Aerospace Science and Technonlogy Dictionary H Section

N: Newton, a unit of force equal to the force required to accelerate a 1-kg mass 1 m per second per second (1m/sec²). Compare with dyne.
N-display: In radar, a display similar to the K-display in which the target appears as a pair of vertical deflections or blips from the horizontal time base. Direction is indicated by the relative amplitude of the vertical deflections; target distance is determined by moving an adjustable signal along the baseline until it coincides with the horizontal position of the vertical deflections. The horizontal control is calibrated in distance. Also called N-scan, N-scope, N-indicator.
N-indicator: = N-display.
N-scan: = N-display.
N-scope: = N-display.
N-unit: A unit of index of refraction in the atmosphere; a mathematical simplification designed to replace rather awkward numbers involved in the values of the index of refraction n for the atmosphere. It is defined by the relation N = (n - 1) 10E6
Thus, if n = 1.000326, then N = 326. Correspondingly, M-units are used for modified index of refraction, and B-units for potential index of refraction.
NACA (abbr): National Advisory Committee for Aeronautics. The predecessor of NASA.
NACA Standard Atmosphere: See standard atmosphere.
nadir: The direction from a spacecraft directly down toward the center of a planet. Opposite the zenith.
nadir: That point on the celestial sphere vertically below the observer, or 180 degrees from the zenith.
Naiad: A natural satellite of Neptune orbiting at a mean distance of 48,000 kilometers.
naked singularities: Singularities in spacetime that will be visible and communicable to the outside world, i.e., singularities that are not shielded by an event horizon from infinity.
nakhlites: Achondritic stony meteorites consisting of a holocrystalline aggregate of diopside (75 percent) and olivine.
nano: (abbr n)
A prefix meaning multiplied by 10^-9.
nanosatellites: Satellites with a total mass smaller than 10 kg incorporating miniaturized electronic and mechanical systems.
nanosecond: (abbr nsec)
10E-9 second. Also called millimicrosecond.
nap-of-the-earth navigation: Low altitude flight of helicopters during night or day utilizing electronic means for detection and recognition of landmarks and targets. Used for NOE navigation.
Napierian base: The logarithmic base, e.
NAPP--National Aerial Photography Program: NAPP was established to coordinate the collection of aerial photography covering the 48 contiguous States and Hawaii every five years. NAPP's goals are to ensure that photography with uniform scale, quality, and cloud-free coverage be made available to meet the requirements of several Federal and State agencies. The program was initiated in 1980 as the National High Altitude Photography (NHAP) program. In 1987, the program was renamed to NAPP when the flying height for the program changed from 40,000 feet to 20,000 feet. NAPP photography is available in black and white, and in most cases, color-infrared. The program is administered by the U.S. Geological Survey's National Mapping Division. NAPP imagery is used by the USGS for photo revision and land use land cover characterization work on the standard series maps at 1:24,000; 1:100,000 and 1:250,000 scales.
narcosis: A state of profound stupor, produced by the toxic effect of certain substances; in diluent gas narcosis, by excessive partial pressure of the diluent.
narrowband: A description of frequency measurement whose frequency band of energy is smaller relative to the rest of the band.
NASA: National Aeronautics and Space Administration.
NASA (abbr): National Aeronautics and Space Administration.
NASA--National Aeronautics and Space Administration: The NASA was formed during the administration of President Dwight D. Eisenhower as a response by the President and the United States Congress to achievements in space by the then Union of Soviet Socialist Republics (USSR) including Sputnik and II. Emphasis was placed on consolidating projects, facilities, and personnel into a national program including the National Advisory Committee for Aeronautics (NACA) which would later become the nucleus for the NASA. The NASA researches solutions to flight problems within and outside of the Earth's atmosphere. The NASA also develops, constructs, tests, and operates aeronautical and space vehicles. These vehicles, both manned and unmanned, are part of the NASA's research activities in the exploration of space. In addition, the NASA coordinates the scientific and engineering resources of the United States with other nations engaged in aeronautical and space activities for peaceful purposes. The NASA was formed during the administration of President Dwight D. Eisenhower as a response by the President and the United States Congress to achievements in space by the then Union of Soviet Socialist Republics (USSR) including Sputnik I and II. Emphasis was placed on consolidating projects, facilities, and personnel into a national program including the National Advisory Committee for Aeronautics (NACA) which would later become the nucleus for the NASA. The NASA researches solutions to flight problems within and outside of the Earth's atmosphere. The NASA also develops, constructs, tests, and operates aronautical and space vehicles. These vehicles, both manned and unmanned, are part of the NASA's research activities in the exploration of space. In addition, the NASA coordinates the scientific and engineering resources of the United States with other nations engaged in aeronautical and space activities for peaceful purposes.
NASA/Ames Research Center (ARC): The ARC's two installations are located at Moffett Field, California, and at the Dryden Flight Research Facility in Edwards, California. The ARC is responsible for managing a diverse series of research and technology development programs in support of NASA activities including flight simulation and research; aerospace and human factors; atmospheric and Earth sciences applications; and space biology and medicine. The ARC also is responsible for maintaining research and test facilities and equipment including wind tunnels, simulators, supercomputers, and aircraft.
