Glossary of Aerospace Terms

Glossary of Aerospace Terms

Compiled by Dave Culley.

This list of aerospace terms and abbreviations was compiled by Dave, I just agreed to host it on my webpage, and tidy up some of the html code for him.

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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

AACS Attitude Articulation Control Subsystem.
AAF Association Astronautique Francaise.
AAS American Astronautical Society.
ABL Allegheny Ballistics Laboratory.
Ablation The erosion of a solid body by a high-temperature gas stream moving with high velocity, e.g. a reentry vehicle's heat shield which melts or chars under the effects of air friction.
ABMA Army Ballistic Missile Agency (USA).
Abort To cancel or cut short a mission.
Absolute zero The temperature at which all heat action ceases, -273.16oC (-459.69oF).
Acceleration A change in velocity, including changes of direction and decreases as well as increases in speed.
Accelerometer A device that senses changes in speed along its axis.
ACS Attitude Control System.
Active heating The use of resistive electric heaters or radioisotope heaters to keep spacecraft components above their minimum allowable temperatures.
Active sun The Sun during times of frequent solar activity such as sunspots, flares, and associated phenomena.
Actuator A device which transforms an electric signal into a measured motion using hydraulic, pneumatic or pyrotechnic (explosive) action.
Aerobraking The process of decelerating by converting velocity into heat through friction with a planetary atmosphere.
Aerodynamic heating The heating of a body due to the passage of air or other gases over the body; caused by friction and compression processes.
Aeronautics The science of building and operating vehicles for flight.
Aerozine 50 A storable liquid fuel 50% hydrazine, 50% UDMH.
AIAA American Institute of Aeronautics and Astronautics (USA).
AIDAA Associazione Italiana di Aeronautica e Astronautica.
Aileron A hinged surface on the wing of an aircraft or spacecraft used to adjust the craft's angle of flight.
Aimpoint The point in the planet's plane that spacecraft aim for to either fly by or enter orbit.
Airglow The visible light that appears at night in the upper atmosphere.
Air lock A compartment separating areas a different environment, especially different air pressures, that is used for entry to and departure from a spacecraft.
Albedo Reflectivity; the ratio of reflected light to incident light. The fraction of the sunlight which is reflected off a planet.
Algae A group of simple organisms, mostly aquatic, which contain chlorophyll and thus provide a means of photosynthesis. They could be used to absorb carbon dioxide and provide nourishment in a spaceship.
ALSEP Apollo Lunar Surface Experiment Package.
Altimeter A device that measures altitude above the surface of a planet or moon. Spacecraft altimeters work by timing the round trip of radio signals bounced off the surface.
Ambient Environmental conditions, such as pressure or temperature.
Analog computer A computing machine that works on the principle of measuring, as distinct from counting, in which the measurements obtained (as voltages, resistances, etc.) are translated into desired data.
Angle of attack The angle between the velocity vector and the longitudinal axis of a missile or rocket.
Angstrom A unit for the measurement of wavelength. Equals one hundred millionth of a centimeter (0.003937 millionth of an inch).
Annular Pertaining to, or having the form of a ring.
Anomaly The angular distance between the position of a planet and its last perihelion, or between that of a satellite and its last perigee.
Anti-matter A hypothetical form of matter of which the atoms are composed of anti-particles, as protons, electrons, etc. assumed to carry charges opposite to those associated with ordinary matter. Particles having such properties have been produced in particle accelerators.
Antipodal Pertaining to, or located on, the opposite side of the Earth.
AOCS Attitude & Orbit Control System.
Aperture The diameter of an opening; the diameter of the primary lens or mirror of a telescope.
Apex The point towards which a body is moving.
Aphelion That point in a solar orbit which is farthest from the Sun.
Apoapsis That point in an orbit which is farthest from the primary.
Apogee That point in a terrestrial orbit which is farthest from the Earth.
Apolune That point in a lunar orbit which is farthest from the Moon.
Argument Angular distance.
Argument of periapsis In an orbit, the angular distance between the point of periapsis and the ascending node.
Arianespace A private limited company established for the purpose of producing, financing and marketing the ESA Ariane launch vehicle. Comprises European companies concerned in the rocket's development, CNES and several banks. US agent is Grumman Aerospace.
ARS American Rocket Society (USA).
Artificial gravity Use of centrifugal force to simulate weight reaction in a condition of free-fall. May be achieved by spinning the vehicle to make the centrifugal force of the outer periphery or bodies within the vehicle to replace the weight reaction experienced at Earth's surface.
ASAT Anti-satellite.
Ascending node The point at which an orbiting object or spacecraft, traveling from south to north, crosses the plane of the equator.
Ascent module That part of a spacecraft that ascends from the surface of a planet or moon to rendezvous and dock with an orbiting spacecraft.
Asteroid A small, usually irregularly shaped body orbiting the sun, most often at least partially between the orbits of Mars and Jupiter.
Asteroid belt A 1/2 AU wide region between the orbits of Mars and Jupiter where most asteroids are found.
Astronaut A person who flies in space, whether as a crew member or passenger.
Astronautics The science and technology of space flight.
Astronomical unit The mean distance of Earth from the Sun, i.e. 92,955,807 miles (149,597,870 km).
Astrophysics Study of the physical and chemical nature of celestial bodies and their environs.
ATDA Agena Target Docking Adapter.
Atmospheric balloon An instrumented package suspended from a buoyant gas bag; deployed in a planet's atmosphere to study wind circulation patterns.
Atmospheric pressure The weight of air on surfaces within Earth's atmosphere, about 14.7 psi (101 kPa) at sea level. Such pressure is also supplied artificially in spacecraft and spacesuits.
Atmospheric probe A small instrumented craft which separates from the main spacecraft prior to closest approach to a planet to study the gaseous atmosphere of the body as it drops through it.
Attenuation The decrease of a propagating physical quantity, such as a radio signal, with increasing distance from the source, or from some obstruction.
Attitude Orientation of a space vehicle as determined by the relationship between its axes and some reference plane, e.g. the horizon.
Attitude & articulation control subsystem The onboard computer that manages the tasks involved in spacecraft stabilization via its interface equipment. For attitude reference, star trackers, star scanners, solar trackers, sun sensors, and planetary limb trackers are used.
Attitude control The system that turns and maintains a spacecraft in the required direction as indicated by its sensors.
AU Astronomical Unit.
Aurora Arcs, rays or swaying curtains of green, yellow or white lights seen in latitudes of about 70o, such as Aurora Borealis or Northern Lights, and Aurora Australis or Southern Lights; caused by streams of electrified particles, emitted by the Sun, trapped in the Earth's magnetic field.
Autopilot A system or device that controls a vehicle's flight at a preset course and altitude.
Azimuth The angular position of an object measured in the observer's horizontal plane, usually from north through east. Bearing or direction in the horizontal plane. As one of the coordinates expressing celestial location, it is sometimes used in tracking spacecraft.

