacecraft wherein the craft directs itself toward the target by means of energy waves transmitted or radiated by the target. See active homing.
passive homing guidance: Guidance in which a craft or missile is directed toward a destination by means of natural radiations from the destination.
passive ranging Doppler system: (abbr Pardop). A trajectory-measuring system similar to Dovap except that no transponder is used in the missile. Space position is computed from several loop ranges between the transmitter, missile, and receivers.
paste (consistency): Mixtures with characteristic soft or plastic consistencies.
pastes: Adhesive compositions having a characteristic plastic-type consistency, that is, high order of yield values, such as that of pastes prepared by heating a mixture of starch and water and subsequently cooling the hydrolyzed product.
path: 1. Of a satellite, the projection of the orbital plane on the earth's surface, the locus of the satellite subpoint.
Since the earth is turning under the satellite, the path of a single orbital pass will not be a closed curve. Path and track are used interchangeably. On a cylindrical map projection, the path is a sine-shaped curve.
2. Of a meteor, the projection of the trajectory on the celestial sphere, as seen by the observer.
3. = flightpath.
Pathfinder: The Mars Environmental Survey (MESUR) engineering prototype.
pathogens: Disease-producing agents, usually referring to living organisms.
Patriot missile: Surface to air, antiaircraft missile.
pattern recognition: The identification of shapes, forms and configurations by automatic means.
Pav, Pavo: International Astronomical Union abbreviations for Pavo. See constellation.
Pavo: (abbr Pav, Pavo)
See constellation.
payload: 1. Originally, the revenue-producing portion of an aircraft's load, e.g., passengers, cargo, mail, etc.
2. By extension, that which an aircraft, rocket, or the like carries over and above what is necessary for the operation of the vehicle for its flight.
payload assist module: Rocket vehicle with a spinning solid propellant motor to attain injection velocity to place payload into intended orbits from the parking orbits of the STS.
payload control: Execution of events involved in operating the payload and supporting systems.
payload delivery (STS): The transport of payloads via the Space Transportation System including ground to Earth orbit delivery by the Space Shuttle and orbit to orbit delivery via orbit transfer vehicles.
payload deployment & retrieval system: System of mechanical and control devices, with associated data systems, for payload handling in space.
payload integration plan: Procedures providing for compatibility of spaceborne experiments with the carrier spacecraft (e.g., shuttle orbiter).
payload mass ratio: (symbol )
Of a rocket, the ratio of the effective propellant mass m_p to the initial vehicle mass m₀ or

Also called mass ratio.
payload stations: The locations in the Space Shuttles' flight decks and cargo bay at which payloads are mounted.
payload transfer: The in-space movement of payloads from point to point.
PCM (abbr): = pulse code modulation.
PCM/FM (abbr): Frequency modulation of a carrier by pulse code modulated information.
PCM/FM/FM (abbr): Frequency modulation of a carrier by subcarrier(s) which is (are) frequency modulated by pulse code modulated information.
PCM/PM (abbr): Phase modulation of a carrier by pulse code modulated information.
PDM (abbr): = pulse duration modulation.
PDM/FM (abbr): Frequency modulation of a carrier by pulses which are modulated in duration by information.
PDM/FM/FM (abbr): Frequency modulation of a carrier by subcarrier(s) which is (are) frequency modulated by pulses which are modulated in duration by information.
PDM/PM (abbr): Phase modulation of a carrier by pulses which are modulated in duration by information.
PDS: Planetary Data System.
PDT: Pacific Daylight Time.
PE: Post encounter phase in flyby mission operations.
peak sound pressure: For any specified time interval, the maximum absolute value of the instantaneous sound pressure in that interval.
In the case of a periodic wave, if the time interval considered is a complete period, the peak sound pressure becomes identical with the maximum sound pressure.
peak-to-peak value: Of an oscillating quantity, the algebraic difference between the extremes of the quantity.
pearlite: An aggregate in steel of ferrite and cementite.
peat: Dark brown or black residuum produced from the partial decomposition and disintegration of mosses, hedges, trees, and other plants that grow in marshes and other wet places.
Peclet number: (symbol N_{P_e})
A nondimensional number arising in problems of heat transfer in fluids. It is the ratio of heat advection to heat diffusion and may be written

where U is a characteristic velocity; l is a characteristic length; and k is the thermometric conductivity. Also,