NASA/Johnson Space Center (JSC): NASA/JSC or JSC supports the acquisition of Earth observations photography by training the astronauts in Earth science and providing real-time mission support to make the crew aware of photo opportunities during the flight (i.e, hurricanes, fires, floods, volcanoes, ice flows, etc.). JSC also trains the crew to use the camera systems and the techniques needed to acquire high quality photography of Earth and environmental phenomena. After each mission JSC catalogs the acquired photography and enters new records into the Space Shuttle Earth Observation Program (SSEOP) data base. This data base is accessible from GLIS through the use of the REMOTE MENU option.
NASC (abbr): National Aeronautics and Space Council.
National Atlas: The National Atlas of the United States of America is a bound collection of full-color maps and charts showing physical features such as landforms, geology, soil, vegetation, and climate. It is produced by the U.S. Geological Survey's Topographic Division in cooperation with other Federal agencies and private organizations. Economic, social, and cultural information is also presented. All maps of the United States contained in the National Atlas are based on the Albers equal area projection.
National Climatic Data Center: The repository for all climatological data in the U.S. located in Asheville, North Carolina.
National Oceanic Satellite System: Joint NASA (Goddard)-DOD venture.
national parks: Areas of scenic beauty or historical importance preserved and maintained by a national government for the enjoyment of the public.
natural coordinates: An orthogonal, or mutually perpendicular, system of curvilinear coordinates for the description of fluid motion, consisting of an axis T tangent to the instantaneous velocity vector and an axis N normal to this velocity vector to the left in the horizontal plane, to which a vertically directed axis Z may be added for the description of three-dimensional flow.
natural frequency: 1. The frequency of free oscillation of a system. For a multiple-degree-of-freedom system, the natural frequencies are the frequencies of the normal modes of vibration.
2. The undamped resonance frequency of a physical system. It is expressed in cycles per unit time. The system may be mechanical, pneumatic, or electrical.
natural gamma: This is a technique of examining the total gamma radiation of rocks in a drill hole.
natural gas: (A) Hydrocarbons that exist as a gas or vapor at ordinary pressures and temperatures. Methane is the momost important, but ethane, propane, or others may be present. (B) Gaseous hydrocarbons trapped in the zone of ground water saturation under pressure from, and partially dissolved in, underlying water or petroleum.
natural gas exploration: Searching the geological features to identify locations for stimulating wells for recovery of natural gas.
natural language (computers): A computer language whose rules reflect and describe current rather than prescribed usage. The language is often loose and ambiguous in interpretation.
natural mode: = normal mode of vibration.
natural year: = tropical year.
nausea: A feeling of discomfort in the region of the stomach, with aversion to food and a tendency to vomit.
Nautical Almanac: An annual publication of the U.S. Naval Observatory and H. M. Nautical Almanac Office, Royal Greenwich Observatory, listing the Greenwich hour angle and declination of various celestial bodies to a precision of 0.1 minute of arc at hourly intervals; time of sunrise, sunset, moonrise, moonset; and other astronomical information useful to navigators. Prior to 1960 separate publications were issued by the two observatories entitled the American Nautical Almanac and the Abridged Nautical Almanac. See American Ephemeris and Nautical Almanac.
nautical charts: Charts and maps of oceans, coasts and harbors now compiled from satellite data for precision and correction of local errors.
nautical mile: A unit of distance used principally in navigation. For practical navigation it is usually considered the length of 1 minute of any great circle of the earth, the meridian being the great circle most commonly used. Also called sea mile.
Because of various lengths of the nautical mile in use throughout the world, due to differences in definition and the assumed size and shape of the earth, the International Hydrographic Bureau in 1929 proposed a standard length of 1852 meters, which is known as the international nautical mile. This has been adopted by nearly all maritime nations. The U.S. Departments of Defense and Commerce adopted this value on July 1, 1954. With the yard-meter relationship then in use, the international nautical mile was equivalent to 6076.10333 feet. Using the yard-meter conversion factor effective July 1, 1959, the international nautical mile is equivalent to 6076.11549 international feet.
nautical twilight: That period when the upper limb of the sun is below the visible horizon and the center of the sun is not more than 12 degrees below the celestial horizon.
nautical year: = tropical year.
Navier-Stokes equations: The equations of motion for a viscous fluid which may be written

where p is the pressure; is the density; t is the time; F is the total external force; N is the fluid velocity; and is the kinematic viscosity. For an incompressible fluid, the term in (divergence) vanishes and the effects of viscosity then play a role analogous to that of temperature in thermal conduction and to that of density in simple diffusion. See viscosity, Ekman spiral.
Solutions of the Navier-Stokes equations have been obtained only in a limited number of special cases. The equations are derived on the basis of certain simplifying assumptions concerning the stress tensor of the fluid; in one dimension they represent the assumption referred to as the Newtonian friction law.
navigation: The practice or art of directing the movement of a craft from one point to another.
Navigation usually implies the presence of a human, a navigator, aboard the craft. Compare guidance.
navigation dome: = astrodome.
navigation technology satellites: Class of navigation satellites utilizing the global positioning system as well as a precise frequency and timing system. Used for NTS.
navigational planets: The four planets commonly used in celestial surface and air navigation: Venus, Mars, Jupiter, and Saturn.
navigational stars: The 57 stars included in the main listing of the Nautical Almanac and Polaris.
The navigational stars include almost all of the stars with common names.
navigational triangle: The spherical triangle solved in computing altitude and azimuth. See celestial triangle.
The celestial triangle is formed on the celestial sphere by the great circles connecting the elevated pole, zenith of the assumed position of the observer, and a celestial body. The terrestrial triangle is formed on the earth by the great circles connecting the pole and two places on the earth. The term astronomical triangle applies to either the celestial or terrestrial triangle used for solving celestial observations.