Index

Backscattering Reflecting light back in the direction of the source.
Back-up An item kept available to replace an item which fails to perform satisfactorily.
Ballistics The science that deals with the motion, behavior, appearance or modification of missiles acted upon by propellants, rifling, wind, gravity, temperature or other modifying conditions of force.
Ballute An aerodynamic braking device which is both balloon and parachute.
Bar A unit of pressure equal to one million dynes per square centimeter, or 0.99 atmospheres.
Barycenter The common center of mass about which two or more bodies revolve.
Basin A large, >200 km, circular depression from the explosive impact of an asteroid or similar sized body on a planet surface, usually rimmed by mountains.
Battery A device with two or more connected cells that produce a direct current by converting chemical energy into electrical energy.
Big Bang theory The theory that the universe was once clustered and at the 'beginning' it exploded out, as shown by the fact that objects are still moving out from the center.
Binary star Two stars revolving around a common center of gravity.
Bi-propellant A rocket propellant consisting of two unmixed or uncombined chemicals (fuel and oxidizer) fed separately into the combustion chamber.
BIS British Interplanetary Society.
Bit A basic unit of computer information; abbreviation of binary digit.
Black hole An object whose gravity is so strong that the escape velocity exceeds the speed of light.
Blackout (physiological) A temporary loss of vision and/or consciousness when a person is subjected to high accelerations.
Blackout (radio) A temporary loss of radio communications which occurs between a spacecraft reentering the atmosphere and ground stations due to an ionized sheath of plasma which develops around the vehicle.
Black powder A mixture of saltpeter (potassium nitrate), sulfur, and charcoal, used in explosives and as an early propellant for rockets.
Boilerplate A metal replica of the flight model (e.g. of a spacecraft) but usually heavier and cruder for test purposes.
BOL Beginning Of Life.
Boost The extra power given to a rocket or space vehicle during liftoff, climb or flight, as with a booster rocket.
Booster The first stage of a missile or rocket.
Bow shock wave The compressed wave that forms in front of a spacecraft or satellite as it moves rapidly through Earth's atmosphere; more generally, any such wave that forms between an object and a fluid medium.
Burn Combustion action in rockets. Propulsion in space is achieved through a sequence of burns.
Bus A major part of the structural subsystem of a spacecraft which provides a place to attach components internally and externally, and to house delicate modules requiring a measure of thermal and mechanical stability. The bus also establishes the basic geometry of the spacecraft.

Index

Calibration Setting an measuring instrument before measuring for accurate results.
Carrier The main frequency of a radio signal generated by a transmitter prior to application of modulation.
CAS Chinese Academy of Sciences.
CAST Chinese Academy of Space Technology.
CAT Capsule Ariane Technologique.
Catalytic decomposition engine A mono-propellant engine in which a liquid fuel decomposes into hot gas in the presence of a catalyst. The fuel is most commonly hydrazine.
C-band A range of microwave radio frequencies in the neighborhood of 4 to 8 GHz, used for spacecraft communications on Mercury and Gemini flights (~5 Ghz).
CCD Charged Coupled Device.
CDS Command & Data Subsystem.
Celestial sphere The apparent sphere of sky that surrounds the Earth; used as a convention for specifying the location of a celestial object.
Centrifugal force A force which is directed away from the center of rotation.
Centripetal force A force which is directed towards the center of rotation.
CEO Close Earth Orbit.
CETI Communication with Extra-Terrestrial Intelligence.
Chaff Metallic foil ejected by a reentry module to enhance its radar image.
Charged coupled device An imaging device consisting of a large-scale integrated circuit which has a two-dimensional array of hundreds of thousands of charge-isolated wells, each representing a pixel.
Cholorella A genus of unicellular green algae, proposed for converting carbon dioxide into oxygen for use in spacecraft.
Chromosphere A reddish layer in the Sun's atmosphere, the transition between the photosphere and the corona.
Cislunar Relating to the space between the Earth and the orbit of the Moon.
CM Command Module.
CNES Centre National d'Etudes Spatiales (France).
CO Circular Orbit.
Coherence The property of being coherent, e.g. waves having similar direction, amplitude and phase that are capable of exhibiting interference.
Coma The cloud of diffuse material surrounding the nucleus of a comet.
Combustion A chemical reaction between two or more substances that releases heat, light, and gases.
Combustion chamber The chamber in a rocket where the fuel and oxidizer are ignited and burned. By common usage the expansion nozzle is included as part of the combustion chamber, particularly for liquid-propelled rocket engines.
Comet A body of small mass but large volume, compared to a planet, often developing a long luminous and partly transparent tail when close to the Sun.
Command & data subsystem The onboard computer responsible for overall management of a spacecraft's activity.
Command module The compartment of a spacecraft which contains the crew and main controls, and is used as the reentry vehicle.
Composites Structural materials of metal alloys or plastics with built-in strengthening agents, e.g. carbon fibers.
Constellation A group of stars that make a shape, often named after mythological characters, people, animals, and things.
Control rocket A vernier or other rocket used to control the attitude of, or slightly change the speed of, a spacecraft.
Coolant A medium, usually a fluid, which transfers heat from a object.
Core The innermost layer of a planet or star.
Coreolis effect Dizziness or nausea experienced when an astronaut in a spinning spacecraft moves his head in the opposite direction.
Corona The Sun's outer layer. The corona's changing appearance reflects changing solar activity.
Coronal mass ejection A huge cloud of hot plasma, expelled sometimes from the Sun. It may accelerate ions and electrons, and may travel through interplanetary space as far as the Earth's orbit and beyond it, often preceded by a shock front. When the shock reaches Earth, a magnetic storm may result.
Cosmic ray An extremely energetic (relativistic) charged particle.
Cosmic year The time it takes the Sun to revolve around the center of the galaxy, approximately 225 million years.
Cosmonaut The Russian term for an astronaut. A space traveler.
COSPAR The Committee on Space Research (established October 1958).
Countdown A count in inverse numerical order, in hours, minutes and finally seconds, of time remaining before the launch of a rocket.
Crater A round impression left in a planet or satellite from a meteoroid.
Crust The outer layer of Earth and other terrestrial planets.
Cryogenic A rocket fuel or oxidizer which is liquid only at very low temperatures, e.g. liquid hydrogen which has a boiling point of -217.2oC (-423oF).
CSA Chinese Society of Astronautics.
CSAA Chinese Society of Aeronautics and Astronautics.
CSM Command/Service Module.
C-stoff A rocket fuel used by Germany in World War II 30% hydrazine hydrate, 57% methanol, 13% water with traces of potassium cuprocyanate. Used in conjunction with T-stoff oxidizer 80% hydrogen peroxide with 1 to 2% oxiquinoline as a stabilizer.
Current The amount of electric charge flowing past a specified circuit point per unit time.
Cut-off The action of stopping a process abruptly, such as shutting off the flow of propellant to a rocket engine.