where N_Re is the Reynolds number and N_Pr is the Prandtl number.
peculiar stars: Stars with spectra that cannot be conveniently fitted into any of the standard spectral classifications. They are denoted by a "p" after their spectral type.
PEEK: A class of semicrystalline polymers called polyayrlene ethers for use as molding compounds and for use as composite matrix materials. Used for polyetheretherketones.
Peg, Pegs: International Astronomical Union abbreviations for Pegasus. See constellation.
Pegasus: (abbr Peg, Pegs)
See constellation.
Pegs: International Astronomical Union abbreviations for Pegasus. See constellation.
Pele's hair: Thin strands of volcanic glass drawn out from molten lava have long been called Pele's hair, named for Pele, the Hawaiian goddess of volcanoes. A single strand, with a diameter of less than 0.5 mm, may be as long as 2 m. The strands are formed by the stretching or blowing-out of molten basaltic glass from lava, usually from lava fountains, lava cascades, and vigorous lava flows (for example, as pahoehoe lava plunges over a small cliff and at the front of an `a`a flow). Pele's hair is often carried high into the air during fountaining, and wind can blow the glass threads several tens kilometers from a vent.
Pele's tears: Small bits of molten lava in fountains can cool quickly and solidify into glass particles shaped like spheres or tear drops called Pele's tears, named after Pele, the Hawaiian goddess of volcanoes. They are jet black in color and are often found on one end of a strand of Pele's hair.
Peltier effect: The production or absorption of heat at the junction of two metals on the passage of an electrical current.
Heat generated by current flowing in one direction will absorbed if the current is reversed. This effect is presently being extensively studied as a possible energy conversion method for space vehicles.
Peltier effects: The effects which result in the production or absorption of heat at the junction of two metals on the passage of an electrical current.
penalty function: In mathematics, a function used in treating maxima and minima problems subject to restraints.
pencil beam: Emission, from an antenna, having the form of a narrow conical beam.
pencil-beam antenna: A unidirectional antenna, so designed that cross section of the major lobe by planes perpendicular to the direction of maximum radiation are approximately circular, and having a very small angular cross section.
penetration: The depths to which one material extends into or penetrates another.
penetrometers: Simple devices for measuring the penetrating power of a beam of x rays or other penetrating radiation by comparing transmission through various absorbers.
peninsulas: Elongated bodies or stretches of land nearly surrounded by water and connected with a larger land area, usually by a neck or an isthmus. The term is derived from the Latin 'paeninsula' "almost island."
Penning discharge: A direct-current discharge where electrons are forced to oscillate between two opposed cathodes and are restrained from going to the surrounding anode by the presence of a magnetic field.
It is sometimes referred to as a pig discharge since the device was originally used as an ionization gage (Penning ionization gage). It is used as a plasma-beam source by permitting the plasma to stream out along the magnetic field through a hole in one of the cathodes.
Penning effect: An increase in the effective ionization rate of a gas due to the presence of a small number of foreign metastable atoms.
For instance, a neon atom has a metastable level at 16.6 volts and if there are a few neon atoms in a gas of argon which has an ionization potential of 15.7 volts, a collision between the neon metastable atom with an argon atom may lead to ionization of the argon. Thus, the energy which is stored in the metastable atom can be used to increase the ionization rate. Other gases where this effect is used are helium, with a metastable level at 19.8 volts, and mercury, with an ionization level at 10.4 volts.
Penning gage: See cold-cathode ionization gage, note.
penumbra: See umbra.
penumbral eclipse: See lunar eclipse, note.
Per, Pers: International Astronomical Union abbreviations for Perseus. See constellation.
perceptual errors: Deviations from accuracy in the perception of objects, shapes, colors, weights, etc., through the use of the senses.
perfect fluid: In simplifying assumptions, a fluid chiefly characterized by lack of viscosity and, usually, by incompressibility. Also called an ideal fluid, inviscid fluid. See perfect gas.
A perfect fluid is sometimes further characterized as homogeneous and continuous.
perfect gas: A gas which has the following characteristics: (a) it obeys the Boyle-Mariotte law and the Charles-Gay-Lussac law; thus satisfying the equation of state for perfect gases; (b) it has internal energy as a function of temperature alone; and (c) it has specific heats with values independent of temperature. Also called ideal gas. Compare perfect fluid.
The normal volume of a perfect gas is 2.24136 X 10E4 centimeters cubed per mole.
perfect gas laws: = gas laws.
perfect radiator: = blackbody.
perfect vacuum: = absolute vacuum.
perfectly diffuse radiator: A body that emits radiant energy in accordance with Lambert law. The radiant intensity emitted in any direction from a unit area of such a radiator varies as the cosine of the angle between the normal to the surface and the direction of the radiation. Compare diffuse radiation, isotropic radiator.
When viewed from a distance, an incandescent perfectly diffuse radiator appears as a uniformly illuminated flat surface regardless of its actual shape or orientation.
perfectly diffuse reflector: A body that reflects radiant energy in such a manner that the reflected energy may be treated as if it were being emitted (radiated) in accordance with Lambert law. The radiant intensity reflected in any direction from a unit area of such a reflector varies as the cosine of the angle between the normal to the surface and the direction of the reflected radiant energy.
perfectly matched layers: In the area of computational electromagnetism, an absorbing boundary condition used for terminating infinite domain calculations in the finite-difference time-domain (FDTD) or finite element methods. The approach has also been extended to the analysis of some problems in acoustics.
peri: A prefix meaning near as in perigee.
periapsis: The point in an orbit closest to the body being orbited.
periapsis: The orbital point nearest the center of attraction. See orbit.
periastron: That point of the orbit of one member of a binary star system at which the stars are nearest to each other.
That point at which they are farthest apart is called apastron.
pericynthian: That point in the trajectory of a vehicle which is closest to the moon.
perifocus: The point on an orbit nearest the dynamical center ( focus). The pericenter is at one end of the major axis of the orbital ellipse.
perigee: That orbital point nearest the earth when the earth is the center of attraction. See orbit.
That orbital point farthest from the earth is called apogee. Perigee and apogee are used by some writers in referring to orbits of satellites, especially artificial satellites, around any planet or satellite, thus avoiding coinage of new terms for each planet and moon.
perigee propulsion: A programmed-thrust technique for escape from a planet, which uses intermittent applications of thrust at perigee (when vehicle velocity is high) and coasting periods.
perigee speed: The speed of an orbiting body when at perigee.
perigee-to-perigee period: = anomalistic period.
perihelion: The point in its orbit where a planet is closest to the Sun. When referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion)
perihelion: That point in a solar orbit which is nearest the sun.
That orbital point farthest from the sun is called aphelion. The term perihelion should not be confused with parhelion, a form of halo.
perihelion distance: The distance between the orbiting body and the sun at it's closest approach.
perijove: Periapsis in Jupiter orbit.
perilune: Periapsis in lunar orbit.
period: 1. The interval needed to complete a cycle.
2. = orbital period.
3. Specifically, the interval between passages at a fixed point of a given phase of a simple harmonic wave; the reciprocal of frequency.
4. The time interval during which the power level (flux) of a reactor changes by a factor of e (2.718, the base of natural logarithms).
period doubling: The bifurcation of a nonlinear system to two stable periodic cycles on its route to chaotic turbulence.
period of moon's node: See nutation, note.
period scrams: Electronic safety circuits that automatically insert safety rods in a reactor when the reactor period decreases below the safe minimum limit.
periodic comets (also short period comets): Any comet with an orbital period of less than 200 years.
periodic quantity: In mathematics, an oscillating quantity whose values recur for certain increments of the independent variable.
periodic terms: See secular terms, note.
peripheral equipment (computers): Equipment that works in conjunction with a computer but is not part of the computer itself. Card or paper-tape readers or punches, magnetic tape handlers, or line printers are among items of peripheral equipment.
periscope: An optical instrument which displaces the line of sight parallel to itself to permit a view which may otherwise be obstructed.
periscopic: Of or pertaining to a periscope, as in periscopic sextant.
periselene: Periapsis in lunar orbit.
permafrost: Any soil, subsoil or other surficial deposit, or even bedrock, occurring in arctic or subarctic regions at a variable depth beneath the Earth's surface in which a temperature below freezing has existed continuously for a long time. Used for frozen soils.
permanent magnetism: Magnetism which is retained for long periods without appreciable reduction, unless the magnet is subjected to demagnetizing force. See induced magnetism.
Because of the slow dissipation of such magnetism, it is sometimes called subpermanent magnetism, but the expression permanent magnetism is considered preferable.
permanent memory: In computer terminology, storage of information which remains intact when the power is turned off. Also called nonvolatile storage.
permeability: 1. Of a magnetic material, the ratio of the magnetic induction to the magnetic field intensity in the same region.
2. The ability to permit penetration or passage. In this sense the term is applied particularly to substances which permit penetration or passage of fluids.
3. = permeability coefficient.
permeability coefficient: The steady-state rate of a flow of gas through unit area and thickness of a solid barrier per unit pressure differential at a given temperature. Also called permeability.
Usually expressed in volume or mass per unit time, per unit area of cross section, per unit thickness, per unit pressure differential across the barrier.
permeation: As applied to gas flow through solids, the passage of gas into, through, and out of a solid barrier having no holes large enough to permit more than a small fraction of the gas to pass through any one hole. The process always involves diffusion through the solid and may involve various surface phenomena, such as sorption, dissociation, migration, and desorption of the gas molecules.
permissible dose: The amount of radiation which may be received by an individual within a specified period with expectation of no harmful result to himself.
perovskites: Minerals with a close-packed lattice and the general formula ABX3 where A and B are metals and X is a nonmetal, usually O.
Pers: International Astronomical Union abbreviation for Perseus. See constellation.
Perseus: (abbr Per, Pers)
See constellation.
persistent train: Remaining glow due to ionization in the upper atmosphere after the passage of a meteoroid. The intensity and duration depend on the meteoroid's atmospheric entry velocity, its size, and its composition. Bright fireballs occasionally caused trains visible for several minutes.
persistent train: A meteor train which endures for an appreciable length of time.
perturbation: 1. Any departure introduced into an assumed steady state of a system, or a small departure from a nominal path such as a desired trajectory. Usually used as equivalent to small perturbation.
2. Specifically, a disturbance in the regular motion of a celestial body, the result of a force additional to that which causes the regular motion, specifically, a gravitational force.
perturbation method: = method of small perturbation.
perturbation quantity: Any parameter of a system, e.g, velocity components or temperature, which may or may not have been assumed to be small perturbations for a mean or steady state value.
perveance: The quotient of the space-charge-limited cathode current by the three-halves power of the anode voltage in a diode. Note: Perveance is the constant G appearing in the Child-Langmuir-Schottky equation.
Petri nets: Abstract, formal models of the information flow in systems with discrete sequential or parallel events. The major use has been the modeling of hardware systems and software concepts of computers.
petrogenesis: Branch of petrology dealing with the origin and formation of rocks, particularly igneous rocks.
petroleum products: Materials derived from petroleum, natural gas, and asphalt deposits. Includes gasolines, diesel and heating fuels, lubricants, waxes, greases, petroleum coke, petrochemicals, and sulfur.
petrology: That branch of geology dealing with the origin, occurrence, structure, and history of rocks, especially igneous and metamorphic rocks.
Pfirsch-Schluter regime: One of the neoclassical transport parameter regimes in a tokamak plasma; characterized by the collisional mean free path being shorter than the connection length. (This is the high-collisionality end of the spectrum; plateau transport is in the middle, and the banana regime is on the low-collisionality end.)
PFM (abbr): = pulse frequency modulation.
phase: 1. Of a periodic quantity, for a particular value of the independent variable, the fractional part of a period through which the independent variable has advanced, measured from an arbitrary reference.
The arbitrary reference is generally so chosen that the fraction is less than unity. In case of a simple harmonic quantity, the reference is often taken as the last previous passage through zero from the negative to positive direction. Thus, if two wave crest one-fourth cycle apart, they are said to be 90 degrees apart in phase, or 90 degrees out of phase. The moon is said to be at first quarter when it has completed one-fourth of its cycle from new moon.
2. The stage of aggregation of a substance, for example solid, liquid, or gas.
3. The extent to which the disk of the moon or the planet, as seen from the earth, is illuminated or not illuminated by the sun.
4. In astronomy = configuration.
phase angle: The phase angle is the Sun-Target-Observer angle. A phase angle of zero indicates that the Sun is, in effect, shining over your shoulder so you see a fully-lit target (like a full Moon). A phase angle of 180 degrees indicates that you are looking back toward the Sun at the "dark side" of a target (like a new Moon). Bodies much larger than the wavelength of light tend to be brightest at low phase angles. However, fine dust is very efficient at forward-scattering light, so dust tends to be brightest at high phase angles. The manner in which a ring scatters light can tell us a great deal about its particle properties.
phase angle: 1. The phase difference of two periodically recurring phenomena of the same frequency, expressed in angular measure.
2. The angle at a celestial body between the sun and earth.
phase change materials: Materials undergoing solid/liquid phase transformations and whose latent heat of fusion properties are used to store and deliver thermal energy, usually solar energy. Used for PCM (materials).
phase conjugation: Technique for the removal of phase distortions during propagation of laser beams through the atmosphere.
phase constant: See propagation constant.
phase detector: A device that continuously compares the phase of two signals and provides an output proportional to their difference in phase.
phase deviation: The peak difference between the instantaneous phase of the modulated wave and the carrier frequency.
The extent of deviation is proportional to the amplitude of the modulating signal.
phase front: A surface of constant phase (or phase angle) of a propagating wave disturbance. Also called wave front.
Generally, phase fronts spread out spherically from their source; but in cases where energy is assumed to travel in parallel rays (as in many radiation problems), phase fronts may be approximated as plane surfaces oriented perpendicularly to the rays.
phase lock: The technique of making the phase of an oscillator signal follow exactly the phase of a reference signal by comparing the phases between the two signals and using the resultant difference signal to adjust the frequency of the reference oscillator. See correlation detection.
phase matching: A way of maximizing the coupling between two systems used in second harmonic generation which happens mostly in crystals.
phase modulation: (abbr PM)
Angle modulation in which the angle of a sine-wave carrier is caused to depart from the carrier angle by an amount proportional to the instantaneous value of the modulating wave.
Combinations of phase and frequency modulation are commonly referred to as frequency modulation.
phase shift: The phase difference of two periodically recurring phenomena of the same frequency, expressed in angular measure. The angle between the lines connecting a celestial body and the sun and a celestial body and the Earth. Used for phase angle and phase response.
phase shift keying: The form of phase modulation in which the modulating function shifts the instantaneous phase of the modulated wave among predetermined discrete values.
phase space: The sum of the three dimensions of ordinary space and the three dimensions of velocity space. See distribution function.
phase speed: = phase velocity.
phase velocity: Of a traveling plane wave at a single frequency, the velocity of an equiphase surface along the wave normal. Also called phase speed, wave speed, wave velocity.
Thus, the component sin represents a wavelength traveling in the positive x-direction with phase velocity c. This concept is to be distinguished from signal velocity, group velocity, and the velocity of fluid parcels. See velocity of propagation.
phase-lock loop: An electronic servo system incorporating phase lock and used either as a tracking filter or as a frequency discriminator.
phase-shaped antenna: = shaped-beam antenna.
phases of the moon: The various appearances of the moon during different parts of the synodical month.
The cycle begins with new moon or change of the moon at conjunction. The visible part of the waxing moon increases in size during the first half of the cycle until full moon appears at opposition, after which the visible part of the waning moon decreases for the remainder of the cycle. First quarter occurs when the waxing moon is at east quadrature; last quarter when the waning moon is at west quadrature. From last quarter to new and from new to first quarter, the moon is crescent; from first quarter to full and from full to last quarter, it is gibbous. The elapsed time, usually expressed in days since the last new moon, is called age of the moon.
Phe, Phoe: International Astronomical Union abbreviations for Phoenix. See constellation.
phenology: A branch of science dealing with the relations between climate and periodic biological phenomena.
Philips gage: A cold-cathode type of vacuum gage wherein an electrical discharge is maintained in the presence of a superposed magnetic field in order to increase the ionization current. See cold-cathode ionization gage.
Phobos: A satellite of Mars orbiting at a mean distance of 9,400 kilometers.
Phobos spacecraft: Two Soviet spacecraft (Phobos 1 and 2, both launched in July 1988) designed to study the plasma environment in the Martian vicinity, the surface and atmosphere of Mars, and the surface composition of the Martian satellite Phobos. Other mission objectives included the study of the interplanetary environment and solar observations.
Phoe: International Astronomical Union abbreviation for Phoenix. See constellation.
Phoebe: A satellite of Saturn orbiting at a mean distance of 12,960,000 kilometers.
Phoenix: (abbr Phe, Phoe)
See constellation.
phon: The unit of loudness level of sound, numerically equal to the sound pressure level in decibels, relative to 0.0002 mircobar, of a simple 1000 cycle per second tone judged by listeners to be equivalent in loudness. Compare sone.
phonometer: An instrument for measuring the intensity or frequency of sounds.
phosphazene: A ring or chain polymer that contains alternating phosphorus and nitrogen atoms, with two substituents on each phosphorus atom.
phosphor: A phosphorescent substance, such as zinc sulfide, which emits light when excited by radiation, as on the scope of a cathode-ray tube. See phosphorescence.
phosphorescence: Emission of light which continues after the exciting mechanism has ceased. See luminescence. Compare fluorescence.
An example of phosphorescence is the glowing of an oscilloscope screen after the exciting beam of electrons has moved to another part of the screen.
phosphoric acid fuel cells: Long life fuel cells for the low to medium wattage range which use phosphoric acid as an electrolyte.
phot: A photometric unit of illuminance or illumination equal to 1 lumen per square centimeter. Compare foot-candle, lux.
photoacoustic spectroscopy: An optical technique for investigating solid and semisolid materials, in which the sample is placed in a closed chamber filled with a gas and illuminated with monochromatic radiation of any desired wavelength, and with intensity modulated at some acoustic frequency. Absorption of radiation results in a periodic heat flow from the sample, which generates sound detectable with a sensitive microphone.
photocathode: An electrode used for obtaining photoelectric emission.
photocell: = photoelectric cell.
photochemical oxidants: Any of the chemicals which enter into oxidation reactions in the presence of light or other radiant energy.
photochemical reaction: A chemical reaction which involves either the absorption or emission of radiation.
photoconductive cell: A photoelectric cell whose electrical resistance varies with the amount of illumination falling upon the sensitive area of the cell.
photoconductivity: The conductivity increase exhibited by some nonmetallic materials, resulting from the free carriers generated when photon energy is absorbed in electronic transitions. The rate at which free carriers are generated, the mobility of the carriers, and the length of time they persist in conducting states (their lifetime) are some of the factors that determine the amount of conductivity change. Used for photoresistivity.
photodiodes: Diodes designed to produce photocurrent by absorbing light. Photodiodes are used for the conversion of optical power to electrical power.
photodissocation: The dissociation (splitting) of a molecule by the absorption of a photon. The resulting components may be ionized in the process (photoionization).
photoelectric: 1. Pertaining to the photoelectric effect.
2. Using a photoelectric cell, as a photoelectric photometer.
photoelectric cell: A transducer which converts electromagnetic radiation in the infrared, visible, and ultraviolet regions into electrical quantities such as voltage, current, or resistance. Also called photocell. See photoelectric effect.
photoelectric effect: The emission of an electron from a surface as the surface absorbs a photon of electromagnetic radiation. Electrons so emitted are termed photoelectrons.
The effectiveness of the process depends upon the surface metal concerned and the wavelength of the radiant energy to which it is expressed. Cesium, for example, will emit electrons when exposed to visible radiation. The energy of the electron produced is equal to the energy of the incident photon minus the amount of work needed to raise the electron to a sufficient energy level to free it from the surface. The resulting energy of the electron, therefore, is proportional to the frequency (i.e., inversely proportional to the wavelength) of the incident radiation.
photoelectric emission: The emission of electrons from atoms or molecules. Used for photocurrents, photoemission, and photoemissivity.
photoelectric emission: See photoelectric effect.
photoelectric photometry: Photometry in which a photoelectric cell is used as the sensing element.
photoelectric transducer: A transducer which converts changes in light energy to changes in electrical energy.
photoelectrochemical devices: Electrochemical devices powered by light or other incident radiation to produce electricity and/or chemical fuels (e.g., hydrogen).
photoelectrochemistry: The study of the interaction between impinging light energy and the electropotential of the chemical changes in the electrode, electrolytic solution, or a photosensitive membrane.
photoelectron: An electron which has been ejected from its parent atom by interaction between that atom and a high-energy photon.
Photoelectrons are produced when electromagnetic radiation of sufficiently short wavelength is incident upon metallic or other solid surfaces (photoelectric effect) or when radiation passes through a gas.
photogrammetry: The art or science of obtaining reliable measurements by means of photography.
photographic emulsions: The light-sensitive coatings on photographic film consisting usually of silver halide.
photographic magnitude: (symbol m_pg)
Stellar magnitudes measured from a photographic plate exposed without filters.
Photographic plates are more sensitive to short wavelengths than the human eye. The zero point of the photographic magnitude scale is such that photographic (m_pg) and visual (m_v) magnitudes are the same for stars of class A0 of magnitudes between 5.5 and 6.5. Photovisual magnitudes (m_pv) are measured from plates exposed through filters which hold back blue and violet thus giving magnitudes in the plate which closely approximate visual magnitudes (m_v).
photographic meteor: A meteor of brightness sufficient to be detected by photography.
photographic observations: A technique for discovering meteors in which images are captured on a photographic film or plate over a long exposure period, resulting in streaks where a moving illuminated body is detected.
photographic transmission density: The common logarithm of opacity. Hence, film transmitting 100 percent of the light has a density of zero, transmitting 10 percent, a density of 1, etc. Density may be diffuse, specular, or intermediate. Conditions must be specified. Also called optical density.
Diffuse transmission density is the value of the photographic transmission density obtained when the light flux impinges normally on the sample and all the transmitted flux is collected and measured. Specular transmission density is the value of the photographic density obtained when the light flux impinges normally on the sample and only the normal component of the transmitted flux is collected and measured.
photography: A process for recording visual images by exposing a light-sensitive substance to radiation such as visible light, infrared radiation, or x-rays.
photoionization: The ionization of an atom or molecule by the collision of a high-energy photon with the particle. See photoelectron.
photolithography: The process of making a printing plate by exposing a design photographically on a sensitized emulsion and removing unwanted portions chemically.
photology: The study of light.
photoluminescence: Luminescence produced by the absorption of radiant flux, distinguished from ordinary reflection by a time delay and usually, an upward shift in a wavelength.
photoluminescence: = fluorescence, see luminescence.
photomasks: In the production of integrated circuit devices, repeated arrays of microphotographs of the circult patterns on glass substrates used to form successive patterns on single wafers often of submicrometer sizes.
photometer: An instrument for measuring the intensity of light or the relative intensity of a pair of lights. Also called illuminometer.
If the instrument is designed to measure the intensity of light as a function of wavelength, it is called a spectrophotometer. Photometers may be divided into two classes: photoelectric photometers in which a photoelectric cell is used to compare electrically the intensity of an unknown light with that of a standard light, and visual photometers in which the human eye is the sensor.
photometry: The study of the measurement of the intensity of light.
At one time photometry referred only to the measurement of luminous intensity, intensity of light in the wavelength to which the eye is sensitive. This restriction has proved difficult to maintain in practice.
photomultiplier: = multiplier phototube.
photomultiplier tubes: Phototubes with one or more dynodes between its photocathode and output electrode. Used for electron multipliers and multiplier phototubes.
photon: According to the quantum theory of radiation, the elementary quantity, or quantum, of radiant energy. It is regarded as a discrete quantity having a momentum equal to hv/c , where h is Planck constant, v is the frequency of the radiation, and c is the speed of light in a vacuum. The photon is never at rest, has no electric charge and no magnetic moment, but does have a spin moment. The energy of a photon (the unit quantum of energy) is equal to hv.
photon engine: A projected type of reaction engine in which thrust would be obtained from a stream of electromagnetic radiation. Compare ion engine.
Although the thrust of this engine would be minute, it may be possible to apply it for extended periods of time. Theoretically, in space, where no resistance is offered by air particles, very high speeds may be built up.
photon gas: A radiation field.
photon rocket: A photon engine; a rocket vehicle powered by a photon engine.
photophoresis: Production of unidirectional motion in a collection of very fine particles, suspended in a gas or falling in a vacuum, by a powerful beam of light.
photopic vision: Vision associated with levels of illumination 0.01 foot-lambert or higher, characterized by the ability to distinguish colors and small detail. Also called foveal vision. Compare scotopic vision.
photopolymers: Polymers created by photochemical processes.
photosensitivity: The property of a material whereby its chemical makeup is altered by exposure to light.
photosphere: The intensely bright portion of the sun visible to the unaided eye.
The photosphere is that portion of the sun's atmosphere which emits the continuum radiation upon which the Fraunhofer lines are superimposed. In one sun model, the photosphere is thought to be below the reversing layer in which Fraunhofer absorption takes place. In another model, all strata are considered equally effective in producing continuous emissions and line absorption.
photosynthesis: A process operating in green plants in which carbohydrates are formed under the influence of light with chlorophyll serving as a catalyst. See closed ecological system.
phototheodolite: An instrument or device incorporating one or more cameras for taking and recording angular measurements.
The phototheodolite, sometimes in conjunction with radar equipment, is used to track rockets and to measure and record attitude, altitude, azimuth and elevation angles, etc.
photothermal conversion: Conversion into thermal energy from optical radiation by a photoabsorptive or photoselective material.
phototube: An electron tube that contains a photocathode and has an output depending on the total photoelectric emission from the irradiated area of the photocathode.
photovisual magnitude: See photographic magnitude, note.
photovoltaic: Materials that convert light into electric current.
photovoltaic cell: A transducer which converts electromagnetic radiation into electric current.
The solar cells used on satellites and space probes are photovoltaic cells employing a semiconductor such as silicon which releases electrons when bombarded by photons from solar radiation.
photovoltaic cells: Photoelectric detectors capable of directly generating an electric current in response to irridation.
photovoltaic effect: The production of a voltage difference across a pn junction resulting from the absorption of photon energy. The voltage difference is caused by the internal drift of holes and electrons.
phugoid oscillation: In a flightpath, a long period longitudinal oscillation consisting of shallow climbing and diving motions about a median flightpath and involving little or no change in angle of attack.
physical chemistry: The application of the laws, principles, and techniques of physics to the study of chemical properties, transformations, and reactions.
physical constant: An abstract number or physically dimensional quantity having a fixed or approximately fixed value; a universal and permanent value, as the constant of gravitation; a characteristic of a substance, as the refractive index of liquid.
A new, consistent set of values for physical constants, which has been recommended by the National Academy of Sciences-National Research Council in 1963, is presented in tables VIII, IX, and X.
physical double star: Two stars in nearly the same line of sight and at approximately the same distance from the observer, as distinguished from an optical double star (two stars in nearly the same line of sight but differing greatly in distance from the observer).
If the stars revolve about their common center of mass, they are called a binary star.
physical equation: = equation of piezotropy.
physical meteorology: That branch of meteorology which deals with optical, electrical, acoustical, and thermodynamic phenomena of atmospheres, their chemical composition, the laws of radiation, and the explanation of clouds and precipitation. As generally accepted, it does not include mathematical theory of the motions of the atmosphere and the forces responsible therefore (which matters fall in the field of dynamic meteorology). Also called atmospheric physics.
Subdivisions of physical meteorology include atmospheric electricity, cloud physics, precipitation physics, atmospheric acoustics, and atmospheric optics.
physical system: = CGS system.
Physics and Chemistry Experiment in Space: A group of Space Shuttle payloads consisting of various space experiments. Used for PACE.
physiognomic: The aspect and character of an abstract entity.
physiological acceleration: The acceleration experienced by a human or an animal test subject in an accelerating vehicle. [Original table not reproduced]
Several different terminologies have been used to describe physiological acceleration. Since the terminology may be based either on the action of the accelerating vehicle or the reaction of the passenger, the terms used are often confusing to a reader without prior knowledge of the system of terminology used. Probably the most easily understood system is the eyeballs in, eyeballs out, eyeballs down, eyeballs up, etc., terminology used by test pilots, which refers to the sensations experienced by the person being accelerated. Thus, the acceleration experienced in an aircraft pullout or inside loop is eyeballs down. Note that, in the NASA vehicle (center of gravity displacement) terminology, this is -az acceleration. Some physiological-acceleration terminologies designate accelerations in terms of the equivalent displacement acceleration of the subject as if he were starting from rest. In such terminologies a man standing up or sitting down on the surface of the earth is experiencing 1 g of headward acceleration because of gravity. Other descriptive terms used in this way are footward, forward (the acceleration experienced by a man pressed into the seat back by an accelerating vehicle), rearward, leftward, rightward, spineward, sternumward, and tailward. One terminology based on reaction uses the terms head-to-foot (the acceleration generated by a pullout in an aircraft), chest-to-back, foot-to-head, and back-to-chest.
physiology: The science that treats of the functions of living organisms or their parts, as distinguished from morphology, anatomy, etc.
phytoplankton: The aggregate of passively floating or drifting plant organisms in aquatic ecosystems.
phytotron: A biotron designed especially for research on plant life.
phytotrons: Apparatus for the growth of plants under a variety of controlled environmental conditions. Used for germinators and growth chambers.
PHz: Petahertz (10¹⁵ Hz).
pi: The constant equal to the ratio of the circumference of a circle to its diameter, which is approximately 3.141593.
PI (acronym): Principal Investigator, scientist in charge of an experiment.
Pic, Pict: International Astronomical Union abbreviations for Pictor. See constellation.
pickling (metallurgy): Preferential removal of oxide or mill scale from the surface of a metal by immersion usually in an acidic or alkaline solution.
pickoff: A sensing device that responds to angular movement to create a signal or to effect some type of control, as a pickoff on a gyro in an automatic pilot.
A pickoff may be a potentiometer, a photoelectric device, a kind of value controlling the fluid flows and pressures in a system, or one of various other devices.
pickup: 1. A device that converts a sound, scene, or other form of intelligence into corresponding electric signals (e.g., a microphone, a television camera, or a phonograph pickup).
2. The minimum current, voltage, power, or other value at which a relay will complete its intended function.
3. Interference from a nearby circuit or electrical system.
pico: A prefix meaning multiplied by 10^-12.
picometer: 10^-12 meter.
Pict: International Astronomical Union abbreviation for Pictor. See constellation.
Pictor: (abbr Pic, Pict)
See constellation.
piezoelectric ceramics: Ceramic material with piezoelectric properties similar to those of some natural crystals.
piezoelectric transducer: A transducer utilizing a piezoelectric element.
piezoelectric transducers: Transducers that depend for their operation on the interaction between electric charge and the deformation of certain materials having piezoelectric properties. Note: Some crystals and specially processed ceramics have piezoelectric properties.
piezoelectricity: The property exhibited by some asymmetrical crystalline materials which when subjected to strain in suitable directions develop electric polarization proportional to the strain.
Inverse piezoelectricity is the effect in which mechanical strain is produced in certain asymmetrical crystalline materials when subjected to an external electric field; the strain is proportional to the electric field.
pig discharge: See Penning discharge, note.
pile = nuclear reactor: The term pile comes from the first nuclear reactor, which was made by piling up graphite blocks and pieces of uranium and uranium oxide. The term reactor is now more commonly used.
pillbox antenna: A cylindrical parabolic reflector enclosed by two plates perpendicular to the cylinder, so spaced as to permit the propagation of only one mode in the desired direction of polarization.
pilot: 1. A person who handles the controls of an aircraft or spacecraft from within the craft, and in so doing, guides or controls it in three-dimensional flight.
2. A mechanical system designed to exercise control functions in an aircraft or spacecraft.
3. To operate, control, or guide an aircraft or spacecraft from within the vehicle so as to move in three-dimensional flight through the air or space.
pilot induced oscillation: Oscillations of a flying aircraft caused by transients and system changeovers, by pilot overreaction upon such transients, or by misleading pilot cues or excessive pilot gain in modern high-gain, high order aircraft control systems.
pilot land data system: A data and information management system developed to support land science research activities by archiving, retrieving, and transferring land data. Abbreviated "PLDS" the program is sponsored by the Communication and Information Systems Office, Land Processes Branch within the Office of Space Science and Applications of NASA, is managed by Goddard Space Flight Center with active participants at Ames Research Center (ARC) and the Jet Propulsion Laboratory (JPL).
pilot ratings: Subjective assessment of the handling and stability characteristics of an aircraft or other flight vehicle.
piloted: Of an aircraft or spacecraft, under, or subject to, continuous control by a person inside the vehicle.
This term is more specific than the term manned.
pinch effect: 1. The result of an electromechanical force that constricts, and sometimes momentarily ruptures, a molten conductor carrying current at high density.
2. The self-contraction of a plasma column carrying large currents due to the interaction of this current with its own magnetic field.
The current required for such an effect is the order of 10E5 amperes. If the current is pulsed on for a short time, a radially imploding shock wave is generated.
pinch reflex diode: A self-insulated ion diode in which the magnetic field from the ion and electron flow alone provide electron control, and the ion source is an anode plasma formed by relexing the electrons through a thinplastic foil.
pinhole cameras: Cameras which have no lenses, but consist essentially of a darkened box with a small hole in one side, so that an inverted image of outside objects is projected on the opposite side where it is recorded on photographic film.
pinning: Sites within a superconducting material that are produced by localizing inclusions, dislocations, voids, etc., which provide a means of resisting flux motion (flux jumps) due to Lorenz forces. SN (limited to electronics).
PIO: JPL's Public Information Office.
Pioneer Venus 1 spacecraft: This orbiter spacecraft is the first of two launched on a seven month journey to observe the planet Venus, its atmosphere and clouds. It was launched May 20, 1978 and is still operational. Used for Pioneer Venus Orbiter.
Pioneer Venus 2 entry probes: Collective term for the five Pioneer Venus atmospheric probes. They are Pioneer Venus 2 day probe, Pioneer Venus 2 night probe, Pioneer Venus 2 North probe, Pioneer Venus 2 sounder probe, and Pioneer Venus 2 transporter bus.
Pioneer Venus 2 spacecraft: This multiprobe spacecraft, launched on its Venus mission in August 1978, comprises a Transporter Bus, a sounder probe, and three identical probes (North, night, and day) which separately investigated and photographed the atmosphere, clouds and related phenomena. The multiprobe spacecraft traveled about 354 million kilometers. It entered Venus atmosphere on December 9, 1978 and all probes transmitted data. Used for Pioneer Venus 2 Multiprobe spacecraft.
pip: Signal indication on the oscilloscope screen of an electronic instrument, produced by a short, sharply peaked pulse of voltage. Also called blip.
pipelining (computers): Processing techniques for improving the capability of computer systems by modelling, sequencing control, resource allocation, etc.
Pirani gage: A thermal conductivity vacuum gage in which an increase of pressure from the zero point causes a decrease in the temperature of a heated filament of material having a large temperature coefficient of resistance, thus unbalancing a Wheatstone bridge circuit (or the circuit is adjusted to maintain the filament temperature constant).
Pisc: International Astronomical Union abbreviation for Pisces. See constellation.
Pisces: (abbr Psc, Pisc)
See constellation.
Piscis Australis: = Piscis Austrinus.
Piscis Austrinus: (abbr PsA, Psc A)
See constellation.
piston engines: Engines, especially internal combustion engines, in which a piston or pistons moving back and forth work upon a crankshaft or other device to create rotational movement. Used for reciprocating engines.
Pit crater: Pit craters are circular-shaped craters formed by the sinking or collapse of the ground. Fissures may erupt from the walls or base of a pit crater, but pit craters are not constructional features built by eruptions of lava or tephra. Pit craters may also partially fill with lava to form a lava lake. They are common along rift zones of shield volcanoes; for example, Mauna Loa and Kilauea volcanoes in Hawai`i. No one has observed the formation of a large pit crater, but they are thought to form as a consequence of the removal of support by withdrawal of underlying magma.
pitch: 1. Of a vehicle, an angular displacement about an axis parallel to the lateral axis of the vehicle.
2. In acoustics, that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from low to high.
Pitch depends primarily upon the frequency of the sound stimulus, but it also depends upon the sound pressure and waveform of the stimulus. The pitch of a sound may be described by the frequency or frequency level of that simple tone having a specified sound pressure level which is judged by listeners to produce the same pitch.
pitch (inclination): Of a vehicle, an angular displacement about an axis parallel to the lateral axis of the vehicle. Used for damping in pitch, phugoid oscillations, and pitch angles.
pitch (material): The residues from the destructive distillation of tars.
pitch angle: For a charged particle moving in a magnetic field, this is the angle arctan (v-perp/v-parallel), where v-parallel is the component of the particle's velocity parallel to the magnetic field, and v-perp is the perpendicular component. The pitch angle is zero when the particle moves purely parallel to the field, and 90-degrees when the particle has no parallel velocity at all.
pitch angle scattering: Scattering (collisional, or due to wave-particle effects) of particles in velocity space, in which the pitch angle is changed.
pitch attitude: The attitude of an aircraft, rocket, etc., referred to the relationship between the longitudinal body axis and a chosen reference line or plane as seen from the side.
pitch axis: A lateral axis through an aircraft, missile, or similar body, about which the body pitches. It may be a body, wind , or stability axis. Also called a pitching axis. See axis, sense 2 and note.
pitching axis: = pitch axis.
pitching moment: A moment about a lateral axis of an aircraft, rocket, airfoil, etc.
This moment is positive when it tends to increase the angle of attack or to nose the body upward.
pitchover: 1. The programmed turn from the vertical that a rocket takes as it describes an arc and points in a direction other than vertical.
2. The point-in-space of this action.
pitot tube: (Pronounced pee-toe. After Henri Pitot, 1695-1771, French scientist.) An open-ended tube or tube arrangement which, when immersed in a moving fluid with its mouth pointed upstream, may be used to measure the stagnation pressure of the fluid for subsonic flow; or the stagnation pressure behind the tube's normal shock wave for supersonic flow.
pitot-static head: = pitot-static tube.
pitot-static tube: A device consisting essentially of a unit combination of a pitot tube and a static tube arranged coaxially or otherwise parallel to one another, used principally in measuring impact and static pressures; also called pitot-static head.
The difference between impact and static pressure is used to measure the velocity of flow past the tube by means of a differential-pressure gage. The static pressure from a pitot-static tube may, in addition, be used in the operation of an altimeter and similar instruments.
pivots: The paths followed by a point in a diameter of a circle as the circle rolls along in a straight line. Used for trochoids.
pixels: Shortened term for 'picture elements.' They are image resolution elements in vidicon-type detectors. Used for picture elements.
plages: Clouds of calcium or hydrogen vapor that show up as bright patches on the surface of the photosphere of the sun.
plains: Any flat areas, large or small, at a low elevation; specifically, extensive regions of comparatively smooth and level or gently undulating land, having few or no prominent surface irregularities. Plains sometime have a considerable slope, and usually at a low elevation with reference to surrounding areas. Plains may be either forested or bare of trees, and may be formed by deposition or by erosion.
plan position indicator: (abbr PPI)
1. A cathode-ray indicator in which a signal appears on a radial line. Distance is indicated radially and bearing as an angle.
2. In radar technique, a cathode-ray indicator on which blips produced by signals from reflecting objects and transponders are shown in plan position, thus forming a maplike display. Also called P-indicator, P-scan, P-scope.
A north-upward plan position indicator has north at the top of the indicator regardless of the heading; a heading-upward plan position indicator has the heading of the craft maintained at the top of the indicator. On a delayed plan position indicator the start of the sweep is delayed so that the center represents a selected range. This allows distant targets to be displayed on a short range scale, thus providing larger scale presentation. An open-center plan position indicator has no signal displayed within a set distance from the center. An off-center plan position indicator is one modified so that the center about which the trace rotates can be moved from the center of the screen to provide a larger scale for distant targets. A master plan position indicator controls remote indicators or repeaters.
plan position indicators: Display devices on which target blips are shown in plan position, thus forming a map-like display, with radial distance from the center representing range and with the angle of the radius vector representing azimuth angle. Used for PPI (position indicators).
Planck constant: (symbol h)
A constant equal to 6.6256 X 10E-27 erg second. It scales the energy of electromagnetic radiation of frequency v so that the radiation appears only in quanta nhv , n being an integer.
Planck distribution law: = Planck law.
Planck equation: The quantum mechanical equation relating the energy of a photon E to its frequency nu: E = h x nu
Planck law: An expression for the variation of monochromatic radiant flux per unit area of source as a function of wavelength of blackbody radiation at a given temperature; it is the most fundamental of the radiation laws. Mathematically, Planck law is