NE: Near encounter phase in flyby mission operations.
nearest neighbor resampling: When correcting image data points, the nearest neighbor technique assigns for each new pixel that pixel value which is closest in relative location to the newly computed pixel location.
nebulae: Clouds of interstellar gas and dust, seen either as a luminous patch of light or a dark cloud against a bright background. The term nebula was coined to describe objects that appeared fuzzy when viewed through early telescopes.
negative acceleration: = deceleration.
negative altitude: See altitude.
negative divergence: = convergence.
negative feedback: Feedback which results in decreasing the amplification.
negative g: In designating the direction of acceleration on a body, the opposite of positive g , for example, the effect of flying an outside loop in the upright seated position. See physiological acceleration.
negative ions: Ions singly or in groups which acquire negative charges by gaining one or more electrons.
negative matter: A hypothetical form of matter whose active-gravitational, passive-gravitational, inertial, and rest mass are opposite in sign to normal, positive matter. Negative matter is not antimatter.
negative temperature coefficient: 1. The partial derivative of any physical variable with respect to temperature, when the value of the variable decreases as temperature increases.
2. The decrease in reactivity of a nuclear reactor with increase in temperature.
Increasing temperature within the reactor increases the average neutron energy. Since the cross section of the fissionable material decrease with increased neutron energy, the net effect of increased temperature is to decrease the number of fissions.
negaton: Variant spelling negatron. See electron.
negatron: A negative electron. Sometimes shortened to negaton. See electron.
negentropy: = average information content.
neper: (abbr n)
A measure of power equal to one-half the Napierian logarithm (logarithm to the base e ) of the power ratio. 1 neper is equal to 8.686 decibels.
Under certain conditions the neper is a measure of the ratio of amplitudes of such physical quantities as voltage, current, particle velocities, and sound pressure. The neper is not commonly used in English-speaking countries.
nephanalysis: A type of analysis using satellite cloud pictures to study the relationship between cloud formation and storm systems.
nephelometer: 1. General name for instruments which measure, at more than one angle, the scattering function of particles suspended in a medium. Information obtained from such instruments may be used to determine the size of the suspended particles an the visual range through the medium.
2. An instrument for chemical analysis by measuring the light-scattering properties of a suspension.
3. = nephometer. See visibility meter.
nephelometry: 1. The study of suspensoids by means of light-scattering techniques. See nephelometer.
2. The study of the scattering properties of small samples of air and its suspensoids.
3. Chemical analysis by use of the nephelometer.
nepheloscope: 1. An instrument for demonstrating the temperature changes which occur in air that is rapidly expanded or compressed.
2. A laboratory instrument for the production of clouds by the condensation process.
3. = nephoscope.
nephometer: A general term for instruments designed to measure the amount of cloudiness. An early type consists of a convex hemispherical mirror mapped into six parts. The amount of cloud coverage on the mirror is noted by the observer. Also called nephelometer. Compare nephoscope.
nephoscope: An instrument for determining the direction of cloud motion. There are two basic designs of nephoscope: the direct-vision nephoscope and the mirror nephoscope. Also called nepheloscope.
Neptune: See planet, table.
Neptune (planet): Eighth planet from the sun, a gas giant or Jovian planet.
Neptune atmosphere: The atmosphere of the planet Neptune which is primarily composed of hydrogen and methane.
Nereid: A satellite of Neptune orbiting at a mean distance of 5,570,000 kilometers.
net radiation factor: The fraction of the total energy emitted by one surface or volume that is absorbed by another surface or volume directly and indirectly.
net thrust: The gross thrust of a jet engine minus the drag due to the momentum of the incoming air.
network: 1. A combination of electrical elements.
2. A group of parts or systems combined to provide a closed information loop, i.e., one that provides for inquiry or command, response, and interpretation of response in relation to inquiry or command.
network control: The management of acquisition, routing, and switching primarily in satellite communication.
neurology: The study of the anatomy, physiology, and pathology of the nervous system.
neuromuscular: Pertaining jointly to nerves and muscles, as neuromuscular junction.
neurotransmitters: Chemical substances secreted by the terminal ends of axons, which stimulate a muscle fiber contraction or an impulse in other neurons.
neutral: Without an electrical charge; neither positive nor negative.
neutral atoms: Atoms in which the number of electrons surrounding the nucleus equals the number of protons in the nucleus resulting in no net electric charge.
neutral beam injection: This is one of the fundamental plasma heating methods. A particle accelerator is used to create fast ion beams (the particle energies are on the order of 100 keV); the ion beam is then passed through a neutral gas region, where the ions neutralize via charge-exchange reactions with the neutral gas. The neutralized beam is then injected into a magnetically confined plasma. The neutral atoms are unaffected (not confined) by the magnetic field, but ionize as they penetrate into the plasma. The high-energy ions then transfer some of their energy to the plasma particles in repeated collisions, and heat the plasma.
neutral currents: Weak interaction currents that carry zero electric charge.
neutral gases: In astronomy, gas clouds of some nebulae which have not been ionized by hot stars.
neutral line: A line along which the magnetic intensity is zero.
neutral line: = Busch lemniscate.
neutral point: A point at which the magnetic intensity is zero.
neutral point: 1. In atmospheric optics, one of several points in the sky for which the degree of polarization of diffuse sky radiation is zero.