Index

Dark matter A form of matter which has not been directly observed but whose existence has been deduced by it's gravitational effects.
Data reduction Conversion of observed values into useful, ordered and simplified information.
DC Direct Current.
Decay The action of air drag upon an artificial satellite causing it to spiral back into the atmosphere, eventually to disintegrate or burn up.
Deceleration Negative acceleration, slowing.
Declination One of the coordinates, measured in degrees, used to designate the location of an object on the celestial sphere. Declination is a north-south value similar to latitude on Earth.
Decompression The relief of pressure. Explosive decompression would occur if the cabin of a spacecraft was punctured in space.
Delta V Difference or change in velocity.
Demodulation To extract information from a modulated carrier wave.
Density Amount of matter per unit volume.
Density Wave A kind of wave induced in a flat plane of a resisting medium (such as the rings of Saturn) by gravitational forces, often assuming the form of a tightly wound spiral.
Descending node The point at which an orbiting object or spacecraft, moving from north to south, crosses the plane of the equator.
Descent engine The rocket used to power a spacecraft as it makes a controlled landing on the surface of a planet or moon.
Descent module That part of a spacecraft that descends from orbit to the surface of a planet or moon.
DGLR Deutsche Gesellschaft fur Luft-und Raumfahrt (German Company for Air and Space Travel).
Digital computer An electronic device for solving numerically a variety of problems.
Dipole A compact source of magnetic force, with two magnetic poles. A bar magnet, coil or current loop, if their size is small, create a dipole field. The Earth's field, as a first approximation, also resembles that of a dipole.
Direct current Electrical current flowing in one direction and substantially constant in value.
Direct sensing Instruments that interact with phenomena in their immediate vicinity, and register characteristics of them.
Dish A reflector for radio waves, usually a paraboloid.
Docking The technique of connecting two or more spacecraft in space.
DoD Department of Defense (USA).
DOF Degrees Of Freedom.
Doppler effect A phenomenon in which waves appear to compress as their source approaches the observer or stretch out as the source recedes from the observer.
Dose A quantity of radiation delivered at a position. In the context of space energetic particle radiation effects, it usually refers to the energy absorbed locally per unit mass as a result of radiation exposure.
Dose equivalent A dose normally applied to biological effects and including scaling factors to account for the more severe effects of certain kinds of radiation.
Downlink The radio signal transmitted from a spacecraft to Earth.
Drag The resistance offered by a gas or liquid to a body moving through it.
Drogue A small parachute used to slow and stabilize a spacecraft returning to the atmosphere, usually preceding deployment of a main landing parachute.
DSN Deep Space Network.
Dust Particulates which have a direct relation to a specific solar system body and which are usually found close to the surface of this body (e.g. Lunar, Martian or Cometary dust).
Dust detector A device for measuring the velocity, mass, charge, flight direction and number of dust particles striking the instrument.
Dynamo process The generation of an electric currents by the flow of an electrically conducting fluid through a magnetic field. For instance, the magnetic field originating inside the Earth is believed to come from a dynamo process involving the flow of molten iron in the Earth's hot core. The energy required by the current is obtained from the motion of the flow.
Dyne A unit of force equal to the force required to accelerate a 1 g mass 1 cm per square second.

Index

Earth Third planet from the Sun, a terrestrial planet.
Earth radius The average radius of the Earth, a convenient unit of distance in describing phenomena and orbits in the Earth's neighborhood in space. 1 RE = 6371.2 km approximately.
Earth-sensor A light-sensitive diode which seeks the direction of Earth and then informs the attitude control system of a spacecraft.
Eccentric Noncircular; elliptical (applied to an orbit).
Eccentricity The amount of separation between the two foci of an ellipse and, hence, the degree to which an elliptical orbit deviates from a circular shape.
Eclipse The obscuring of one celestial body by the passage of another in front of it.
Ecliptic The great circle on the celestial sphere which traces the path of the Sun during the year.
ECM Electromagnetic Countermeasures.
EDT Eastern Daylight Time.
Ejection seat A seat fitted with an explosive charge and designed to eject the occupant clear of an aircraft during an in-flight emergency.
ELDO European Launcher Development Organization.
Electric propulsion A form of rocket propulsion which depends on some form of electric acceleration of propellant to achieve low thrust over long periods of time. E.g. an ion or magnetohydrodynamic engine.
Electromagnetic Relating to the interplay between electric and magnetic fields.
Electromagnetic waves A wave propagated through space by simultaneous periodic variation in the electric and magnetic field intensity at right angles to each other and to the direction of propagation. The electromagnetic spectrum includes radio waves, microwaves, infrared, visible and ultraviolet radiation, X-rays, gamma rays and cosmic rays.
Elevation The angular measure of the height of an object above the horizon; with azimuth, one of the coordinates defining celestial location and sometimes used in tracking spacecraft.
ELV Expendable Launch Vehicle.
EMU Extravehicular Mobility Unit.
Energetic particle Particles that can penetrate outer surfaces of spacecraft. For electrons, this is typically above 100 keV, while for protons and other ions this is above 1 Mev. Neutrons, gamma-rays and X-rays are also considered energetic particles in this context.
Engine In spacecraft, a rocket or thruster that burns liquid propellants and can be throttled to adjust thrust.
EOL End Of Life.
Ephemeris Table of predicted positions of bodies in the solar system.
Ephemeris time A measurement of time defined by orbital motions. Equates to Mean Solar Time corrected for irregularities in Earth's motions.
Epoch An instant in time that is arbitrarily selected as a point of reference, e.g. for a set of orbital elements.
Equator An imaginary circle around a body which is everywhere equidistant from the poles, defining the boundary between the northern and southern hemispheres.
Equatorial orbit An orbit in the plane of the equator.
ESA European Space Agency.
Escape tower A rocket-powered framework designed to separate spacecraft modules from their booster rockets in case of accident. Escape towers are mounted atop the spacecraft and jettisoned after launch.
Escape velocity The precise velocity necessary to escape from a given point in a gravitational field. A body in a parabolic orbit has escape velocity at any point in that orbit. The velocity necessary to escape from the Earth's surface is 6.95 miles/sec. (11.2 km/sec.).
ESMC Eastern Space and Missile Center.
EST Eastern Standard Time.
Eurospace Non-profit-making industrial association with headquarters in Paris (founded September 1961).
EVA Extravehicular Activity.
Exhaust velocity The velocity of the exhaust leaving the nozzle of a rocket.
Exosphere The part of the Earth atmosphere above the thermosphere which extends into space. H and He atoms can attain escape velocities at the outer rim of the exosphere.
Extravehicular activity Action performed by an astronaut or cosmonaut outside a vehicle in space; a spacewalk.

Index

Fairing A structure whose main function is to streamline and smooth the surface of an aircraft or space vehicle.
Fault A crack or break in the crust of a planet along which slippage or movement can take place.
Fault protection Algorithms, which reside in a spacecraft's subsystems, that insure the ability of the spacecraft to both prevent a mishap and to reestablish contact with Earth if a mishap occurs and contact is interrupted.
Ferret Satellite using electromagnetic surveillance techniques.
Fission The release of energy through splitting atoms.
Fluorescence The phenomenon of emitting light upon absorbing radiation of an invisible wavelength.
Flux The amount of radiation crossing a surface per unit of time, often expressed in "integral form" as particles per unit area per unit time.
Flyby Space flight past a heavenly body without orbiting.
Flyby spacecraft A spacecraft which follows a continuous trajectory past a target object, never to be captured into an orbit. It must carry instruments that are capable of observing passing targets by compensating for the target's apparent motion.
FOBS Fractional Orbit Bombardment System. A Soviet method of delivering a warhead from partial satellite orbit and thus approaching from any direction.
Force A vector quantity that tends to produce an acceleration of a body in the direction of its application.
Forward scattering Reflecting light approximately away from the source.
FOV Field Of View.
Free-fall The motion of any unpowered body moving in a gravitational field.
Free-return trajectory Path of a spacecraft that provides for a return to Earth.
Frequency The number of oscillations per second of an electromagnetic (or other) wave.
Fuel A substance that when combined with an oxidizer burns to produce thrust in rockets.
Fuel cell A cell in which chemical reaction is used directly to produce electricity.
Fussion The release of nuclear energy through the uniting of atoms.
FY Fiscal Year.