where dw is the radiant flux from a blackbody in the wavelength interval , centered around wavelength , per unit area of blackbody surface at temperature T; c₁ and c₂ are radiation constants. This law was derived theoretically by M. Planck in 1901.
Plancks constant: The fundamental constant equal to the ratio of the energy of a quantum of energy to its frequency. It is the quantum of action.
plane of the ecliptic: The orbital plane of the Earth around the Sun. The line of the ecliptic on the celestial sphere is formed by the intersection of the plane of the ecliptic with that sphere. The reason the major planets and Moon appear in the sky close to the ecliptic is that the solar system is flat, and its orbital planes are very close to each other. We observe their motion (very nearly) edge-on.
plane polarized sound wave: At a point in an elastic medium, a transverse wave in which the displacements at all times lie in a fixed plane which is parallel to the direction of propagation. Also called linearly polarized sound wave.
The above definition is equivalent to stating that, in a plane polarized sound wave, the displacement vector at any point lies in a fixed straight line passing through the point.
plane strain: A deformation of a body in which the displacement of all points in the body are parallel to a given plane, and the displacement values are not dependent on the distance perpendicular to the plane.
plane wave: A wave in which the wave fronts are everywhere parallel planes normal to the direction of propagation.
planet: A celestial body of the solar system, revolving around the sun in a nearly circular orbit, or a similar body revolving around a star. See Table XII. See also astronomical constant, tables II and III [not reproduced], noting that some values differ in the three tables.
The larger of such bodies are sometimes called principal planets to distinguish them from asteroids, planetoids, or minor planets, which are comparatively very small. The larger planets are accompanied by satellites, such as the moon. An inferior planet has an orbit smaller than that of the earth. The four planets nearest the sun are called inner planets; the others, outer planets. The four largest planets are called major planets. The four planets commonly used for celestial observations are called navigational planets. The word planet is of Greek origin, meaning, literally, wanderer, applied because the planets appear to move relative to the stars.
planetary aberration: A displacement in the apparent position of a planet in the celestial sphere due to the relative movement of the observer and the planet. See aberration.
planetary boundary layer: The layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.
planetary boundary layer: The layer of the atmosphere from the Earth's surface to the geostrophic wind level, including the surface boundary layer and the Ekman layer.
planetary boundary layer: That layer of the atmosphere from a planet's surface to the geostrophic wind level including, therefore, the surface boundary layer and the Ekman layer. Above this layer lies the free atmosphere. Also called friction layer, atmospheric boundary layer.
planetary circulation: 1. The system of large-scale disturbances in a planet's troposphere when viewed on a hemispheric or world-wide scale.
2. The mean or time-averaged hemispheric circulation of a planetary atmosphere; also called general circulation.
planetary configurations: Apparent positions of the planets relative to each other and to other bodies of the solar system, as seen from the earth.
planetary cores: The centers of planets.
planetary craters: Collective term for craters on any of the planetary surfaces.
planetary crusts: The outermost layers of planets. The planetary crusts are on top of the mantle and are modified by various processes of weathering, sedimentation, metamorphosis, volcanism, and bombardment by meteorites.
planetary geology: Study or science of a planet, its history, and its life as recorded in the rocks. Includes the study of the surface features, the geometry of rock formations, weathering and erosion, and sedimentation.
planetary limb: In astronomy, the circular outer edge of a planet.
planetary magnetospheres: The magnetospheres of planets, especially of Jupiter, Saturn, Uranus and Neptune, all of which have dipole-like magnetic fields stronger than the Earth´s.
planetary magnetotails: The portion of the magnetosphere extending from a planet in the direction away from the sun for a variable distance of the order of 1,000 planet radii.
planetary nebulae: A shell of gas ejected from, and expanding about, a certain kind of extremely hot star.
planetary precession: That component of general precession caused by the effect of other planets on the equatorial protuberance of the earth, producing an eastward motion of the equinoxes along the ecliptic. See precession of the equinoxes.
Planetary precession is approximately 0.1247 second of arc per year.
planetary systems: Systems consisting of a star and the planets and other objects in orbit around it.
planetary waves: Waves on uniform currents in two-dimensional nondivergent fluid systems rotating with varying angular speeds about the local vertical (beta plane). These waves represent a special case of barotropic disturbance, conserving absolute vorticity. As applied to atmospheric flow, the planetary waves takes into account the variability of the Coriolis parameter while assuming the motion to be two-dimensional. Used for long waves (meteorology) and Rossby waves.
planetesimals: Primordial bodies of intermediate size that accreted into planets or asteroids.
planetocentric: 1. Of or pertaining to a planet's center of mass.
2. Of or pertaining to the planet as a center of a system.
planetographic: Referring to positions on a planet measured in latitude from the planet's equator and in longitude from a reference meridian.
planetoid: = asteroid.
See planet.
planets: Celestial bodies of the solar system, revolving around the sun in nearly circular orbits, or similar bodies revolving around stars. The larger of such bodies are sometimes called principal planets to distinguish them from asteroids, planetoids, or minor planets, which are comparatively small. The larger planets are accompanied by satellites such as the moon. Inferior planets have orbits smaller than that of the Earth; superior planets have orbits larger than that of the Earth. The four planets nearest the sun are called inner planets; the others, outer planets. The four largest planets are called major planets. The four planets commonly used for celestial observations are called navigational planets. The word planet is of Greek origin, meaning, literally, wanderer, applied because the planets appear to move relative to the stars.
planform: The shape or form of an object, such as an airfoil , as seen from above, as in a plan view.
planimetric: Two dimensional. The measurement of plane surfaces. A map representing only horizontal features. Parts of a map that represent everything except relief.
plankton: The aggregate of passively floating or drifting plant and animal organisms which provide the major source of sustenance for animal life in the aquatic ecosystem. Used for plankton bloom.
plant design: Encompasses all design consideration of physical plants, i.e., airports, industrial plants, test facilities, etc. Structural is just one aspect of this design. SN (excludes biological plants)
plant stress: Stimulus or a series of stimuli of such magnitude as to disrupt the growth and/or survival of plants.
plasma (physics): An electrical conductive gas comprised of neutral particles, ionized particles, and free electrons but which, taken as a whole, is electrically neutral.
A plasma is further characterized by relatively large intermolecular distances, large amounts of energy stored in the internal energy levels of the particles, and the presence of a plasma sheath at all boundaries of the plasma. Plasmas are sometimes referred to as a fourth state of matter.
plasma antennas: An air plasma made by ionizing the atmosphere which acts as the conducting element of an RF antenna.
plasma arc cutting: Use of plasma torches for cutting hard materials at extremely high temperatures.
plasma bubbles: Pockets of very low electron density in the equatorial F region of the ionosphere in which the plasma density is lower than the ambient density.
plasma cloud: Specifically, a mass of ionized gas flowing out of the sun.
plasma clouds: Specifically, a mass of ionized gas flowing out of the sun.
plasma compression: Decrease in volume and consequent increase in density of a plasma usually by the applicaton of an intense magnetic field.
plasma cooling: Temperature control of plasmas in controlled fusion operations.
plasma core reactors: Nuclear reactors utilizing fissionable plasmas (such as uranium fluoride) for the fuel.
plasma currents: Electric currents induced in plasmas by injection of fast ion beams or some other means.
plasma display devices: Digital matrix flat panel devices in which small gas discharge plasma cells are used as light emitting sources.
plasma drift: Movement in the ionosphere of ion and plasma concentration by electric field variations in the upper atmosphere.
plasma engine: A reaction engine using magnetically accelerated plasma as propellant.
A plasma engine is a type of electrical engine.
plasma equilibrium: Condition of plasma in which the constituent particles or fluid elements are unaccelerated or collectively at rest in steady flow.
plasma etching: Removal of material by use of a focused plasma beam.
plasma focus: A highly compressed plasma.
plasma frequencies: The natural collective oscillation frequency of a charge species (electrons, ions, etc.) in a plasma, in the absence of (or at least parallel to) a magnetic field. Also known as Langmuir or Langmuir-Tonks frequency.
plasma frequency: The natural frequency for motion of electrons in a plasma. The plasma frequency

