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

Aerospace Science & Technology Dictionary

Aerospace Science
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Technology Dictionary