Index

g The symbol for the acceleration of a freely moving body due to gravity at the surface of the Earth (9.81 m/s2). Alternatively, 1 g.
Galaxy A very large system of stars, gas and dust isolated from its neighbors by an immensity of space; an "island universe".
GALCIT Guggenheim Aeronautical Laboratory of the California Institute of Technology.
Gamma rays Very short, highly-penetrative electromagnetic radiation with a shorter wavelength than X-rays; produced in general by emission from atomic nuclei.
Gas generator A chamber in which propellant is burned to produce high pressure gas that is then used to drive a turbine, e.g. turbopump.
Gas giant A large planet composed mostly of gas, e.g. the Jovian planets.
GATV Gemini-Agena Target Vehicle.
Gauss CGS unit of magnetic induction (after the German mathematician Karl F. Gauss).
GE General Electric Company of the USA.
GEO Geostationary Orbit. Also abbreviated GO.
Geo- Prefix referring to the Earth.
Geocentric Earth centered.
Geodesy The science of the Earth's shape.
Geomagnetic storm A worldwide disturbance of the Earth's magnetic field, distinct from regular diurnal variations.
Geospace Also called the solar-terrestrial environment, geospace is the domain of Sun-Earth interactions. It consists of the particles, fields, and radiation environment from the Sun to Earth's space plasma environment and upper atmosphere. Geospace is considered to be the fourth physical geosphere (after solid earth, oceans, and atmosphere).
Geostationary orbit A circular orbit in which a satellite moves from west to east at such velocity as to remain fixed above a particular point on the equator. Also called geosynchronous.
Geostationary transfer orbit An elliptical orbit used to transfer a space vehicle from low earth orbit to geostationary orbit.
Geosynchronous orbit See geostationary orbit.
g-Force A force caused by acceleration expressed in g's.
GH2 Gaseous Hydrogen.
GHz Gigahertz, equal to one billion hertz.
Gimbal A mechanical frame for a gyroscope or power unit, usually with two perpendicular axes of rotation.
GMT Greenwich Mean Time.
GN&C Guidance, Navigation and Control.
GO Geostationary Orbit. Also abbreviated GEO.
GOX Gaseous Oxygen.
Grain The rubberlike mass of chemical propellant that provides propulsion in solid fuel rockets. The shape of the grain determines the rate and pattern of burn and thus controls thrust.
Gravitational waves Einsteinian distortions of the space-time medium predicted by general relativity theory (not yet detected as of November 1995).
Gravity The force responsible for the mutual attraction of separate masses.
Gravity assist trajectory A trajectory in which angular momentum is transferred from an orbiting planet to a spacecraft approaching from behind. The result is an increase in the spacecraft's velocity.
Gravity field survey The mapping of a planet's mass distribution by studying variations in the in the planet's gravity field strength made evident by minute Doppler shifts in an orbiting spacecraft's radio signal.
Gravity waves Certain atmospheric waves within a planet's atmosphere.
Great circle An imaginary circle on the surface of a sphere whose center is at the center of the sphere.
Greenwich mean time See universal time.
GSFC NASA Goddard Space Flight Center (Greenbelt, Maryland).
GSO Geosynchronous Orbit.
GTO Geostationary Transfer Orbit.
Guillotine A device equipped with explosive blades used to cut cables, water lines, wires, etc. during separation of spacecraft modules.
Gyration The circular motion of ions and electrons around magnetic field lines.
Gyroscope A spinning, wheel-like device that resists any force that tries to tilt its axis. Gyroscopes are used for stabilizing the attitude of rockets and spacecraft in motion.

Index

H2O2 Hydrogen Peroxide.
Hatch Door or doorway, usually hermetically sealed.
Heat shield A device which protects people or equipment from heat, such as a shield in front of a reentry capsule.
Helio- Prefix referring to the Sun.
Heliocentric Centered on the Sun.
Heliopause The boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence, perhaps 100 AU from the Sun.
Heliosphere The space within the boundary of the heliopause, containing the Sun and solar system.
Hertz A unit of frequency equal to one cycle per second, named after Heinrich Hertz.
Heterosphere The Earth atmosphere above 105 km altitude where species-wise concentration profiles establish due to diffusive equilibrium, with N2 dominance below 200 km, O dominance from 200 to 600 km, and He dominance as of 600 km altitude.
HGA High-Gain Antenna.
High-energy particle detector A device for measuring the energy spectra of trapped energetic electrons, and the energy and composition of atomic nuclei.
High-gain antenna A dish-shaped spacecraft antenna principally used for high rate communication with Earth This type of antenna is highly directionally and must be pointed to within a fraction of a degree of Earth.
Hohmann transfer orbit An interplanetary trajectory in which a spacecraft is launched into an elliptical solar orbit whose perihelion (inner planet) or aphelion (outer planet) reaches the orbit of the target planet on the opposite side of the Sun. Uses least propellant.
Homosphere The Earth atmosphere below 105 km altitude where complete vertical mixing yields a near-homogeneous composition of about 78.1% N2, 20.9% O2, 0.9% Ar, and 0.1% CO2 and trace constituents. The homopause (or turbopause) marks the ceiling of the homosphere. The homosphere can be broadly divided into three distinct regimes the troposphere (0 to 12 km), the stratosphere (12 to 50 km) and the mesosphere (50 to 90 km)
Horizon The line marking the apparent junction of Earth and sky.
Horizon scanner A scanner which automatically seeks the horizon for purposes of a spacecraft's orientation and control, e.g. one that detects the sharp discontinuity in infrared intensity at the outer edge of the Earth's tropopause.
Hour angle The angular distance of a celestial object measured westward along the celestial equator from the zenith crossing.
HTP High Test Peroxide.
HTPB Hydroxy-terminated polybutadiene. A polymeric fuel binder.
Hydrazine A rocket fuel which burns spontaneously with nitric acid or nitrogen tetroxide. Can also be used as a mono-propellant: when passed through an iridium-bearing catalyst, it decomposes at high temperature into constituent gases of ammonia, nitrogen and hydrogen. Used in small thrusters for orbit modification and attitude control of spacecraft. Also see MMU and UDMH.
Hydrosphere The water on or around the surface of a planet.
Hydyne A rocket fuel comprised of 60% UDMH and 40% diethylene-triamine.
Hyperbolic A trajectory path to a planet shaped like a hyperbola.
Hypergolic A term applied to an oxidizer and a fuel which ignite spontaneously with each other.