where e is charge on the electron; m is mass of the electron; and N is number of electrons per cubic centimeter. See critical frequency.
plasma generator: 1. A machine, such as an electric-arc chamber, that will generate very high heat fluxes to convert neutral gases into plasma.
2. A device which uses the interaction of a plasma and electrical field to generate a current.
plasma interaction experiment: A NASA Lewis experiment, the first of which was launched piggyback with Landsat 3 in 1978 to study the charged particle space plasma environment and its effect on spacecraft surfaces operating at high voltages. The experiment lasted several hours as planned. The second was launched piggyback with Iras in 1983. Used for PIX.
plasma length: = Debye length.
plasma oscillations: Class of electrostatic oscillations which occur at/near the plasma frequency and involve oscillations in the plasma charge density. Also known asLangmuir Oscillations;
plasma physics: The study of the nature and properties of highly ionized gases (comprised of ions and free electrons).
plasma physics: The study of the properties of plasmas.
plasma pumping: Application of radiation of appropriate frequencies to plasma, to increase the population of atoms or molecules in the higher energy states.
plasma rocket: A rocket using a plasma engine. Also called electromagnetic rocket.
plasma sheath: 1. The boundary layer of charged particles between a plasma and its surrounding walls, electrodes, or other plasmas.
The sheath is generated by the interaction of the plasma with the boundary material. Current flow may be in only one direction across the sheath (single sheath), in both directions across the sheath (double sheath), or when the plasma is immersed in a magnetic field, it may flow along the sheath surface at right angles to the magnetic field (magnetic current sheath).
2. An envelope of ionized gas that surrounds a body moving through an atmosphere at hypersonic velocities.
The plasma sheath affects transmission, reception, and diffraction of radio waves; thus it is important in operational problems of spacecraft.
plasma sheet: A near-equatorial layer of denser plasma in the tail of the Earth´s magnetosphere. It separates the two tail lobes, the two bundles of magnetic field lines connected to the regions around the Earth´s magnetic poles.
plasma temperature: A measure of the random (thermal) kinetic energy of the ions or electrons in the plasma. The temperature of each component of a plasma depends on the mean kinetic energy of that component. An example of this is the fluorescent light bulb, which is an example of a weakly-ionized plasma where the electrons are at temperatures of tens of thousands of degrees, whereas the ions and neutrals are much cooler
plasma torches: Burners which attain 50,000 degrees C temperatures by the use of plasma gas injected into an electric arc. Plasma torches are used for welding, spraying molten metal, and cutting hard rock or hard metals.
plasma waves: A disturbance of a plasma away from equilibrium, involving oscillations of the plasma's constituent particles and of an electromagnetic field. Plasma waves can propagate from one point in the plasma to another without net motionof the plasma.
plasmadynamic lasers: Stimulated emission devices in which the lasing gas flow has been replaced with a lasing plasma flow of atoms or ions.
plasmasphere: Envelope of highly ionized gases surrounding the Earth or another planet.
plastic properties: The tendency of a loaded body to assume a deformed state other than its original state when the load is removed. Used for plasticity.
plasticity: The tendency of a loaded body to assume a deformed state other than its original state when the load is removed.
plastics: Materials that contain as an essential ingredient one or more organic polymeric substances of large molecular weight, are solid in their finished state, and at some stage in their manufacture or processing into finished articles can be shaped by flow.
plastisols: A suspension of a finely divided polymer in a plasticizer.
plate: 1. A planar body whose thickness is small compared with its other dimensions.
2. A common name for the principal anode in an electron tube.
plateaus: Broadly, and comparatively flat areas of great extent and elevation; specifically, extensive land regions considerably elevated (more than 150-300 meters in altitude) above the adjacent country or above sea level. They are commonly limited on at least one side by an abrupt descent, have a flat or nearly smooth surface but are often dissected by deep valleys and surmounted by high hills or high mountains, and have a large part of their total surface at or near the summit level. Plateaus are usually higher and have more noticeable relief than plains (they often represent elevated plains), and are usually higher and more extensive than mesas. They may be tectonic, residual, or volcanic in origin.
plates (tectonics): Rigid divisions of the outer surface of the Earth (lithosphere) which moves over a weaker layer (asthenosphere). The plates are about 100 km thick, and the continents, which are 40 km thick, rest on the plates and moves with them.
Platonic year: = great year.
Plinian eruption: Plinian eruptions are large explosive events that form enormous dark columns of tephra and gas high into the stratosphere (>11 km). Such eruptions are named for Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79 A.D. This eruption generated a huge column of tephra into the sky, pyroclastic flows and surges, and extensive ash fall.
PLL: Phase-lock-loop circuitry in telecommunications technology.
plus count: In the launch of a rocket, a count in seconds (plus 1, plus 2, etc.) that immediately follows T-time, used to check on the sequence of events after the action of the countdown has ended.
Pluto: See planet, table.
Pluto (planet): Ninth planet from the sun, sometimes classified as a small terrestrial planet.
plutonium: Radioactive metallic element (Pu). The primary isotope, plutonium-239, is a product of neutron absorption by U-238, esp. in fission reactors. Pu is used in nuclear weapons and as a fission reactor fuel.
ply orientation: The arrangement of bonded layers comprising laminated materials to obtain optimal strength or other characteristics.
pm: Picometer (10^-12 meter).
PM: Post meridiem (Latin: after midday), afternoon.
PM (abbr): = phase modulation.
PMR (abbr): = Pacific Missile Range.
PN10: Pioneer 10 spacecraft.
PN11: Pioneer 11 spacecraft.
pneumatic-probe pyrometer: A thermometer for high-temperature gases, in which the gas is sucked through a nozzle and then cooled. Reliance is place principally on knowledge of the law of gas expansion through the nozzle and on measurement of pressure and mass flow rate of the gas.
pneumatics: The branch of physics dealing with the mechanical properties of gases with particular emphasis on gas statics in closed systems.
pod: An enclosure, housing, or detachable container of some kind, as an engine pod.
point discharge: A silent, nonluminous, gaseous electrical discharge from a pointed conductor maintained at a potential which differs from that of the surrounding gas. Compare corona discharge, spark discharge.
point of inflection: See inflection.
point spread functions: Mathematical functions involved in image processing.
poise: The unit of viscosity in the cgs system equal to 1 dyne second per square centimeter.
Poiseuille flow: The steady laminar flow of a fluid through a narrow horizontal circular cylinder according to the relation