In an ideal Rayleigh atmosphere there would be just two such points, the solar point and the antisolar point. Because of effects of multiple scattering and because of the action of scattering particles larger than molecules, the actual atmosphere is typically found to have quite different neutral points. In the vertical plane containing the observer's zenith and the sun, three neutral points are commonly found: the Arago point, the Babinet point, and the Brewster point.
2. In aircraft, that location of the center of gravity at which the aircraft would exhibit neutral aerodynamic stability.
3. = Lagrangian point.
neutrino: A subatomic particle of zero, or near zero, rest mass, having no electric charge, postulated by Fermi (1934) in order to explain apparent contradictions to the law of conservation of energy in beta-particle emission.
According to Fermi theory, the atomic nucleus in beta decay releases energy partly in the form of electrons (beta particles) and partly in the form of neutrinos.
neutrino beams: Organized collections of neutrinos traveling outward from the source.
neutrinos: Subatomic particles of zero, or near zero, rest mass, having no electric charge, postulated by Fermi (1934) in order to explain apparent contradictions to the law of conservation of energy in beta particle emission.
neutron: A subatomic particle with no electric charge, and with a mass of 1.67482 X 10E-24 gram.
Protons and neutrons comprise atomic nuclei; and they are both classed as nucleons.
neutron flux: The sum of the distances traveled by all the neutrons in 1 cubic centimeter in 1 second. Normally the figure must be energy qualified, e.g., thermal, intermediate, or fast neutron flux.
neutron flux density: A measure of the intensity of neutron radiation within a given range of neutron energies; the product of the neutron density and velocity, measured in neutrons per square meter-second or neutrons per square centimeter-second.
neutron radiography: Nondestructive testing and inspection utilizing neutron beams from nuclear reactors, particle accelerators, and/or radioisotopes. Imagery displaying structural defects utilizes neutron image recorders or screens.
neutron stars: The imploded core of a massive star produced by a supernova explosion. (typical mass of 1.4 times the mass of the sun, radius of about 5 miles, density of a neutron.) Neutron stars can be observed as pulsars.
neutrons: Subatomic particles with no electric charge, and with a mass of 1.67482 times 10 to the minus 24 gram.
neutrosphere: The atmospheric shell from the earth's surface upward in which the atmospheric constituents are for the most part not ionized, i.e., it is electrically neutral.
The region of transition between the neutrosphere and the ionosphere is somewhere between 70 and 90 kilometers, depending on latitude and season.
new moon: The moon at conjunction, when little or none of it is visible to an observer on the earth because the illuminated side is away from him. Also called change of the moon. See phases of the moon.
Newcomb tables of the sun: See ephemeris time.
newton: The unit of force in the SI system; that force which gives to a mass of l kilogram an acceleration of 1 meter per second squared.
newton: The unit of force in the MKSA system; that force which gives to a mass of 1 kilogram an acceleration of 1 meter per second squared.
Newton equations of motion: = Newton laws of motion.
Newton first law: For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.
Newton formula for the stress: = Newtonian friction law.
Newton law of cooling: See thermal conductivity.
Newton law of gravitation: Every particle of matter in the universe attracts every other particle with a force, F, acting along the line joining the two particles, proportional to the product of the masses m₁m₂of the particles and inversely proportional to the square of the distance r between the particles, or

where G = gravitational constant.
Newton laws of motion: A set of three fundamental postulates forming the basis of the mechanics of rigid bodies, formulated by Newton in 1687.
The first law is concerned with the principle of inertia and states that if a body in motion is not acted upon by an external force, its momentum remains constant (law of conservation of momentum). The second law asserts that the rate of change of momentum of a body and is in the direction of the applied force. A familiar statement of this is the equation F = ma where F is vector sum of the applied forces, m is the mass, and a is the vector acceleration of the body. The third law is the principle of action and reaction, stating that for every force acting upon a body there exists a corresponding force of the same magnitude exerted by the body in the opposite direction.
Newton second law: For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.
Newton third law: In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.
Newtonian friction law: The statement that the tangential force (i.e., the force in the direction of the flow) per unit area acting at an arbitrary level within a fluid contained between two rigid horizontal plates, one of which is motionless and the other of which is in steady motion, is proportional to the shear of the fluid motion at that level. Mathematically, the law is given by

where is the tangential force per unit area, usually called the shearing stress; is a constant of proportionally called the dynamic viscosity; and is the shear of the fluid flow normal to the resting plate. Also called Newton formula for the stress.
Newtonian mechanics: The system of mechanics based upon Newton laws of motion in which mass and energy are considered as separate, conservative, mechanical properties, in contrast to their treatment in relativistic mechanics.
Newtonian speed of sound: An approximation to the speed of sound a in a perfect gas given by the relation

where p is pressure and is density. Compare Laplacian speed of sound. See acoustic velocity.
Newton derived this expression by assuming the propagation of sound to be an isothermal process. It leads to values about 16 percent below those observed.
Newtonian telescope: A reflecting telescope in which a small plane mirror reflects the convergent beam from the objective to an eyepiece at one side of the telescope. After the second reflection the rays travel approximately perpendicular to the longitudinal axis of the telescope. See Cassegrain telescope.
Newtonian universal constant of gravitation: = gravitational constant.