Index

IAA Indian Astronautical Association.
IAA International Academy of Astronautics (established August 1960).
IAF International Astronautical Federation (formally inaugurated 1951).
IC Integrated Circuit.
ICBM Inter-Continental Ballistic Missile (range >5,500 km).
IGY International Geophysical Year (1957-58).
Impulse The product of the average force acting on a body and the interval of time during which it acts, being a vector quantity equal to the change of momentum of the body during the same time interval.
IMU Inertial Measurement Unit.
Inclination The angular distance between a satellite's orbital plane and the equator of its primary.
Inertial guidance An on-board system for launch vehicles and spacecraft where gyroscopes, accelerometers and other devices satisfy guidance requirements.
Inertial measurement unit An on-board instrument system that measures the attitude of a spacecraft. It includes accelerometers and gyroscopes.
Inferior conjunction Alignment of Earth, Sun, and an inferior planet on the same side of the Sun.
Inferior planets Planets whose orbits are closer to the Sun than Earth's, i.e. Mercury and Venus. Also called inner planets.
Infrared Electromagnetic radiation of wavelengths between 7500 A, the limit of the visible light spectrum at the red end, and centimetric radio waves.
Infrared radiometer A telescope based instrument that measures the intensity of infrared energy radiated by the targets.
Injection angle The angle at which a spacecraft's return trajectory intersects the Earth's atmosphere.
Injector Typically, a perforated plate through which liquid fuel and oxidizer are injected into the combustion chamber at a controlled rate.
Intelsat Organization of 105 countries (July 1980) owning or operating systems of satellites used by 144 countries and territories around the world for international communications, and by 16 countries for domestic communications.
Interferometer Any of several optical, acoustic, or radio frequency instruments that use interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to determine wavelengths and wave velocities, measure very small distances and thicknesses, and measure indices of refraction.
Interplanetary magnetic field The weak magnetic field filling interplanetary space, with field lines usually connected to the Sun. The IMF is kept out of the Earth's magnetosphere, but the interaction of the two plays a major role in the flow of energy from the solar wind to the Earth's environment.
Interplanetary probe Unmanned instrumented spacecraft capable of reaching the planets.
Interplanetary shock The abrupt boundary formed at the front of a plasma cloud (e.g. from a coronal mass ejection) moving much faster than the rest of the solar wind, as it pushes its way through interplanetary space.
Interstellar ark Hypothetical space colony capable of transporting human intelligence to the stars.
Interstallar probe Unmanned instrumented spaceship with artificial intelligence capable of reaching the nearer stars.
Inverse-square law The mathematical description of how the strength of some forces, including gravity, changes in inverse proportion to the square of the distance from the source.
Ion An atom that has lost or acquired one or more electrons.
Ion engine A rocket engine, the thrust of which is obtained by the electrostatic acceleration of ionized particles.
Ionization Formation of electrically charged particles. Can be produced by high-energy radiation such as light or UV rays, or by collision of particles in thermal agitation.
Ionosphere An atmospheric layer dominated by charged, or ionized, atoms that extends from about 38 to 400 miles above the Earth's surface.
IR Infrared.
IRBM Intermediate Range Ballistic Missile (range 500-5,500 km).
IRFNA Inhibited Red Fuming Nitric Acid; 85% HNO3 + < 5% H2O + 6-15% NOX
ISAS Institute of Space and Aeronautical Science; University of Tokyo (Japan).
Isotropic A property of a distribution of particles where the flux is constant over all directions.
Isp Specific Impulse. Also abbreviated SI.
ISRO Indian Space Research Organization.
IWFNA Inhibited White Fuming Nitric Acid; 97.5% HNO3 + 2% H2O + < 0.5% NOX

Index

Jansky Unit used to express flux. 1 Jansky = 10-26 W m-2 Hz-1 Bandwith.
Jet propulsion Reaction propulsion in which the propulsion unit obtains oxygen from the air as distinguished from rocket propulsion in which the unit carries its own oxygen-producing material.
Jovian planet Any of the four biggest planets Jupiter, Saturn, Uranus, and Neptune.
JPL NASA Jet Propulsion Laboratory (Pasadena, California).
JSC NASA Johnson Space Center (Houston, Texas).
Jupiter Fifth planet from the Sun, a gas giant or Jovian planet.

Index

K-band A range of microwave radio frequencies in the neighborhood of 12 to 40 GHz, used for high speed data transmission on shuttle flights (~15 Ghz).
Kelvin Scale of temperature named after the English physicist Lord Kelvin, based on the average kinetic energy per molecule of a perfect gas. Absolute zero is equivalent to -273.16oC (-459.4oF).
Kerosene A mixture of hydrocarbons distilled from crude petroleum; see RJ-1, RP-1.
KHz Kilohertz, equal to 1,000 Hertz.
Kilogram The standard unit of mass in the metric system.
Kinetic energy An object's energy of motion; for example, the force of a falling body.
Kosmobuksir Russian name for "space tug".
Kosmolyot Russian name for "spaceplane".
KSC Kennedy Space Center (Florida).
KT Kilotonne, equal to 1,000 tonnes.
Kuiper belt A swarm of cometary bodies thought to orbit the Sun beyond Neptune at distances between 30 and 50 AU.

Index

L-band A range of microwave radio frequencies in the neighborhood of 1 to 2 GHz.
Lander spacecraft A spacecraft designed to reach the surface of a planet or moon and survive long enough to telemeter data back to Earth.
Langrangian point In a system dominated by two attracting bodies (such as Sun and Earth), a point at which a third, much smaller body (such as a satellite) keeps the same position relative to the other two. Theoretically, the Sun-Earth system has 5 Lagrangian points, but only two are important: L1, on the sunward side of Earth, about 4 times the distance of the Moon, and L2 at approximately the same distance on the midnight side. The only two lagrangian stable points, L4 and L5, lie in the orbit of the primary body, leading and trailing it by a 60-degree arc. Jupiter's trojan asteroids can be found orbiting around the Jupiter-Sun L4 and L5 points.
Lanyard Small rope or cord.
Laser Light Amplification by the Stimulated Emission of Radiation. A device for producing a coherent monochromatic high-intensity beam of light.
Latitude Circles in parallel planes to that of the equator defining north-south measurements, also called parallels.
Launch complex The complex of site, facilities and equipment used to launch a missile or space rocket.
Launch pad The load-bearing base from which a rocket or spacecraft positioned on its launcher is fired.
Launch window An interval of time during which a space vehicle can be launched to accomplish a given mission, e.g. a flight to Venus or Mars.
LC Launch Center.
Leading side For a satellite that keeps the same face toward the planet, the hemisphere that faces forward, into the direction of motion.
Leap year Every fourth year, in which a 366th day is added since the Earth's revolution takes 365 days 5 hr 49 min.
LEO Low Earth Orbit.
LGA Low-Gain Antenna.
LH2 Liquid Hydrogen.
Liftoff The start of a rocket's flight from its launch pad. Colloquially, "blast-off".
Light Electromagnetic radiation that is visible to the eye, in the neighbourhood of 1 nanometer wavelength.
Light speed 299,792,458 meters per second +/- 1.2 m/sec (186,282.39 miles/sec). U.S. National Bureau of Standards, 1971.
Light time The amount of time it takes light or radio signals to travel a certain distance at light speed.
Light year The distance light travels in one year, approximately 9.46 trillion km (5.88 trillion miles).
LiOH Lithium Hydroxide.
Liquid hydrogen A cryogenic rocket fuel which becomes liquid at -423oF.
Liquid oxygen A cryogenic oxidizer which becomes liquid at -279oF.
Lithosphere The crust of a planet.
LM Lunar Module.
LO2 Liquid Oxygen. Also abbreviated LOX.
Local time Time adjusted for location around the Earth or other planets in time zones.
LOI Lunar Orbit Insertion.
Longitude Great circles that pass through both the north and south poles, also called meridians.
Longitude of ascending node In an orbit, the celestial longitude of the ascending node.
Longitudinal axis The fore-and-aft line through the center of a space vehicle.
Low-energy charged particle detector A device designed to characterize the composition, energies, and angular distributions of charged particles in interplanetary space and within planetary systems.
Low-gain antenna An omni-directional spacecraft antenna that provides relatively low data rates at close range, several AU for example.
LOX Liquid Oxygen. Also abbreviated LO2.
LRBM Long Range Ballistic Missile.
LRV Lunar Roving Vehicle.
LT Launch Time.
Lunar Of or pertaining to the Moon.
Lunar module The craft used by Apollo missions for Moon landings. The lunar module consisted of a decent stage, used to land on the Moon and as a platform for liftoff, and an ascent stage, used as crew quarters and for returning to the orbiting command module.
Lunar roving vehicle A battery powered wheeled vehicle used by Apollo astronauts to explore the lunar surface.
LV Launch Vehicle.