where u is the fluid velocity along the cylinder's axis at a distance r from the cylinder's axis; µ is the dynamic viscosity of the fluid; a is the cylinder radius; and is the pressure gradient along the axis of the cylinder. The velocity profile across the cylinder is seen to be parabolic, and this relation affords a convenient experimental means of determining a fluid's viscosity. Also called Hagen-Poiseuille flow. Compare Couette flow.
poison: In a nuclear reactor, those atoms (of such elements as boron) other than fuel that have large capture cross section for thermal neutrons. In capturing thermal neutrons unproductively, these atoms decrease the number available to cause fission.
poisoning (reaction inhibition): Buildup of ash and impurities in a fusion plasma tends to reduce the quality of the plasma and reduce the fusion output; this sort of process is sometimes called "poisioning" the reactor or the plasma.
Poisson constant: (symbol µ)
The ratio of the gas constant to the specific heat of a gas at constant pressure. See Poisson equation, sense 2.
Poisson distribution: A one-parameter discrete frequency distribution giving the probability the n points (or events) will be (or occur) in an interval (or time) x , provided that these points are individually independent and that the number occurring in a subinterval does not influence the number occurring in any other nonoverlapping subinterval. It has the form

The mean and variance are both , and is the average density (or rate) with which the events occur. When is large, the Poisson distribution approaches the normal distribution. The binomial distribution approaches the Poisson when the number of events n becomes large and the probability of success P becomes small in such a way that .
Poisson equation: 1. The partial differential equation

where is the Laplacian operator; is a scalar function of position; and F is a given function of the independent space variables. For the special case F = 0, the Poisson equation reduces to the Laplace equation. See relaxation method.
2. The relationship between the temperature T and pressure p of a perfect gas undergoing an adiabatic process; given by

where µ is the Poisson constant.
This equation defines a family of process lines, called isentropes or dry adiabats, each of which represents the changes of state possible in a fluid with a constant value of entropy.
polar blackout: = blackout.
polar caps: In magnetospheric usage, the regions around the Earth´s magnetic poles, inside the auroral oval. The field lines in these regions extend into the tail lobes of the Earth; they reach great distances and do not close in the magnetosphere.
polar coordinates: 1. In a plane, a system of curvilinear coordinates in which a point is located by its distance r from the origin (or pole) and by the angle which a line (radius vector) joining the given point and the origin makes with a fixed reference line, called the polar axis. The relations between rectangular Cartesian coordinates and polar coordinates are

where the origins of the two systems coincide and the polar axis coincides with the X-axis.
2. In three dimensions, short for space polar coordinates.
polar distance: Angular distance from a celestial pole; the arc of an hour circle between a celestial pole, usually the elevated pole, and a point on the celestial sphere, measured from the celestial pole through 180 degrees.
If the declination, d, and the celestial pole are of the same name, the polar distance is 90 degrees - d, but if of contrary name, it is 90 degrees + d. See codeclination.
polar orbit: The orbit of an earth satellite that passes over or near the earth's poles.
Polar Orbiting Environmental Satellite: The Polar Orbiting Environmental Satellite (POES) system began in 1960 with the launch of TIROS-1. Later satellites in the Improved TIROS Operational Satellite (ITOS) program were expanded to capture concurrent multiple-channel data on a daily basis. The Advanced TIROS-N satellites (renamed NOAA-6, 7, 8, etc., after launch) offer 4 or 5 channel multispectral daily repetitive global coverage.
polar orbits: A satellite orbit passing over both poles of the Earth. During a 12-hour day, a satellite in such an orbit can observe all points on Earth.
polar plume: Bright structure of out-flowing gas which occur along magnetic field lines in coronal holes. These field lines extend into the solar system. Although plumes usually occur at the poles, they can appear anywhere there is a coronal hole.
polar rain: A drizzle of electrons observed inside the polar caps, apparently from the high end of the energy distribution of solar wind electrons. Its origin in the solar corona is revealed by the fact that in general only one polar cap receives it at any time--the one which (depending on IMF polarity) is linked to the Sun.
polar wandering (geology): Migration during geologic time of the Earth's poles of rotation and magnetic poles. Also known as polar migration. Used for Chandler motion.
Polar Year: See International Polar Year.
polarimeter: An instrument for determining the degree of polarization of electromagnetic radiation, specifically the polarization of light.
polarimetry: The measurement and study of the polarization of light reflected off of a surface.
polariscope: An instrument for detecting polarized radiation and investigating its properties.
polarity: The sign of the electric discharge associated with a given object, as an electrode or an ion.
polarizability: A measure of the degree to which any given atom or ion undergoes polarization in the presence of an external electric field.
polarization: 1. The state of electromagnetic radiation when transverse vibrations take place in some regular manner, e.g., all in one plane, in a circle, in an ellipse, or in some other definite curve.
Radiation may become polarized because of the nature of its emitting source, as is the case with many types of radar antennas, or because of some processes to which it is subjected after leaving its source, as that which results from the scattering of solar radiation as it passes through the earth's atmosphere.
2. With respect to particles in an electric field, the displacement of the charge centers within a particle in response to the electric force acting thereon. See polarizability.
3. The response of the molecules of a paramagnetic medium (such as iron) when subjected to a magnetic field.
A right-handed polarized wave is defined as one receding from the observer and radiated by an electric vector rotating clockwise in a fixed plane that is in front of the observer and at right angles to the direction of propagation of the wave in question. Left-handed polarization is the rotation in a counter-clockwise manner. This recommended definition of circular (or elliptical) polarization sense is according to that of the Institute of Radio Engineers. The definition of classical physics is exactly the opposite.
polarizer: A device for polarizing radiant energy. See polarization.
pole: 1. The origin of a system of polar coordinates.
2. For any circle on the surface of a sphere, the point of intersection of the surface of the sphere and the normal line through the center of the circle. See geographical pole, celestial pole, elevated pole, depressed pole, ecliptic pole, fictitious pole.
3. A point of concentration of electric charge. See dipole.
4. A point of concentration of magnetic force. See magnetic pole.
pole of the Milky Way: The pole in the galactic system of coordinates.
pollution transport: Dispersing or diffusion of atmospheric or water pollutants. Used for atmospheric loading.
poloidal flux: Plasma confinement concept with multipole magnetic fields.
polyacetylene: An aliphatic organic polymer that has high semiconductor properties which can be enhanced by doping.
polybrominated biphenyls: A group of 209 chemicals whose toxicity varies and includes principally one fire retardant called firemaster. Used for PBB.
polyesters: Polymers in which the repeated structural unit in the chain is of the ester type.
polymer matrix composites: Materials consisting of reinforcing fibers, filaments, and/or whiskers embedded in polymeric bonding matrices for increased mechanical and physical properties.
polymerization: A chemical reaction in which the molecules of monomers are linked together to form polymers.
polynuclear organic compounds: Hydrocarbon molecules with two or more nuclei and with or without oxygen, nitrogen, or other elements.
polynucleotides: Linear sequences of esters of nucleotides and phosphoric acid.
polypeptides: In organic chemistry, chains of amino acids linked by peptide bonds but with lower molecular weights than proteins; obtained by synthesis or by partial hydrolysis of proteins.
polytropic atmosphere: A model atmosphere in hydrostatic equilibrium with a constant nonzero lapse rate. The vertical distribution of pressure and temperature is given by

where p is the pressure; T is the Kelvin temperature; g is the acceleration of gravity; R is the gas constant for the gases concerned; and is the environmental lapse rate. The subscript 0 denotes values at the planet's surface.
polytropic process: A thermodynamic process in which changes of pressure p and density are related according to the formula

where is a constant and the subscript 0 denotes initial values of the variables. Therefore pressure and temperature are similarly related:

where k is the coefficient of polytropy. For isobaric processes, k = 0; for isosteric process, k = 1; for adiabatic processes k = c_p/R , where c_p is the specific heat at constant pressure and R is the gas constant.
In meteorology this formula is applied to individual gas parcels and should be distinguished from that for a polytropic atmosphere, which describes a distribution of pressure and temperature in space. See equation of piezotropy.
polyvinyl fluoride: DuPont's Tedlar, unplasticized PVF films with outstanding resistance to ultraviolet radiation. Used for Tedlar (trademark).
ponds: Natural bodies of standing fresh water occupying small surface depressions, usually smaller than lakes and larger than pools.
population: In statistical usage, any definite class of individuals or objects. Also called universe. Compare sample.
porosity: The volume fraction of voids contained in a solid, often expressed as a percent.
port: 1. A place of access to a system where energy may be supplied or withdrawn or where system variables may be observed or measured.
In any particular case, the ports are determined by the way in which the system is used, and not by the structure alone. A designated pair of terminals is an example of a port.
2. An opening, as the port in a solid rocket.
Portevin-le Chatelier effect: A discontinuous yielding in crystalline solids characterized by a serrated or step-like deformation curve.
posigrade rocket: An auxiliary rocket which fires in the direction in which the vehicle is pointed, used, for example, in separating two stages of vehicle.
position: 1. A point in space.
2. A point defined by stated or implied coordinates, particularly one on the surface of the earth.
3. = attitude.
4. A crew member's station aboard an aircraft or spacecraft. See line of position, band of position, surface of position.
position angle: = parallactic angle.
position angle (PA): The PA of a tail or other cometary feature represents the direction on the sky (in degrees from north) toward which it is pointing.
position vector: See vector.
positional notation: Any scheme for representing quantities characterized by the arrangement of digits in sequence with the understanding that successive digits are to be interpreted as coefficients of successive powers of an integer called the base or radix.
The base determines the name of the notation, as, binary (base 2), decimal (base 10), or duodecimal (base 12).
positive acceleration: 1. Acceleration such that speed increases.
2. Accelerating force in an upward sense or direction, e.g., from bottom to top, seat to head, etc.; acceleration in the direction that this force is applied. See physiological acceleration.
positive column: The luminous glow, often striated, which occurs between the Faraday dark space and the anode in a glow discharge plasma tube.
positive feedback: Feedback which results in increasing the amplification.
positive g or positive G: See physiological acceleration.
positive ions: Group of atoms which has acquired a positive electric charge by the loss of one or more electrons.
positron: A subatomic particle which is identical to the electron in atomic mass, theoretical rest mass, and energy, but opposite in sign. Compare proton.
The positron is short lived and can exist only when in motion. When it combines with an electron, both particles are annihilated and two photons result, equal in energy to the combined masses of the annihilated particles. Production of positrons can occur only in pair formation with the electron, the inverse of the annihilation process.
positron emission: Form of nuclear decay where a proton disintegrates into a neutron, positron, and some sort ofneutrino.
positrons: Subatomic particles which are identical to electrons in atomic mass, theoretical rest mass, and energy, but opposite in sign.
posthypoxia paradox: An abrupt convulsive incident which may occur when a marked oxygen deficiency is relieved by sufficient oxygen; this is in contrast to the normal rapid recovery from lack of oxygen. Also called oxygen paradox.
postlaunch reports: Memoranda issued following spacecraft launchings to report launch data, the launch vehicle performance, orbital elements (expected and measured), and current status.
postmission analysis (spacecraft): A broader term than postflight analysis which deals with the scientific aspects of a mission.
potential: (symbol )
1. A function of space, the gradient of which is equal to a force. In symbols, F = - , where F is the force; is the del-operator; and is the potential. A force which may be so expressed is said to be conservative, and the work done against it in motion from one given equipotential surface to another is independent of the path of the motion. See Gibbs function, potential energy.
In celestial mechanics and geodesy, the negative of the potential, sometimes called force function, is usually employed.
2. Applied to the value that an atmospheric thermodynamic variable would attain if processed adiabatically from its initial pressure to the standard pressure of 1000 millibars. See potential density, potential temperature.
3. Short for electric potential.
potential density: The density a parcel of air would attain if compressed adiabatically by descent to the standard pressure of 1000 millibars. The potential density is most easily defined in relation to the potential temperature as