Next Generation Space Telescope project: Project in the NASA Origins program with the goal of developing a spaceborne observatory to succeed the Hubble Space Telescope after 2005. The telescope is foreseen to have an aperture of 8 meters and be optimized for near infrared wavelengths (0.6-10+ microns) in order to enable the exploration of the most remote high redshift universe.
nickel cadmium batteries: Alkaline storage batteries in which the positive active material is nickel oxide and the negative contains cadmium.
nickel iron batteries: Alkaline-type electric cells using potassium hydroxide as the electrolyte and anodes of steel wool substrate with active iron material and cathodes of nickel plated steel wool substrate with active nickel material.
nickel steels: Steels containing nickel as a main alloying element.
Nimbus: The Nimbus satellite program, initiated by the National Aeronautics and Space Administration (NASA) and later operated by both NASA and the National and Oceanic Atmospheric Administration (NOAA), was developed in the early 1960s to meet research and development needs of the atmospheric and Earth scientists, providing global surveillance of the atmospheric structure at low Earth orbital altitudes for the world's weather services.
Nimbus 7 satellite: One in a series of meteorological satellites.
NIMS: Near-Infrared Mapping Spectrometer onboard the Galileo spacecraft.
nitinol alloys: Shape memory alloys of titanium and nickel.
nitrogen cycle: The exchange of nitrogen between animals and plants, in which plants convert urea or nitrates to protein, animals digest protein and excrete its nitrogen content an urea, which is taken up again by plants.
nitrogen desaturation: The reduction of the nitrogen content of the tissues of the body by breathing gases not containing nitrogen.
nitrogen lasers: Stimulated emission devices in which the nitrogen molecule is the lasing medium.
nm: Nanometer (10^-9 meter).
nm: Nautical Mile, equal to the distance spanned by one minute of arc in latitude, 1.852 km.
NOAA 4 satellite: One of a series of meteorological satellites launched by NASA for the National Oceanic and Atmospheric Administration.
NOAA 5 satellite: One of a series of environmental satellites launched by NASA for the National Oceanic and Atmospheric Administration for the sensing and recording of atmospheric, hydrological, and oceanographic environmental data.
NOAA 6 satellite: Designation for a NOAA meteorological satellite conforming to the TIROS N configuration.
NOAA 7 satellite: Designation for the seventh NOAA meteorological satellite conforming to the TIROS N configuration.
noctilucent clouds: Clouds of unknown composition which occur at great heights, 75 to 90 kilometers. They resemble thin cirrus, but usually with a bluish or silverish color, although sometimes orange to red, standing out against a dark night sky. Sometimes called luminous clouds.
These clouds have been seen rarely, and then only during twilight, especially with the sun between 5 and 13 degrees below the horizon; they have been observed only during summer months in both hemispheres (between latitudes 50 to 75 degrees N and 40 to 60 degrees S), and only in some parts of these latitude belts.
nocturnal radiation: = effective terrestrial radiation.
nodal point: = node.
node: 1. One of the two points of intersection of the orbit of a planet, planetoid, or comet with the ecliptic, or of the orbit of a satellite with the plane of the orbit of its primary. Also called nodal point. See regression of the nodes.
That point at which the body crosses to the north side of the reference plane is called the ascending node; the other, the descending node. The line connecting the nodes is called line of nodes.
2. A point, line, or surface in a standing wave where some characteristic of the wave field has essentially zero amplitude.
The appropriate modifier should be used before the word node to signify the type that is intended; e.g., displacement node, velocity node, pressure node.
3. A terminal of any branch of a network or a terminal common to two or more branches of a network. Also called junction point, branch point , or vertex.
nodes (standing waves): Points, lines, or surfaces in standing waves where some characteristic of the wave field has essentially zero amplitude.
nodical month: = dracontic month.
nodical period: The interval between successive passages of a satellite through the ascending node.
NOESS: The acronym for the National Operational Environmental Satellite System. This term is no longer in use. Used for National Operational Environmental Sat Sys.
noise: Any undesired signal (electrical or acoustic) that tends to interfere with the reception, interpretation, or processing of the desired signal.
noise: The random fluctuations that are always associated with a measurement that is repeated many times over. Noise appears in astronomical images as fluctuations in the image background. These fluctuations do not represent any real sources of light in the sky, but rather are caused by the imperfections of the telescope.
noise: 1. Any undesired sound. By extension, noise is any unwanted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device.
When caused by natural electrical discharges in the atmosphere, noise may be called static.
2. An erratic, intermittent, or statistically random oscillation.
3. In electrical circuit analysis, that portion of the unwanted signal which is statistically random, as distinguished from hum, which is an unwanted signal occurring at multiples of the power-supply frequency.
If ambiguity exists as to the nature of the noise, a phrase such as acoustic noise or electric noise should be used. Since the above definition are not mutually exclusive, it is usually necessary to depend on context for the distinction. See white noise.
noise level: See level.
noise pollution: Objectional or harmful levels of noise.
noise prediction: Estimation of intensity and frequencies based on analyses of probable oscillation of vibration producing components.
noise prediction (aircraft): Estimating or forecasting of aircraft noise. Used for aircraft noise prediction.
nomographs: On charts or graphs, lines of constant value of given quantities with respect to either space or time. Used for isopleths and nomograms.
non-coherent: Communications mode wherein a spacecraft generates its downlink frequency independent of any uplink frequency.
nonadiabatic conditions: In thermodynamics, changes in volume, temperature, flow, etc., accompanied by a transfer of heat.
nonadiabatic process: See diabatic process.
noncoherent echo: See coherent echo.
noncondensable gas: A gas whose temperature is above its critical temperature, so that it cannot be liquefied by increase of pressure alone.
nondestructive tests: Testing that does not alter the material under test in a permanent manner or introduce any permanent changes to the material properties and structure.
nondimensional number: A pure number not involving any physical dimensions, e.g., a ratio of two velocities or two lengths.