Index

Mach The ratio of the speed of a vehicle (or of a liquid or gas) to the local speed of sound.
Magnetic field A region of space near a magnetized body where magnetic forces can be detected.
Magnetic field line Lines everywhere pointing in the direction of the magnetic force, used as a device to help visualize magnetic fields. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.
Magnetic pole Two meanings: (1) the points on Earth towards which the compass needle points. (2) A concentrated source of magnetic force, e.g. a bar magnet has two magnetic poles near its end.
Magnetic storm A disturbance of the Earth's magnetic field initiated by a solar flare or sunspot.
MagnetoHydroDynamics The study of plasma motion and dynamics in the presence of a magnetic field.
Magnetometer A device for measuring the strength and direction of the interplanetary and solar magnetic fields.
Magnetopause The boundary of the magnetosphere, lying inside the bow shock. The location in space where Earth's magnetic field balances the pressure of the solar wind. It is located about 63,000 km from Earth in the direction of the Sun.
Magnetosphere That region of space surrounding the Earth which is dominated by the magnetic field.
Magnetron A vacuum tube in which the flow of electrons is subject to the control of an external magnetic field.
Major axis The maximum diameter of an ellipse.
Manned maneuvering unit A portable jet-pack device used by astronauts to propel themselves through space independent of a spacecraft.
Mantle Middle layer of the Earth; between the crust and the core.
Maria Dark areas on the Moon, actually lava plains, once believed to be seas.
Mars Fourth planet from the Sun, a terrestrial planet.
Mass The quantity of matter in a body. It can be determined by measuring the force of gravity (weight) acting on it and dividing this by the gravitational acceleration at that point. Thus, the mass of a given body remains the same everywhere, while its weight changes with the gravitational attraction.
Mass ratio Ratio of the total mass of a rocket vehicle to the mass remaining when all the propellant is consumed.
Max Q Maximum dynamic pressure; the point during launch when the vehicle is subjected to its greatest aerodynamic stress.
Mean Synonym for average.
Mean solar time Time based on an average of the variations caused by Earth's non-circular orbit.
Medium-gain antenna A spacecraft antenna that provides greater data rates than a low-gain antenna, with wider angles of coverage than a high gain antenna, about 20-30 degrees.
Memory The faculty of an electronic device to record and store data and/or instructions for future action on a command.
Mercury First planet from the Sun, a terrestrial planet.
Meridian Great circle that passes through both the north and south poles, also called line of longitude.
Mesosphere A division of the Earth's atmosphere extending from altitudes ranging 18-30 miles to 48-55 miles.
Meteor The luminous phenomenon seen when a meteoroid enters the atmosphere, commonly known as a shooting star.
Meteorite A part of a meteoroid that survives through the Earth's atmosphere.
Meteoroid A solid body, moving in space, that is smaller than an asteroid and at least as large as a speck of dust. Nearly all meteoroids originate from asteroids or comets.
MeV One million electron volts.
MHz Megahertz, equal to one million hertz.
Microgravity An environment of very weak gravitational forces, such as those within an orbiting spacecraft. Microgravity conditions in space stations may allow experiments or manufacturing processes that are not possible on Earth.
Micrometeoroid Meteoroid less than 1/250th of an inch in diameter.
Micrometeoroid protection Shielding used to protect spacecraft components from micrometeroid impacts. Interplanetary spacecraft typically use tough blankets of Kevlar or other strong fabrics to absorb the energy from high-velocity particles.
Microwaves Radio waves having wavelengths of less than 20 centimeters.
Milky Way The galaxy which includes the Sun and Earth.
Minor planet An asteroid.
Missile An object or a weapon that is fired, thrown, dropped, or otherwise projected at a target; a projectile.
Mixture ratio Ratio of the masses of the fuel to the oxidizer at any given time.
MMH Monomethyl Hydrazine. A liquid hypergolic fuel.
MMU Manned Maneuvering Unit.
Mock-up A full-size replica or dummy of a vehicle, e.g. a spacecraft, often made of some substitute material such as wood to assess design features.
Modulation The variation of a property of an electromagnetic wave or signal, such as its amplitude, frequency, or phase.
Module A self-contained unit of a spacecraft or space station which serves as a building block for the total structure.
Momentum The product of the mass of a body and its velocity.
Mono-propellant A rocket propellant consisting of a single substance, especially a liquid containing both fuel and oxidizer, either combined or mixed together.
Moon A small natural body which orbits a larger one. A natural satellite.
Motor In spacecraft, a rocket that burns solid propellants.
MRBM Medium Range Ballistic Missile.
MSFC NASA Marshall Space Flight Center (Huntsville, Alabama).
MT Megatonne, equal to 1 million tonnes.
MT Moscow Time.
Multiplexer A mechanical or electrical device for sharing a circuit by two or more coincident signals.
Multistage rocket A rocket having two or more stages which operate in succession each being discarded as its job is done.