where p is a pressure of 1000 millibars and R is the gas constant, in appropriate units. See adiabatic process.
potential energy: Energy possessed by a body by virtue of its position in a gravity field in contrast with kinetic energy, that possessed by virtue of its motion.
potential gradient: In general, the local space rate of change of any potential, as the gravitational potential gradient or the velocity potential gradient.
potential index of refraction: An atmospheric index of refraction so formulated that it would have no height variation in an adiabatic atmosphere. Also called potential refractive index. Compare modified index of refraction.
The potential index of refraction is usually expressed in terms of B-units.
potential refractive index: = potential index of refraction.
potential temperature: The temperature a parcel of dry air would have if brought adiabatically from its initial state to the (arbitrarily selected) standard pressure of 1000 millibars. Its mathematical expression is

where is the potential temperature; T is the Kelvin temperature; p is pressure, millibars; R is the gas constant for dry air; and c_p is the specific heat or dry air at constant pressure. See equivalent potential temperature, adiabatic process.
potentiometer: 1. An instrument for measuring differences in electric potential by balancing the unknown voltage against a variable know voltage. If the balancing is accomplished automatically, the instrument is called a self-balancing potentiometer.
2. A variable electric resistor.
potentiometric transducer: A transducer in which the displacement of the force summing members is transmitted to the slider in a potentiometer, thus changing the ratio of output resistance to total resistance.
pound: (abbr lb)
1. A unit of mass equal in the United States to 0.45359237 kilogram, exactly.
2. Specifically, a unit of measurement of the thrust or force of a reaction engine representing the weight the engine can move, as an engine with 100,000 pounds of thrust. See poundal, pound mass.
3. The force exerted on 1 pound mass by the standard acceleration of gravity. See gravity, sense 2.
pound force: = pound, sense 3.
pound mass: 1. A mass equal to 0.45359237 kilogram.
2. A unit of measure of the inertial property equal to the mass of a body weighing 1 pound at the standard acceleration of gravity (980.665 centimeters per second squared).
pound weight: A force equal to the earth's attraction for a mass of 1 pound. This force, acting on a 1- pound mass, will produce an acceleration of 32.1747 feet per second squared.
poundal: A unit of force; that unbalanced force which, acting on a body of 1 pound mass, produces an acceleration of 1 foot per second squared. See pound, pound mass.
powder (particles): An aggregate of discrete particles that are usually within the size range 1 to 1,000 mm.
powder metallurgy: The art of producing metal powders and of the utilization of metal powders for the production of massive materials and shaped objects.
power: A measure of the amount of work done per second, expressed in Watts.
power: 1. (symbol P). Rate of doing work.
2. Luminous intensity.
3. The number of times an object is magnified by an optical system, such as a telescope. Usually called magnifying power.
4. The result of multiplying a number by itself a given number of times, as the third power of a number is its cube ; the superscript which indicates this process as in 2E3 = 2 X 2 X 2.
power density: The rated power of a reactor or isotopic power source per unit volume. Power density is often stated in kilowatts per cubic centimeter of core volume.
power factor controllers: A solid state electronic device that reduces excess energy waste in AC induction motors by providing only the amount of voltage required to satisfy a given load.
power gain: 1. The ratio of the power that a transducer delivers to a specified load, under specified operating conditions, to the power absorbed by its input circuit.
If the input and/or output power consist of more than one component, such as multifrequency signal or noise, then the particular components used and their weighting must be specified. This gain is usually expressed in decibels.
2. Of an antenna, in a given direction, 4pi times the ratio of the radiation intensity in that direction to the total power delivered to the antenna.
power loading: The ratio of the gross weight of a propeller-driven aircraft to its power, usually expressed as the gross weight of the aircraft divided by the rated horsepower of the power plant corrected for air of standard density. With turboprop engines, the equivalent shaft horsepower is used. Compare thrust loading.
power modules (STS): Modules for providing power for payloads for STS and mission dependent equipment.
power package: An engine, especially a reciprocating engine, together with its accessories, lines, cowling, etc., ready for quick installation on an aircraft.
power plant: 1. The complete assemblage or installation of engine or engines with accessories (induction system, cooling system, ignition system, etc.) that generates the motive power for a self-propelled vehicle or vessel such as an aircraft, rocket, etc.
2. An engine or engine installation regarded as a source of power.
power series: An infinite series of increasing power of the variable, of the form

Both the variable and the coefficients may take on complex values. The totality of values of x for which a power series is convergent is called the interval of convergence of the series.
power spectrum: The square of the amplitude of the (complex) Fourier coefficient of a given periodic function. Thus if f(t) is periodic with period T , its Fourier coefficients are

where and the power spectrum of f(t) is . Here n take integral values and the spectrum is discrete. The total energy of the periodic function is infinite, but the power, or energy per unit period, is finite.
powered models: Models that can be tested in complete force equilibrium, including propulsion.
Poynting-Robertson effect: The gradual decrease in orbital velocity of a small particle such as a micrometeorite in orbit about the sun due to the absorption and reemission of radiant energy by the particle.
Poynting-Robertson effect: A dissipative force caused by the anisotropic loss of momentum by a particle through re-radiation of solar energy. This causes aphelion collapse such that a circular orbit is soon attained; thereafter the particle spirals slowly towards the Sun. Small particles (below 1cm) are most severely affected because the force varies as the reciprocal of its size.
PPI (abbr): = plan position indicator.
PPI reflectoscope: See beam splitter.
PPI scope: = plan position indicator.
PPM (abbr): = pulse position modulation.
PPM/AM (abbr): Amplitude modulation of a carrier by pulses which are position modulated by information.
Prandtl number: (symbol N_Pr, P_r)
(After Ludwig Prandtl, 1875-1953, German scientist). A dimensionless number representing the ratio of momentum transport to heat transport in a flow, defined by the equation

where µ is the viscosity coefficient; c_p is the specific heat at constant pressure; and k is the coefficient of thermal conductivity.
The Prandtl number may also be defined as the product of the Reynolds and Peclet numbers.
pre-Imbrian period: One of four stratigraphic classifications adopted for displaying (on maps) the geological ages of major features on the moon.
pre-main sequence stars: Stars in which nuclear reactions that take place in its core have not yet occurred.
preamplifier: 1. An amplifier, the primary function of which is to raise the output of a low-level source to an intermediate level so that the signal may be further processed without appreciable degradation in the signal-to-noise ratio of the system.
A preamplifier may include provision for equalizing and/or mixing.
2. In radar an amplifier separated from the remainder of the receiver and located so as to provide the shortest possible input circuit path from the antenna so as to avoid deterioration of the signal-to-noise ratio.
precession: Change in the direction of the axis of rotation of a spinning body, as a gyro, when acted upon by a torque. See apparent wander, precession of the equinoxes.
The direction of motion of the axis is such that it causes the direction of spin of the gyro to tend to coincide with that of the impressed torque. The horizontal component of precession is called drift, and the vertical component is called topple.
precession in declination: The component of general precession along a celestial meridian, amounting to about 20 seconds of arc per year.
precession in right ascension: The component of general precession along the celestial equator, amounting to about 46.1 seconds of arc per year.
precession of the equinoxes: A slow motion of the axis of the Earth around a cone, one cycle in about 26000 years. As a result, the celestial pole moves around a circle in the sky, and in ancient times, for instance, was quite far from Polaris. Discovered by Hipparchus around 130 BC as a slow shift of the vernal equinox around the ecliptic (i.e. around the zodiac).
precession of the equinoxes: The conical motion of the earth's axis about the normal to the plane of the ecliptic, caused by the attractive force of the sun, moon, and other planets on the equatorial protuberance of the earth.
The effect of the sun and moon, called lunisolar prescession, is to produce a westward motion of the equinoxes along the ecliptic. The effect of other planets, called planetary precession, tends to produce a much smaller motion eastward along the ecliptic. The resultant motion, called general precession, is westward along the ecliptic at the rate of about 50.3 seconds of arc per year. The component of general precession along the celestial equator, called precession in right ascension, is about 46.1 seconds of arc per year; and the component along a celestial meridian, called precession in declination, is about 20.0 seconds of arc per year.
precipitation (chemistry): The separation of a new phase from solid or liquid solution, usually with changing conditions of temperature or pressure or both.
precipitation (meteorology): The precipitation of water from the atmosphere in the form of hail, mist, rain, sleet, and snow. Deposits of dew, fog, and frost are excluded.
precipitation attenuation: The loss of radio energy due to the passage through a volume of the atmosphere containing precipitation. Part of the energy is lost by scattering and part by absorption. See cloud attenuation, range attenuation.
Radars operating at wavelengths of 10 centimeters and higher are generally unaffected, whereas even the smallest precipitation rates will seriously attenuate radar energy of wavelengths less than 1 centimeter. For rain and snow diameter-to-wavelength ratios less than 0.07, the loss is due primarily to absorption. Scattering becomes important for ratios near 0.1 and greater. Attenuation by dry snow is small for most radar wavelengths.
precision: Precision is a statistical measurement of repeatability that is usually expressed as a variance or standard deviation, root-mean-square or RMS, of repeated measurements. These are expressed as x,y coordinates of arcs, label points, and tics in either single or double precision in ARC/INFO. Single-precision coordinates have up to seven significant digits of precision. This allows for a level of accuracy of approximately 10 meters for a region whose extent is 1,000,000 meters across. Double-precision coordinates have up to 15 significant digits; this allows for the precision necessary to represent any desired map accuracy at a global scale.
precision: The quality of being exactly or sharply defined or stated. A measure of the precision of a representation is the number of distinguishable alternatives from which it was selected, which is sometimes indicated by the number of significant digits it contains. Compare accuracy.
precision guided projectiles: Missiles guided by precise laser radiation.
precombustion chamber: In a rocket, a chamber in which the propellants are ignited and from which the burning mixture expands torchlike to ignite the mixture in the main combustion chamber.
predissociation: A process by which a molecule that has absorbed energy separates into constituents before it loses energy by radiation. See dissociation.
prelaunch summaries: Summaries prior to launch of the preparations and parameters of the mission.
preliminary stage: = prestage.
premixing: The mixing of ingredients prior to a specified action (mixing of fuel and air prior to ignition in combustion, for example).
prepolymers: Polymers of degrees of polymerization between that of the monomer or monomers, and the final polymer.
prepregs: The reinforcing materials containing or combined with the full complement or resin before molding operations in the production of composite materials.
presentation: In electronics, the act or process of displaying radar echoes on a cathode-ray screen; the echo or images displayed on a cathode-ray screen.
preset guidance: A type of guidance in which devices in the aircraft or spacecraft, adjusted before launching, control the path of the missile.
Presidential reports: Formal reports originated by the President or his office.
pressure: Force or load per unit area. Used for surface pressure.
pressure: (symbol p )
1. In a gas, the net rate of transfer of momentum in the direction of the positive normal to an imaginary plane surface of specified area located in a specified position in the gas by molecules crossing the surface in both directions, momentum transmitted in the opposite direction being counted as negative, divided by the area of the surface.
In general, it is assumed that the area of the imaginary plane surface is small enough so that the pressure with respect to any part of the surface is equal (within narrow limits) to the pressure based on the whole surface. Different kinds of pressure (static, dynamic, partial, total, vaport, etc.) are distinguished by the orientation of the surface with respect to mass-flow velocity vectors or by the restriction to a specified set of molecular species crossing the imaginary surface.
2. On a boundary surface, the force applied per unit area and equal to the pressure in the gas as determined by molecules crossing an imaginary surface located at a fixed distance of molecular magnitude in front of the real surface, the positive normal being drawn from the imaginary surface toward the real surface.
The term pressure when used alone can be assumed to refer to the total pressure in a gas at rest or else to refer to the static pressure in a gas flowing under steady-state conditions.
3. = atmospheric pressure.
4. As measured in a vacuum system, the quantity measured at a specified time by a so-called vacuum gage, whose sensing element is located in a cavity (gage tube) with an opening oriented in a specified direction at a specified point within the system, assuming a specified calibration factor.
The sensitivity of the sensing element is, in general, not the same for all molecular species, but the gage reading is frequently reported using the calibration factor for air regardless of the composition of the gas. The opening to the gage tube is often carelessly oriented with respect to mass-flow vectors in the gas (which is seldom at rest), and errors due to variations in wall temperatures of tube and system are frequently neglected. The actual total pressure in a high-vacuum system cannot usually be measured by a single gage, but in vacuum technology the term total pressure is sometimes used to refer to the reading of a single untrapped gage which responds to condensable vapors as well as permanent gases.
pressure altimeter: An altimeter that utilizes the change of atmospheric pressure with height to measure altitude. It is commonly an aneroid altimeter. Also called barometric altimeter. See aneroid, sense 1.
pressure altitude: 1. Altitude in the earth's atmosphere above the standard datum plane, standard sea level pressure, measured by a pressure altimeter.
2. The altitude in a standard atmosphere corresponding to atmospheric pressure encountered in a real atmosphere.
3. The stimulated altitude created in an altitude chamber.
pressure amplitude: = maximum sound pressure.
pressure breathing: The breathing of oxygen or of a suitable mixture of gases at a pressure higher than the surrounding pressure. See continuous pressure breathing, intermittent pressure breathing.
pressure broadening: The process in which the width of the lines in an emission spectrum or absorption spectrum of a gaseous radiative medium is increased due to perturbations of the energy states by collisions of the molecules or atoms within the gas. The extent of this line-broadening effect is directly proportional to the number of impacts experienced by the emitter or absorber per unit time, and hence is proportional to the pressure. Compare Doppler broadening.
pressure dependence: Study of how a rate constant changes with pressure.
pressure height: = pressure altitude.
pressure microphone: A microphone in which the electric output substantially corresponds to the instantaneous sound pressure of the impressed sound wave.
pressure modulator radiometers: A cell containing a known quantity of a gas is placed in the single optical path of the radiometer and subjected to cyclical pressure changes which alter the absorption lines in the infrared spectrum of the gas. A narrow band signal results from the different voltages at the detector at high and low cell pressures. A wideband signal is generated by physically chopping a percentage of the input beam with a rotating chopper blade.
pressure ratio: The relationship of a force to the deformation of a system whose deformation varies in some proportion to the force.
pressure stabilized: Referring to membrane-type structures that require internal pressure for maintenance of a stable structure; for example, the Atlas missile structure.
pressure suit: A garment designed to provide pressure upon the body so that respiratory and circulatory functions may continue normally, or nearly so, under low-pressure conditions, such as occur at high altitudes or in space without benefit of a pressurized cabin.
A pressure suit is distinguished from a pressurized suit, which inflates, although it may be fitted with inflating parts that tighten the garment as ambient pressure decreases. Compare g-suit.
pressure tensor: A generalized pressure (can be anisotropic) which plays a role in MHD analogous to that of pressure in ordinary fluid mechanics.
pressure thrust: In rocketry, the product of the cross-sectional area of the exhaust jet leaving the nozzle exit and the difference between the exhaust pressure and the ambient pressure.
pressure transducer: A transducer which produces an output related to imparted pressure.
pressure wave: 1. In meteorology, a short period oscillation of pressure such as that associated with the propagation of sound through the atmosphere; a type of longitudinal wave. See sound wave, compression wave.
These waves are usually recorded on sensitive microbarographs capable of measuring pressure changes of amounts down to 10E-4 millibar. Typical values for the period and wavelength of pressure waves are 1/2 to 5 seconds and 100 to 1500 meters, respectively. Pressure waves produced by explosions in the upper atmosphere are of value in determining the high-altitude temperatures and winds.
2. A wave or periodicity which exists in the variation of atmospheric pressure on any scale, usually excluding normal diurnal and seasonal trends. See barometric wave.
Such waves can persist for an indefinite length of time only if they coincide approximately with the free oscillations of the atmosphere. Waves of a period longer than that associated with the passage of large-scale weather disturbances are difficult to isolate, since they usually have such a small amplitude that they can be extracted from the data and only by means of precise statistical methods.
pressure-breathing system: An oxygen system in which oxygen is injected inside the respiratory ducts through a pressure higher than the surrounding pressure.
pressure-demand oxygen system: A demand oxygen system that furnishes oxygen at a pressure higher than atmospheric pressure above a certain altitude.
pressurization: The process of producing pressures higher than ambient, as in a pressurized cabin.
pressurized: Containing air, or other gas, at a pressure higher than ambient.
pressurized suit: A suit designed to be inflated so as to provide pressure directly upon the body, not to air surrounding the body. Compare pressure suit.
pressurizing gas: Specifically, a gas used to expel propellant from a fuel tank.
prestage: 1. A step in the action of igniting a large liquid rocket taken prior to the ignition of the full flow, and consisting of igniting a partial flow of propellants into the thrust chamber.
2. The partial flow thus ignited. Also called preliminary stage.
prevaporization: The phase transformations of liquids to gases prior to some physical or chemical reaction.
primary: 1. Short for primary body.
2. Short for primary cosmic ray.
primary body: The celestial body or central force field about which a satellite or other body orbits, or from which it is escaping, or towards which it is falling.
The primary body of the moon is the earth; the primary body of the earth is the sun.
primary circle: = primary great circle.
primary circulation: In meteorology, the prevailing fundamental atmospheric circulation on a planetary scale which must exist in response to radiation differences with latitude, to the rotation of the planet, and to the particular distribution of land and oceans; and which is required from the viewpoint of conservation of energy.
primary cosmic ray: High-energy particles originating outside the earth's atmosphere.
Primary cosmic rays appear to come from all directions in space. Their energy appears to range from 10E9 to more than 10E17 electron volts.
primary great circle: A great circle used as the origin of measurement of a coordinate; particularly, such a circle 90 degrees from the poles of a system of spherical coordinates, as the equator. Also called primary circle, fundamental circle.
primary radar: Radar using reflection only, in contrast with secondary radar which uses automatic retransmission on the same or a different radio frequency.
primary scattering: Any scattering process in which radiation is received at a detector, such as the eye, after having been scattered just once; to be distinguished from multiple scattering.
primary standard: A unit directly defined and established by some authority, against which all secondary standards are calibrated, as the prototype kilogram.
prime meridian: 1. The meridian of longitude 0 degrees, used as the origin for measurement of longitude. The meridian of Greenwich, England, is almost universally used for this purpose.
2. Any meridian in any coordinate system used as an origin for measurement of longitude.
prime vertical: The vertical circle through the east and west points of the horizon. It may be true, magnetic, compass , or grid depending upon which east or west points are involved. Also called prime vertical circle.
prime vertical circle: = prime vertical.
primers (coatings): Coatings designed to enhance adhesion.
primitive atmosphere: The atmosphere of a celestial body as it existed in the early stages of its formation; specifically, the earth's atmosphere of 3 billion or more years ago, through to consist of water vapor, carbon dioxide, methane, and ammonia gas.
primitive equations: The Eulerian equations of motion of a fluid in which the primary dependent variables are the fluid's velocity components. These equations govern a wide variety of fluid motions and form the basis of most hydrodynamical analysis.
primitive period: Of a periodic quantity, the smallest increment of the independent variable for which the function repeats itself.
If no ambiguity is likely, the primitive period is simply called the period of the function.
principal planets: The larger bodies revolving about the Sun in nearly circular orbits. See planet.
The known principal planets, in order of their distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
principal stresses: The normal stresses on three mutually perpendicular planes on which there are no shear stresses.
principal vertical circle: The vertical circle through the north and south points of the horizon, coinciding with the celestial meridian.
principle of reciprocity: If an electromotive force at one point in a circuit produces a current at a second point in the circuit, then the same voltage acting at the second point will produce the same current at the first point.
prisms: Transparent bodies with at least two polished plane faces inclined with respect to each other, from which light is reflected or through which light is refracted. When light is refracted by a prism whose refractive index exceeds that of the surrounding medium, it is deviated or bent toward the thicker part of the prism.
privacy: Freedom from observation and/or intrusion. Applies to such things as communications, personal records, photographs.
probability: The chance that a prescribed event will occur, represented as a pure number P in the range 0<P<1. The probability of an impossible event is zero and that of an inevitable event is unity.
Probability is estimated empirically by relative frequency, that is, the number of times the particular event occurs divided by the total count of all events in the class considered.
probability integral: The classical form (still widely used in engineering work) of the definite integral of the special normal distribution for which the mean µ = 0 and standard deviation . Geometrically, the probability integral equals the area under this density curve between -z and z , where z is an arbitrary positive number. Often denoted by the symbol erf z (read error function of z ) the probability integral is defined thus:

Also called error function, erf.
probable error: (symbol e_p)
In statistics, that value e_p for which there exists and even probability (0.5) that the actual error exceeds e_p. The probable error e_p is 0.6745 times the standard deviation .
The probable error is not 'probable' in the normal sense of the word.
probe: 1. Any device inserted in an environment for the purpose of obtaining information about the environment.
2. In geophysics, a device used to make a sounding.
3. Specifically, an instrumented vehicle moving through the upper atmosphere or space or landing upon another celestial body in order to obtain information about the specific environment.
In sense 3, almost any instrumented spacecraft can be considered a probe. However, earth satellites are not usually referred to as probes. Also, almost any instrumented rocket can be considered a probe. In practice, rockets which attain an altitude of less than 1 earth radius (400 miles) are called sounding rockets, those which attain an altitude of more that 1 earth radius are called probes or space probes. Spacecraft which enter into orbit around the sun are called deep-space probes. Spacecraft designed to pass near or land on another celestial body are often designated lunar probe, Martian probe, Venus probe, etc.
4. Specifically, a slender device or apparatus projected into a moving fluid, as for measurement purposes; a pitot tube.
5. Specifically, a slender projecting pipe on an aircraft which is thrust into a drogue to receive fuel in inflight refueling.
procedures: Detailed instructions for the performance of a process or function.
process control (industry): The ways and means by which continuous manufacturing and other industrial processes are monitored and maintained to create products of planned, uniform dimension and quality.
process heat: Increase in enthalpy accompanying chemical reactions or phase transformations at constant pressure (heat of crystallization and heat of sublimation are examples).
process lapse rate: The rate of decrease of the temperature T of an air parcel as it is lifted, -dT/dz , when z is altitude, or, occasionally, dT/dp , where p is pressure.
The concept may be applied to other atmospheric variables, e.g., the process lapse rate of density. The process lapse rate is determined by the character of the fluid processes and should be carefully distinguished from the environmental lapse rate, which is determined by the distribution of temperature in space. In the atmosphere the process lapse rate is usually assumed to be either the dry-adiabatic lapse rate or the saturation-adiabatic lapse rate.
production costs: The process of fabrication, from raw materials through the finished products, including packaging and other prorated costs.
profile: 1. Of a variable, a curve representing corresponding values of two or more variables which may occur.
A profile accounts for the correlation from point to point on the curve and has some possibility, not necessarily specified, of actual occurrence.
2. The contour or form of a body, especially in a cross section; specifically, an airfoil profile.
3. Something likened to a profile (sense 1), such as a line on a graph, as a flight profile.
profiles: A method of making Digital Elevation Models (DEMs). In this technique a stereo pair of photographs is set up in a photogrammetric instrument and referenced to the ground using ground control points. After this process is completed the instrument automatically moves a computer cursor across the stereo model. As the cursor is being driven across the model, the operator controls the motion of the cursor while a recording device captures the elevation figures. Each swath across the stereo model is called a profile.
program: 1. In computer operations, a plan for the solution of a problem.
2. To create a plan for the solution of a problem.
A complete program includes plans for the transcription of data, coding for the computer, and plans for the absorption of the results into the system. The list of coded instructions, called a routine, plans a computation or process from the asking of a question to the delivery of the result, including the integration of the operation into an existing system. Thus, programming consists of planning and coding, including numerical analysis, systems analysis, specification of printing formats, and any other functions necessary to the integration of a computer in a system.
Project SETI: A program to search for extraterrestrial intelligence by means of radio communication. Used for Search for Extraterrestrial Intelligence and SETI.
projectile: 1. Any object, especially a missile, fired, thrown, launched, or otherwise projected in any manner, such as a bullet, a guided rocket missile, a sounding rocket, a pilotless airplane, etc.
2. Originally, an object, such as a bullet or artillery shell, projected by an applied external force.
projection: Orderly system of lines on a plane representing a corresponding system of imaginary lines on an adopted terrestrial or celestial datum surface. Also, the mathematical concept of such a system. For maps of the Earth, a projection consists of (1) a graticule of lines representing parallels of latitude and meridians of longitude or (2) a grid.
prolate spheroid: An ellipsoid of revolution, the longer axis of which is the axis of revolution.
An ellipsoid of revolution, the shorter axis of which is the axis of revolution, is called an oblate spheroid.
prolate spheroids: Ellipsoids of revolutions, the longer axis of which is the axis of revolution.
Prometheus: A natural satellite of Saturn, orbiting at a mean distance of 139,350 kilometers.
prominence: A filamentlike protuberance from the chromosphere of the sun. See flocculi. Compare flare.
Prominences can be observed visually (optically) whenever the sun's disk is masked, as during an eclipse or by using a coronagraph; and can be observed instrumentally by filtering in certain wavelengths, as with a spectroheliograph. A typical prominence is 6,000 to 12,000 kilometers thick, 60,000 kilometers high, and 200,000 kilometers long.
prominences: A cloud of cooler plasma extending high above the Sun"s visible surface, rising above the photosphere into the corona.
prompt neutrons: In nuclear fission, those neutrons released coincident with the fission process, as opposed to the neutrons subsequently released.
prompt radiation: See radioactivity, note.
prop-fan technology: Technology of a small diameter, highly loaded, many-bladed variable pitch advanced turboprop.
propagation: The spreading abroad or sending forward, as of radiant energy.
propagation constant: Of a traveling plane wave at a given frequency, the complex quantity whose real part is the attenuation constant in nepers per unit length and whose imaginary part is the phase constant in radians per unit length.
propagation error: For ranging systems, the algebraic sum of propagation velocity error and curved-path error.
Except at long ranges and low angles, the curved-path component of propagation error is generally negligible.
propagation ratio: For a wave propagating from one point to another, the ratio of the complex electric-field strength at the second point to that at the first point.
propagation velocity: = velocity of propagation.
propagation velocity error: The difference between the effective value of propagation velocity, over a ray path, and the assumed value. See effective propagation velocity.
propargyl groups: Crosslinking agents for certain aromatic polyamides used as matrix resins in fiber composites.
propellant: (symbol p , used as a subscript). Any agent used for consumption or combustion in a rocket and from which the rocket derives its thrust, such as a fuel, oxidizer, additive, catalyst, or any compound or mixture of these; specifically, a fuel, oxidant, or a combination or mixture of fuel and oxidant used in propelling a rocket. See fuel.
Propellants are commonly in either liquid or solid form.
propellant explosions: Detonations of propellants as a result of motor malfunction.
propellant mass fraction: (symbol )
Of a rocket, the ratio of the effective propellant mass m_p to the initial vehicle mass m₀ or

Also called mass ratio, propellant mass ratio.
propellant mass ratio: = propellant mass fraction.
propellants: Any agents used for consumption or combustion in rockets and from which the rockets derive their thrust, such as fuels, oxidizers, additives, catalysts, or any compounds or mixture of these; specifically, fuels, oxidants, or a combination or mixture of fuels and oxidants used in propelling rockets. Propellants are commonly in either liquid or solid form.
propeller pitch: The angle at which the propeller blade (or part of it) "bites" into the air, its angle of attack.
proper motion: That component of the space motion of a celestial body perpendicular to the line of sight, resulting in the change of a star's apparent position relative to other stars. Proper motion is expressed in angular units.
proportional control: Control of an aircraft, rocket, etc. in which control-surface deflection is proportional to the movement of the remote controls. Compare flicker control.
proportional navigation: The control of the angular rate of the velocity vector of a vehicle in proportion to the apparent relative angular velocity of its moving target.
proprioceptive stimulation: Stimulation originating within the deeper structures of the body (muscles, tendons, joints, etc.) for sense of body position and movement and by which muscular movements can be adjusted with a great degree of accuracy and equilibrium can be maintained.
propulsive efficiency: (symbol _p)
The efficiency with which energy available for propulsion is converted into thrust by a rocket engine.