Such numbers are fundamental descriptive quantities of a physical system. Nondimensional numbers involving several variables often are interpreted as estimates of characteristics velocity ratios, force ratios, heat transfer ratios, frequency ratios, etc. Usually several different ratio interpretations are possible and useful for the same number. (See Mach number, Reynolds number, Boussinesq number, Cauchy number, Prandtl number, Peclet number, Rayleigh number, Strouhal number, Richardson number, Nusselt number, Grashof number, Taylor number, Froude number.)
nondimensional parameter: Any parameter of a problem which has the dimensions of a pure number, usually rendered so deliberately. See nondimensional number.
nonimpinging injector: An injector used in rocket engines which employs parallel streams of propellant usually emerging normal to the face of the injector.
In this injector, mixing is usually obtained by turbulence and diffusion. The V-2 used a nonimpinging injector.
nonisothermal processes: In thermodynamics, compression or expansion of substances at nonuniform temperatures.
nonlinear damping: Damping due to a damping force that is not proportional to velocity.
The words force and velocity should be treated in the generalized sense. For example, they can be replaced by voltage and current, respectively.
nonlinear distortion: Distortion caused by a deviation from a proportional relationship between specified measures of the output and input of a system.
The related measures need not be output and input values of the same quantity; e.g., in a linear detector, the desired relation is between the output signal voltage and the input modulation envelope.
nonlinear optics: Study of the interaction of radiation with matter in which certain variables describing the response of the matter are not proportional to variables describing the radiation.
nonlinear programming: An optimization problem in which any or all of the following are nonlinear in the variables: (A) the objective functions, (B) the the defining relationships among the variabes, the plant description, (C) the constraints.
nonNewtonian flow: The rate of flow of a material that is not proportional to the degree of force applied.
nonNewtonian fluids: Fluids that exhibit a viscosity which varies with changing shear stress or shear rate.
nonpoint sources: Undetermined or general areas from which pollutants, contaminants, and/or other unwanted materials or wastes enter the environment.
nonthermal particle: A particle that is not part of a thermal gas. These particles cannot be described by a conventional temperature.
nonvolatile: Of a computer or computer component. The ability to retain information in the absence of power as nonvolatile memory , nonvolatile storage.
nonvolatile storage: = permanent memory.
noon: The instant at which a time reference is over the upper branch of the reference meridian.
Noon may be solar or sidereal as either the sun or vernal equinox is over the upper branch of the reference meridian. Solar noon may be further classified as mean or apparent as the mean or apparent sun is the reference. Noon may also be classified according to the reference meridian, either the local or Greenwich meridian or additionally in the case of mean noon, a designated time zone meridian.
Nor, Norm: International Astronomical Union abbreviations for Norma. See constellation.
Norma: (abbr Nor, Norm)
See constellation.
normal: 1. Equivalent to usual, regular, rational, or standard conditions.
2. Perpendicular.
A line measured to another line or a plane when it is perpendicular to it. A line is normal to a curve or curved surface when it is perpendicular to the tangent line or plane at the point of tangency.
3. Referring to thermal radiation properties, in a direction perpendicular to the surface.
4. The line normal, sense 2, to a surface.
normal dispersion: Dispersion of an electromagnetic wave characterized by an increase in refractive index with increase in frequency.
normal distribution: The fundamental frequency distribution of statistical analysis. A continuous variety x is said to have a normal distribution or to be normally distributed if it possesses a density function f(x) which satisfies the equation

where µ is the arithmetic mean (or first moment) and is the standard deviation. Also called Gaussian distribution.
About two-thirds of the total area under the curve is included between 1 and . The corresponding frequency distribution of vectors is the normal circular distribution in which the frequencies of vector deviations are represented by a series of circles centered on a vector mean. When applied to error distribution, this function is the normal law of errors, and the distribution is called the normal curve of error.
normal emittance: Emittance in a direction perpendicular to the surface or in a small solid angle whose axial ray is perpendicular to the surface.
normal functions: See orthogonal functions.
normal gravity: See acceleration of gravity.
normal mode of vibration: A mode of free vibration of an undamped system. In general, any composite motion of a vibrating system ban be analyzed into a summation of its normal modes. Also called natural mode, characteristic mode , and eigenmode.
normal plane: In aerodynamics, a plane at right angles to the longitudinal axis of an aerodynamic vehicle.
normal shock: = normal shock wave.
normal shock wave: A shock wave perpendicular, or substantially so, to the direction of flow in a supersonic flow field. Sometimes shortened to normal shock.
normalize: 1. To change in scale so that the sum of squares, or the integral of the squares of the transformed quantity is unity. See orthogonal functions".
2. To transform a random variable so that the resulting random variable so that the resulting random variable has a normal distribution.
3. In computer operations, to adjust the exponent and coefficient of a floating-point result so that the coefficient is in the prescribed normal range. Also called standardize.
north polar sequence: A list of stars near the north celestial pole arranged in order of photographic magnitude, used as reference stars in stellar photometry.
North Polar Spur (astronomy): One of the largest sources of diffuse radio emission outside the galactic plane. The Spur, a ridge of enhanced emission, may be the remnant of the shells of supernovae which exploded over 100,000 years ago.
north pole: 1. In astronomy, that end of the axis of rotation of a celestial body at which, when viewed from above, the body appears to rotate in a clockwise direction. See celestial pole, ecliptic pole, geographical pole, geomagnetic pole, magnetic pole.