Index

N2O4 Nitrogen Tetroxide. Also abbreviated NTO.
Nadir The direction from a spacecraft directly down toward the center of a planet. Opposite the zenith.
NASA National Aeronautics and Space Administration (USA).
NASDA National Space Development Agency (Japan).
Nautical mile The distance spanned by one minute of arc in latitude, defined internationally as 1,852 meters (6,076.1033 feet).
Neptune Eighth planet from the Sun, a gas giant or Jovian planet.
Neutron Atomic particles having approximately the same mass as a hydrogen atom; very penetrating.
Newton That force which gives a mass of 1 kilogram an acceleration of 1 meter per second per second; equal to 100,000 dynes.
NiCd Nickel Cadmium.
Nitric acid A liquid oxidizer that reacts spontaneously with hydrazine. Also see IRFNA and IWFNA.
Nitrogen tetroxide A liquid oxidizer that reacts spontaneously with hydrazine.
Noctilucent clouds Weakly-luminous clouds, seen at night at heights of about 50 miles (80 km) above the Earth.
Non-coherent Communications mode wherein a spacecraft generates its downlink frequency independent of any uplink frequency.
NORAD North American Air Defense Command (USA).
Nose shroud A cover on the nose of a rocket or spacecraft which jettisons before insertion into orbit.
Nozzle The projecting aperture at the end of a combustion chamber serving as an outlet for the exhaust gases.
NRC National Research Council (USA).
NTO Nitrogen Tetroxide. Also abbreviated N2O4.
Nucleus The central body of a comet.

Index

Occultation The passage of a celestial body across a line between an observer and another celestial object; and the progressive blocking of light, radio waves, or other radiation from a celestial source during such a passage.
OKB Experimental Construction Bureau.
Omnidirectional Capable of transmitting or receiving signals in all directions, as an antenna.
OMS Orbital Maneuvering System.
One-way Communications mode consisting only of downlink received from a spacecraft.
One-way light time The elapsed time it takes for light, or a radio signal, to reach a spacecraft or other body from Earth, or vice versa.
Oort cloud A large swarm of comets theorized to orbit the sun in the neighborhood of 50,000 AU.
Orbit The path of a body acted upon by the force of gravity. Under the influence of a single attracting body, all orbital paths trace out simple conic sections. Although all ballistic or free-fall trajectories follow an orbital path, the word orbit is more usually associated with the continuous path of a body which does not impact with its primary.
Orbit insertion The placing of a spacecraft into orbit around a planet or moon.
Orbit trim maneuver The firing of control rockets to refine a spacecraft's speed and trajectory.
Orbital elements Six quantities used to mathematically describe an orbit; i.e. semi-major axis, eccentricity, inclination, argument of periapsis, time of periapsis passage and longitude of ascending node.
Orbital mechanics The study of the motions of artificial satellites and space vehicles moving under the influence of forces such as gravity, drag, and thrust. Also called flight mechanics.
Orbital module That part of a spacecraft which allows additional volume for crew relaxation and/or experiments. Discarded prior to reentry.
Orbital period The time taken by an orbiting body to complete one orbit.
Orbital velocity The velocity necessary to overcome the gravitational attraction of the Earth and so keep a satellite in orbit, about 17,450 mph (28,080 km/hr) close to the Earth.
Orbiter spacecraft A spacecraft designed to travel to a distant planet or moon and enter orbit. It must carry a substantial propulsive capability to decelerate it at the right moment to achieve orbit insertion.
O-stage Rocket boosters which operate during part of the burning time of the first stage of a launch vehicle to provide additional thrust.
OTM Orbit Trim Maneuver.
OTRAG Orbital Transport- und Raketen- Atktiengesellsschaft.
OWLT One-Way Light Time.
Oxidizer An agent that releases oxygen for combination with another substance, creating combustion and gas for propulsion. Alternatively oxidants.

Index

P & W Pratt and Whitney (USA).
Parachute An apparatus used to retard free fall, consisting of a light, usually hemispherical canopy attached by cords and stored folded until deployed in descent.
Parallel Circle in parallel planes to that of the equator defining north-south measurements, also called line of latitude.
Parking orbit Orbit in which a space vehicle awaits the next phase of its planned mission.
Parsec Measure of distance, 1 parsec = approximately 3.26 light years.
Pascal A unit of pressure equal to one newton per square meter.
Passive cooling The use of painting, shading, reflectors and other techniques to cool a spacecraft.
Payload Revenue-producing or useful cargo carried by a spacecraft; also, anything carried in a rocket or spacecraft that is not part of the structure, propellant, or guidance systems.
PBAN Polybutadiene acrylic acid acrylonitrile. A polymetric fuel binder.
PDT Pacific Daylight Time.
Pegasus A rocket-vehicle concept for transportation of commercial high-priority freight or 172 passengers.
Periapsis That point in an orbit which is nearest to the primary.
Perigee That point in a terrestrial orbit which is nearest to the Earth.
Perihelion That point in a solar orbit which is nearest to the Sun.
Perilune That point in a lunar orbit which is nearest to the Moon.
Period of revolution Time of one complete cycle in orbital motion - referred to as a year when applied to Earth.
Period of rotation Time of one complete cycle - referred to as a day when applied to Earth.
Perturbation Modifications to simple conic section orbits caused by such disturbances as air drag, non-uniformity of the Earth, and gravitational fields of more distant bodies such as the Moon.
Phase Two meanings (1) The particular appearance of a body's state of illumination, such as the full phase of the moon. (2) As applied to electromagnetic waves, phase is the relative measurement of the alignment of two waveforms of similar frequency.
Phase angle The angle in which waves come to a body.
Photometer An optical instrument that measures the intensity of light from a source.
Photometry The measurement of light intensities.
Photon A quantum of radiant energy.
Photon propulsion The propulsion of a vehicle by the emission of photons, which possess momentum.
Photosphere The visible surface of the Sun.
Photovoltaic cells Crystalline wafers called solar cells which convert sunlight directly into electricity without moving parts.
Pitch The rotation of a vehicle about its lateral (Y) axis, i.e. movement in elevation.
Planet A nonluminous celestial body larger than an asteroid or a comet, illuminated by light from a star, such as the sun, around which it revolves. The only known planets are those of the Sun but others have been detected on physical (non-observational) grounds around some of the nearer stars.
Planetoid An asteroid.
Plasma A gaslike association of ionized particles that responds collectively to electric and magnetic fields.
Plasma detector A device for measuring the density, composition, temperature, velocity and three-dimensional distribution of plasmas that exist in interplanetary regions and within planetary magnetospheres.
Plasma engine A rocket engine in which thrust is obtained from the acceleration of a plasma with crossed electrical and magnetic fields.
Plasma wave An oscillation or wave in a plasma that falls in the audio range of frequency.
Plasma wave detector A device for measuring the electrostatic and electromagnetic components of local plasma waves in three dimensions.
Plasmasphere The region of the atmosphere consisting of cold dense plasma originating in the ionosphere and trapped by the Earth's magnetic field.
PLSS Portable Life Support System.
Plug nozzle A doughnut-shaped combustion chamber which discharges engine gases against the surface of a short central cone (the plug). Adapted in the form of an LH2 cooled heat shield, it can be used as a combination rocket/ aerodynamic braking device.
Pluto Ninth planet from the Sun, considered by many a minor planet.
Plutonium-238 A form of the radioactive element plutonium, characterized by high energy emissions.
PO Polar Orbit.
Polar orbit An orbit which passes over the poles.
Polarimeter An optical instrument that measures the direction and extent of the polarization of light reflected from its targets.
Polymer A compound used as a binder for solid rocket propellant systems; more generally, a compound consisting of repeating structural units.
Potential energy The energy of a body due to its position in a field.
Pound The U.S. customary unit of force defined as the weight of the standard pound at sea level and at the latitude of 45o.
ppm Parts per million.
Precession A change in the direction of the axis of spin of a rotating body.
Pressure suit A suit, with helmet attached, which is inflated to provide body pressure and air, worn by the crew of certain spacecraft and aircraft which fly at great altitudes.
Pressurized Containing air or other gas at a pressure higher than the pressure outside the chamber.
Primary The body around which a satellite orbits.
Primitive Used in a chemical sense, indicating an unmodified material representative of the original composition of the solar nebula.
Probe An unmanned instrumented vehicle sent into space to gather information.
Prograde Orbital motion in the same direction as the primary's rotation.
Propellant A chemical or chemical mixture burned to create the thrust for a rocket or spacecraft.
Propulsion The process of driving or propelling.
PST Pacific Standard Time.
PTC Passive Thermal Control.
Pulsar Discovered in 1967. Pulsars emit radio signals the pulsations of which are extremely precise. The evidence suggests that pulsars are fast-spinning neutron stars.
Pyrotechnics The use of electrically initiated explosive devices to operate valves, ignite solid rocket motors, and explode bolts to separate from or jettison hardware, or to deploy appendages.