where u is the absolute vehicle velocity, and c is the effective exhaust velocity with respect to the vehicle. Propulsive efficiency is a maximum when u = c.
protein synthesis: Process by which protein molecules are formed.
Proteus: A natural satellite of Neptune, orbiting at a mean distance of 117,600 kilometers.
proton: A positively charged subatomic particle having a mass of 1.67252 x 10^-24 gram, slightly less than that of a neutron but about 1836 times greater than that of an electron.
proton storm: The flux of protons sent into space by a solar flare.
proton-proton reaction: A thermonuclear reaction in which two protons collide at very high velocities and combine to form a deuteron. The resultant deuteron may capture another proton to form tritium and the latter may undergo proton capture to form helium. Compare carbon cycle.
The proton-proton reaction is believed to be the principal source of energy within the sun and other stars of its class. A temperature of the order of 5 million degrees Kelvin and high hydrogen (proton) concentrations are required for this reaction to proceed at rates compatible with energy emission by such stars.
protoplanet: Transition objects formed during primeval cloud condensation into stellar systems (stars, planets, etc.) which form the nucleus of planetary accretion. Used for planetesimals.
protoplanet: Any of the sun's planets as it emerged or existed in the formative period of the solar system.
protostars: Very dense regions (or cores) of molecular clouds where stars are in the process of forming.
protosun: The sun as it emerged in the formation of the solar system.
prototype: 1. Of any mechanical device, a production model suitable for complete evaluation of mechanical and electrical form, design, and performance.
2. The first of a series of similar devices.
3. A physical standard to which replicas are compared, as the prototype kilogram.
prototypes: Preliminary type, form, or instance of systems that serve as models for later stages or for the final, completed version of the systems.
proving stand: A test stand for reaction engines, especially rocket engines. See test stand.
proximity effect (electricity): Redistribution of current in a conductor caused by the presence of another conductor.
PsA, Psc A: International Astronomical Union abbreviations for Piscis Austrinus. See constellation.
Psc A: International Astronomical Union abbreviations for Piscis Austrinus. See constellation.
Psc, Pisc: International Astronomical Union abbreviations for Pisces. See constellation.
pseudo-wet-bulb potential temperature: = wet-bulb potential temperature.
pseudoadiabatic expansion: A saturation-adiabatic process in which the condensed water substance is removed from the system, and therefore best treated by the thermodynamics of open systems. See adiabatic process.
Meteorologically, this process corresponds to rising air from which the moisture is precipitating. Descent of air so lifted becomes by definition a dry-adiabatic process.
pseudocode: An arbitrary code not directly understandable by a computer. Also called interpreter code.
pseudoequivalent temperature: = equivalent temperature, sense 2.
pseudopotentials: Factors in an approximate method for calculation of energy bands in solids by the use of approximation which includes the many body effect.
PST: Pacific Standard Time.
PSU: Pyrotechnic Switching Unit onboard a spacecraft.
psycholinguistics: Study of linguistic behavior such as conditioning by psychological factors including the speaker's and listener's culturally determined categories of expression and comprehension.
psychology: The science which studies the functions of the mind, such as sensation, perception, memory, through, and, more broadly, the behavior of an organism in relation to its environment.
psychomotor ability: Of or pertaining to muscular action ensuing directly from a mental process, as in the coordinated manipulation of aircraft or spacecraft controls.
psychomotor performance: Of or pertaining to muscular action ensuing directly from a mental process, as in the coordinated manipulation of aircraft or spacecraft controls.
psychopharmacology: The science that deals with the action of drugs on mental function.
psychophysical quantity: A physical measurement, as a threshold, dependent on human attributes or perception.
psychrometers: Instruments for measuring humidity through the use of wet and dry bulb thermometers.
PTM (abbr): = pulse time modulation.
Ptolemy's system: The explanation given by ancient Greek astronomers to the motion of planets around the sky, described in a book by the Greek Ptolemy, around 150 AD. It regarded Earth as the center of the universe and assumed the motion of planets was a superposition of circular motions .
Puck: A natural satellite of Uranus, orbiting at a mean distance o of 86,010 kilometers.
pulmonary: Pertaining to, or affecting, the lungs or any component of the lungs.
pulsars: A rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, x-ray, and gamma-ray energies.
pulse: 1. A variation of a quantity whose value is normally constant; this variation is characterized by a rise and a decay, and has a finite duration.
The word pulse normally refers to a variation in time; when the variation is in some other dimensions, it should be so specified, such as space pulse. This definition is so broad that it covers almost any transient phenomenon. The only features common to all pulses are rise, finite duration, and decay. It is necessary that the rise, duration, and decay be of a quantity that is constant (not necessarily zero) for sometime before the pulse and has the same constant value for some time afterwards. The quantity has a normally constant value and is perturbed during the pulse. No relative time scale can be assigned.
2. Radar, sense 2.
3. The intermittent change in the shape of an artery due to an increase in the tension of its walls following the contraction of the heart. The pulse is usually counted at the wrist (radial pulse), but may be taken over any artery that can be felt.
pulse amplitude: A general term indicating the magnitude of a pulse.
For specific designation, adjectives such as average, instantaneous, peak, root-mean-square (effective), etc., should be used to indicate the particular meaning intended. Pulse amplitude is measured with respect to the normally constant value unless otherwise stated.
pulse amplitude modulation: Null
pulse charging: Rapid and efficient method for charging electric batteries.
pulse code: 1. A sequence of pulses so modulated as to represent information.
2. Loosely, a code consisting of pulses, such as Morse code, binary code.
pulse code modulation: (abbr PCM). Any modulation which involves a pulse code.
This is a generic term, and additional specification is required for a specific purpose.
pulse compression: The coding and processing of a signal pulse of long time duration to one of short time duration and high range resolution, while maintaining the benefits of high pulse energy.
pulse decay time: The interval between the instants at which the instantaneous amplitude of a pulse last reaches specified upper and lower limits, namely, 90 percent and 10 percent of the peak pulse amplitude unless otherwise stated.
pulse Doppler radar: A pulse radar system which utilizes the Doppler effect for obtaining information about the target (not including simple resolution from fixed targets).
pulse duration: The time interval between the first and last instants at which the instantaneous amplitude reaches a stated fraction of the peak pulse amplitude.
pulse duration modulation: A form of pulse time modulation in which the duration of a pulse is varied.
The terms pulse width modulation and pulse length modulation are also used to designate this system of modulation but the term pulse duration modulation is preferred.
pulse frequency modulation: (abbr PFM). A form of pulse time modulation in which the pulse repetition rate is the characteristic varied.
A more precise term for pulse frequency modulation would be pulse repetition rate modulation.
pulse length modulation: = pulse duration modulation.
pulse modulation: 1. Modulation of a carrier by a pulse train. Compare frequency modulation.
In this sense, the term is used to describe that process of generating carrier frequency pulses.
2. Modulation of one or more characteristics of a pulse carrier.
In this sense, the term is used to describe methods of transmitting information on a pulse carrier.
pulse packet: In radar, the volume of space occupied by the radar pulse energy.
pulse phase modulation: (abbr PPM) = pulse position modulation.
pulse position modulation: (abbr PPM). A form of pulse time modulation in which the position in time of a pulse is varied. Also called pulse phase modulation.
pulse radar: A type of radar, designed to facilitate range measurement, in which the transmitted energy is emitted in periodic short pulses. Also called pulsed radar. Compare continuous-wave radar.
The distance to any target a detectable echo can be determined by measuring one-half the time interval between transmitted pulse and received echo and multiplying this number by the speed of light. This is by far the most common type of radar.
pulse repeater: In a transponder, a device used for receiving pulses from one circuit and transmitting corresponding pulses into another circuit. It may also change the frequency and wave forms of the pulses and perform other functions.
pulse spike: An unwanted pulse of relatively short duration superimposed on the main pulse.
pulse storm: A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds.
pulse time modulation: (abbr PTM). Modulation in which the values of instantaneous samples of the modulating wave are caused to modulate the time of occurrence of some characteristic of a pulse carrier.
pulse train: In radio, a sequence of pulses.
pulse width: The time interval during which a pulse exceeds a reference level.
For measuring pulse width, the reference level is generally taken at the half-power points.
pulse width modulation: (abbr PWM) = pulse duration modulation.
pulsed Doppler system: A pulse radar system which utilizes the Doppler effect for obtaining information about the target (not including simple resolution from fixed targets).
pulsed power: The technology of using electrical energy stores for producing multi-terawatt (10^12 Watts or higher) pulses of electrical power for inertial confinement fusion, nuclear weapon effects simulation, and directed energy weapons. High efficiency and cost effectiveness make it desirable technology for large energy experiments.
pulsed radar: = pulse radar.
pulsejet: A pulsejet engine.
pulsejet engine: A type of compressorless jet engine in which combustion takes place intermittently, producing thrust by a series of explosions, commonly occurring at the approximate resonance frequency of the engine. Often called a pulsejet.
pulses: Short-wave trains of mechanical vibration.
pultrusion: Process of pulling continuous lengths of resin impregnated fiber through a shaped, heated die to produce lengths of reinforced plastic.
pumice: Pumice is a light, porous volcanic rock that forms during explosive eruptions. It resembles a sponge because it consists of a network of gas bubbles frozen amidst fragile volcanic glass and minerals. All types of magma (basalt, andesite, dacite, and rhyolite) will form pumice. Pumice is similar to the liquid foam generated when a bottle of pressurized soda is opened--the opening depressurizes the soda and enables dissolved carbon dioxide gas to escape or erupt through the opening. During an explosive eruption, volcanic gases dissolved in the liquid portion of magma also expand rapidly to create a foam or froth; in the case of pumice, the liquid part of the froth quickly solidifies to glass around the glass bubbles.
pumice: A light-colored, vesicular, glassy rock commonly having the composition of a rhyolite.
punched cards: Cards on which a pattern of holes or cuts is used to represent data.
punched tapes: Tapes on which a pattern of holes or cuts is used to represent data.
Pup, Pupp: International Astronomical Union abbreviations for Puppis. See constellation.
Puppis: (abbr Pup, Pupp)
See constellation.
purge: To rid a line or tank of residual fluid, especially of fuel or oxygen in the tanks or lines of a rocket after a test firing or simulated test firing.
Purkinje effect: The response of the human eye which makes it less sensitive to lights of loner wavelengths under conditions of decreased illumination, e.g., red appears darker at night than blue having the same brightness under photopic conditions. See color index, dark adaptation, photopic vision.
push-pull = balanced: See balanced amplifier, balanced circuit.
push-pull amplifier: = balanced amplifier.
push-pull amplifiers: Amplifiers in which there are two identical signal branch circuits so as to operate in phase opposition and with input and output connections each balanced to ground. Used for balanced amplifiers.
push-push circuit: A circuit employing two similar tubes with grids connected in phase opposition and plates in parallel to a common load, and usually used as a frequency multiplier to emphasize even-order harmonics.
pushbroom sensor modes: Spacecraft instrument arrangements in which large numbers of detectors comprising linear arrays are swept by the forward motion of the spacecraft to attain increased fidelity and high sensitivity in the data captured.
PWM (abbr): = pulse width modulation.
pyranometer: An actinometer which measures the combined intensity of incoming direct solar radiation and diffuse sky radiation. The pyranometer consist of a recorder and a radiation sensing element which is mounted so that it views the entire sky. Sometimes called solarimeter. See pyrheliometer, solarimeter, Robitzsch actinograph, albedometer.
pyrazines: Compounds that contain a six-membered heterocyclic ring containing nitrogen atoms in the 1 and 4 positions.
pyrgeometer: An instrument developed by K. Ångström for measuring the effective terrestrial radiation. It consists of four manganin strips, of which two are blackened and two are polished. The blackened strips are allowed to radiate to the atmosphere while the polished strips are shielded. The electrical power required to equalize the temperature of the four strips is taken as a measure of the outgoing radiation. See actinometer, pyrgeometer. Compare Ångström compensation pyrheliometer.
pyrgeometer: An actinometer which measures the effective terrestrial radiation.
pyrheliometer: An actinometer which measures the intensity of direct solar radiation, consisting of a radiation sensing element enclosed in a casing which is closed except for a small aperture, through which the direct solar rays enter, and a recorder unit. See silver-disk pyrheliometer, water-flow pyrheliometer, Eppley pyrheliometer, spectropyrheliometer, Michaelson actinograph.
pyrheliometry: The science and study of pyrheliometric measurements. See pyrheliometer.
pyridines: Compounds that contain a six-membered heterocyclic ring containing one nitrogen atom.
pyrimidines: Compounds that contain a six-membered heterocyclic ring containing nitrogen atoms in the 1 and 3 positions.
pyroheliometers: See pyrheliometer
pyrohydrolysis: Decomposition by the action of heat and water vapor.
pyrolysis: Chemical decomposition by the action of heat.
pyrometer: An instrument for the measurement of temperatures; generally applied to instruments measuring temperatures above 600 degrees C.
pyrometers: Instruments that measure high temperature, e.g., of molten lavas, by electrical or optical means.
pyrometric photography: The derivation of flame temperature measurements by means of comparative photography with a calibrated light source.
pyrometry: High-temperature thermometry, the technique of measurement of temperatures, generally above 600 degrees C, at a distance.
pyron: A unit of radiant intensity of electromagnetic radiation equal to 1 calorie per square centimeter per minute.
pyrophoric fuel: A fuel that ignites spontaneously in air. Compare hypergolic propellants.
pyrophyllite: A white, greenish, gray, or brown phyllosilicate mineral that resembles talc.
pyroxenes: A group of dark, rock-forming silicate minerals.
pyrrhotite: A common reddish-brown to bronze hexagonal mineral.
pyrroles: Compounds that contain a five-membered heterocyclic ring containing one nitrogen atom.
Pyx, Pyxi: International Astronomical Union abbreviations for Pyxis (= Malus ). See constellation.
Pyxis: (= Malus)
(abbr Pyx, Pyxi). See constellation.

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