2. The north-seeking end of a magnet.
north-upward plan position indicator: See plan position indicator.
northbound node: =ascending node.
northern lights: = aurora borealis.
northern sky: That part of the sky visible from the northern hemisphere.
nose cones: The cone-shaped leading ends of some rocket vehicles, consisting of (a) a chamber or chambers in which a satellite, instruments, animals, plants, or auxiliary equipment may be carried, and (b) an outer surface built to withstand high temperatures generated by aerodynamic heating.
In a satellite vehicle, the nose cone may become the satellite itself after separating from the final stage of the rocket or it may be used to shield the satellite until orbital speed is accomplished, then separating from the satellite. Used for nose caps.
nose gear: That part of a landing gear which is located at the forward end of the vehicle.
nose tips: The foremost, sharp points of bombs, rockets, missiles, and other symmetrical bodies.
NOT circuit: In computers, a device or circuit which inverts the polarity of a pulse. Also called inverter.
notation: A manner of representing quantities. See positional notation, binary notation, biquinary notation, decimal notation, fixed point, floating point, hexidecimal notation, octal notation, sexidecimal notation, ternary notation.
nova (plural novae): A star which suddenly becomes many times brighter than previously, and then gradually fades.
Nova computers: A series of minicomputers built by Data General.
Nova Laser System: Laser fusion system utilizing large neodymium glass lasers for irradiating DT pellets.
Nova satellites: A second generation Navy navigation satellite which replaces the transit satellites.
novae: A celestial body that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by hundreds to thousands of times before fading back to its original luminosity.
nowcasting: A self contained short period meteorological forecast for the immediate future covering a period of up to six hours.
Nozomi Mars Orbiter: A Japanese Mars mission spacecraft designed to study the Martian upper atmosphere and its interaction with the solar wind, and to develop technologies for use in future planetary missions. Specifically, instruments on the spacecraft enable the measurement of the structure, composition and dynamics of the ionosphere; aeronomy effects of the solar wind; the escape of atmospheric constituents; the intrinsic magnetic field; and dust in the upper atmosphere and in-orbit around Mars.
nozzle: (symbol n used as subscript)
1. A duct, tube, pipe, spout, or the like through which a fluid is directed and from the open end of which the fluid is discharged, designed to meter the fluid or to produce a desired direction, velocity, or shape of discharge. See de Laval nozzle, jet nozzle, supersonic nozzle.
2. Specifically, that part of a rocket thrust chamber assembly in which the gases produced in the chamber are accelerated to high velocities.
nozzle blade: Any one of the blades or vanes in a nozzle diaphragm. Also called a nozzle vane.
nozzle diaphragm: A ring of stationary, equally spaced blades or vanes, forming an annulus of nozzles through which fluid is directed onto a turbine wheel. Sometimes called a nozzle ring.
nozzle efficiency: The efficiency with which a nozzle converts potential energy into kinetic energy, commonly expressed as the ratio of the actual change in kinetic energy to the ideal change at the given pressure ratio.
nozzle exit area: The cross-sectional area of a rocket nozzle available for gas flow measured at the nozzle exit.
nozzle ring: = nozzle diaphragm.
nozzle throat: The portion of a nozzle with the smallest cross section.
nozzle throat area: The area of the minimum cross section of a rocket nozzle.
nozzle thrust coefficient: (symbol C_F). A measure of the amplification of thrust due to gas expansion in a particular nozzle as compared with the thrust that would be exerted if the chamber pressure acted only over the throat area. Also called thrust coefficient.
nozzle vane: = nozzle blade.
nozzle-contraction area ratio: (symbol _c). Ratio of the cross-sectional area for gas flow at the nozzle inlet to that at the throat.
nozzle-divergence loss factor: (symbol). The ratio between the momentum of the gases in a nozzle with an angle 2a and the momentum of an ideal nozzle. In mathematical form = ½(1 + cos a).
nozzle-expansion area ratio: (symbol _e). Ratio of the cross-sectional area for gas flow at the exit of a nozzle to the cross-sectional area available for gas flow at the throat.
nuclear binding energy: The difference between the total energy ( = mc^2) of the bound nucleus, and the energies of the individual constituent particles ( = sum of masses * c^2). The nuclear binding energy *per nucleon* is a maximum for iron. Fusion releases energy because light nuclei are less tightly bound than medium-weight nuclei, and thus energy is liberated when they become more tightly bound after fusing. Fission releases energy for the same reason - heavy nuclei are also less tightly bound than medium-weight nuclei, and energy is liberated when heavy nuclei split into lighter nuclei.
nuclear Bohr magneton: = nuclear magneton.
nuclear cross section: (symbol )
A measure of the probability that the reaction will take place which is defined by

dI = In dx
where I is the intensity of the particle beam; n is the number of target nuclei per cubic centimeter of target; is the cross section for the specified process, expressed in square centimeters; and x is the target thickness in centimeters. See barn.
nuclear devices: Devices whose explosive potency is derived from nuclear fission of atoms of fissionable material with the consequent conversion of part of their mass into energy.
nuclear emulsion: A very thick photographic emulsion used in the study of cosmic rays and other energetic particles. The path of the particle through the thick emulsion is recorded in three dimensions.
nuclear energy: All forms of energy released in the course of a nuclear fission or a nuclear transformation.
nuclear forces: The short-range forces acting on protons and neutrons in atomic nuclei. Two types actually exist, the "strong force" which holds nuclei together, and the "weak force" which determines the ratio between the numbers of protons and neutrons.
nuclear fuel: Fissionable material of reasonable long life, used or usable in producing energy in a nuclear reactor.
nuclear fuel reprocessing: Periodic chemical, physical, and metallurgical treatment of materials used as fuel elements in nuclear reactors to recover and purify residual fissionable and fertile materials.
nuclear fusion: A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones.
nuclear magneton: (symbol M_N)
A unit of magnetic moment of the proton equal to 5.0505 x 10^-24 erg per gauss.
nuclear medicine: That branch of medicine dealing with the effect of radiation such as x rays, gamma rays, and energetic particles on the body and with the prevention and cure of physiological injuries resulting from such radiation. Used for radiation medicine.
nuclear pumped lasers: Lasers in which the excitation is supplied by a nuclear reactor as a high flux source or by the kinetic energy of the fission fragments only.
nuclear pumping: Laser-like pumping produced by electrons generated in nuclear reactions or, in general, by beams of charged particles.
nuclear radiation: Corpuscular emissions, such as alpha and beta particles, or electromagnetic radiation, such as gamma rays, originating in the nucleus of an atom.
nuclear reactor: An apparatus in which nuclear fission may be sustained in a self supporting chain reaction. Commonly called reactor. Formerly called pile.
nuclear research and test reactors: A class of nuclear reactors used to do research into nuclear physics, reactor materials and design, and nuclear medicine.
nuclear rocket engine: A rocket engine in which a nuclear reactor is used as a power source or as a source of thermal energy.
nuclear vulnerability: The resistance of structures or materials to nuclear radiation or explosions.
nuclear-electric rocket engine: A rocket engine in which a nuclear reactor is used to generate electricity which is used in an electric propulsion system or as a heat source for the working fluid.
nuclei (nuclear physics): The positively charged cores of atoms with which are associated practically the whole mass of each atom but only a minute part of its volume.
nucleon: In the classification of subatomic particles according to mass, the second heaviest type of particle; its mass is intermediate between that of the meson and the hyperon.
Examples of the nucleon are the proton and neutron.
nucleus: The tiny core of an atom, positively charged, containing protons and neutrons (except for simple hydrogen, which has only a single proton). In an atom, electrons "orbit" the nucleus, forming a cloud around it.
nuclide: An individual atom of given atomic number Z and mass number A, for example, ₉₂U²³⁵.
A nuclide is any species of atom that exist for a measurable length of time and has a nuclear structure distinct from that of any other species of atom.
null: In direction-finding systems wherein the output amplitude is a function of the direction of arrival of the signal, the minimum output amplitude (ideally zero).
The null is frequently employed as a means of determining bearing. The term minimum is often used to indicate an imperfect null.
number: In computer operations, (a) amount of units by count, (b) a magnitude or quantity represented by group of digits.
The term quantity is preferred to number in sense (b).
number system: A scheme for representing magnitudes or quantities by a group of digits. See numeric coding, positional notation.
numeric: Composed wholly or partly of digits. See alphanumeric, number.
numeric coding: A system of coding in which information is represented by digits. See alphabetic coding, alphanumeric.
numerical analysis: Study of approximation methods using arithmetic techniques for the solution of mathematical problems.
numerical aperture: The sine of the vertex angle of the largest cone of meridional rays that can enter or leave an optical system or element, multiplied by the refractive index of the medium in which the vertex of the cone is located. Generally measured with respect to an object or image point, and will vary as that point is moved.
numerical aperture: The quantity n sin , the product of the index of refraction of the object medium, usually air, multiplied by the sine of the slope angle of the outermost ray from an axial point on the object.
numerical differentiation: Approximate estimation of a derivative of a function by numerical techniques.
Nusselt number: (symbol N_Nu)
(After Wilhelm Nusselt, 1882- , German engineer). A number expressing the ratio of convective to conductive heat transfer between a solid boundary and a moving fluid, defined as hl/k where h is the heat-transfer coefficient, l is the characteristic length, and k is the thermal conductivity of the fluid.
nutating feed: In a tracking radar an oscillating antenna feed for producing an oscillating deflection of the beam in which the plane of polarization remains fixed.
nutation: The short-period oscillations in the motion of the pole of rotation of a freely rotating body that is undergoing torque from external gravitational forces. Nutation of the Earth's pole is discussed in terms of components in obliquity and longitude.
nutation: 1. The oscillation of the axis of any rotating body, as a gyroscope rotor.
2. Specifically, in astronomy, irregularities in the precessional motion of the equinoxes because of varying positions of the moon and, to a lesser extent, of other celestial bodies with respect to the ecliptic.
Because of nutation, the earth's axis nods like a top, describing a slightly wavy circle about the ecliptic pole. The maximum displacement is about 9.21 seconds (constant of nutation) and the period of a complete cycle is 18.60 tropical years (period of moon's node, nutation period).
nutation period: See nutation, note.
nutator: A mechanical device for gyrating the antenna feed horn or dipole or a radar about the axis of the reflector without changing its polarization.
Nyquist frequency: The highest frequency which can be determined in a Fourier analysis of a discrete sampling of data. If a time series is sampled at interval t, this frequency is 1/2t cycles per second. Also called turnover frequency.
nystagmus: An involuntary oscillation of the eyeballs, especially occurring as a result of eye fixations and stimulations of the inner ear during rotation of the body.