Index

Quasars Quasi-stellar objects. They are believed to be among the most distant objects in the observable Universe, emitting more energy than some of the most powerful galaxies.

Index

Radar System or technique for detecting the position, motion, and nature of a remote object by means of radio waves reflected from its surface.
Radian Unit of angular measurement equal to the angle at the center of a circle subtended by an arc equal in length to the radius. Equals about 57.296 degrees.
Radiation Energy in the form of electromagnetic waves or particles.
Radiation belt The region of high-energy particles trapped in the Earth's magnetic field, also known as the Van Allen belts.
Radio The least energetic form of electromagnetic radiation, having the lowest frequency and the longest wavelength.
Radio astronomy The science of astronomy using radio waves instead of light waves.
Radio guidance A system which is dependent on outside signals for information.
Radioisotopes Atomic particles which decay by natural radioactivity.
Radioisotope thermoelectric generator A device that converts the heat produced by the radioactive decay of plutonium-238 into electricity by an array of thermocouples made of silicon-germanium junctions. The Pu-238 is contained within a crash resistant housing.
Radiometry The detection and measurement of radiant electromagnetic energy, usually in the infrared.
Rankine A temperature scale, having a degree equal to the Fahrenheit degree but having a zero point at absolute zero. The freezing point of water is at 459.69oR.
RCS Reaction Control System.
RD Reaktivnyi Dvigatel. Russian for reaction motor.
RE Unit of distance equal to the radius of the Earth, 6371.2 km.
Reaction control system System of thrusters used to control spacecraft attitude.
Reaction wheels Electrically-powered wheels mounted in three orthogonal axes aboard a spacecraft. To rotate the vehicle in one direction, you spin up the proper wheel in the opposite direction. To rotate the vehicle back, you slow down the wheel.
Readout The action of a radio transmitter sending data either at the same time as data are acquired or by playback from an electronic memory.
Receiver An electronic device that receives incoming radio signals and converts them to perceptible forms.
Red dwarf A small star, on the order of 100 times the mass of Jupiter.
Redundancy The duplication of certain critical components in a space vehicle.
Reentry The descent into Earth's atmosphere from space.
Reentry interface An altitude 400,000 feet; the point at which reentering spacecraft are considered to enter the Earth's atmosphere.
Refraction The deflection or bending of electromagnetic waves when they pass from one kind of transparent medium into another.
Regenerative cooling Circulation of a propellant through a jacket around the combustion chamber in order to cool the chamber wall, the propellant subsequently being injected into the combustion chamber.
Relay An electrical switch employing an armature to open and close circuits.
Rem Roentgen Equivalent Man. A measure of nuclear radiation causing biological damage.
Remote sensing Instruments that record characteristics of objects at a distance, sometimes forming an image by gathering, focusing, and recording reflected light from the Sun, or reflected radio waves emitted by the spacecraft.
Rendezvous A place of meeting at a given time, for example, a spaceship with a space station.
Resolution Ability to distinguish visual detail, usually expressed in terms of the size (in kilometers) of the smallest features that can be distinguished.
Resonance A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
Retrograde Orbital motion in the direction opposite to the primary's rotation.
Retrorocket A rocket fired to reduce the speed of a spacecraft.
Revolution Orbital motion about a primary.
RF Radio Frequency.
Right ascension With declination, one of the coordinates used to designate the location of an object on the celestial sphere. Right ascension is measured in hours, minutes, and seconds and is similar to longitude on Earth.
Ring current A very spread-out electric current circling around the Earth, carried by trapped ions and electrons.
RJ-1 A hydrocarbon rocket fuel (a refined kerosene).
RLV Reusable Launch Vehicle.
Rocket A missile or vehicle propelled by the combustion of a fuel and a contained oxygen supply. The forward thrust of a rocket results when exhaust products are ejected from the tail.
Rocketdyne A division of Rockwell International concerned with the design and development of rocket engines (USA).
Roll The rotational movement of a vehicle about a longitudinal (X) axis.
Rotation Rotary motion about an axis.
Round-trip light time The elapsed time is takes for light, or a radio signal, to travel from Earth, be received and immediately transmitted or reflected, and return to the starting point.
RP-1 A hydrocarbon rocket fuel (a refined kerosene).
RTG Radioisotope Thermoelectric Generator.
RTLT Round-Trip Light Time.
RV Reentry Vehicle.

Index

SAR Synthetic Aperture Radar.
Satellite Any body, natural or artificial, in orbit around a planet. The term is used most often to describe moons and spacecraft.
Saturn Sixth planet from the Sun, a gas giant or Jovian planet.
S-band A range of microwave radio frequencies in the neighborhood of 2 to 4 GHz, used for communicating with piloted space missions (~2 Ghz).
SCADA Supervisory Control And Data Acquisition.
Scan platform An articulated, powered appendage to the spacecraft bus which points in commanded directions, allowing optical observations to be taken independently of the spacecraft's attitude.
Seismometer A device for measuring movements of the ground.
Semi-major axis Half the major axis of an ellipse. The mean distance of a planet or satellite from its primary.
Sensor An electronic device for measuring or indicating a direction or movement.
SEP Societe Europeene de Propulsion (France).
Sequencer A mechanical or electrical device which may be set to initiate a series of events and to make events follow a sequence.
Service module That part of a spacecraft which usually carries a maneuvering engine, thrusters, electrical supply, oxygen and other consumables external to the descent module. Discarded prior to reentry.
SETI Search for Extra-Terrestrial Intelligence.
Sextant An instrument that measures angular distances from fixed celestial objects.
Shepherd moon Moon which gravitationally confines ring particles.
Sidereal time Time relative to the stars other than the Sun.
Simulator A device that mimics the operational conditions of equipment or vehicles.
SIS Satellite Interceptor System.
SL Sea Level.
SLBM Submarine Launched Ballistic Missile.
Slug The U.S. customary unit of mass defined as the mass which receives an acceleration of 1 foot per second per second when a force of 1 pound is applied to it.
SM Service Module.
Solar Of or pertaining to the Sun.
Solar array See solar panel.
Solar cell A cell that converts sunlight into electrical energy. The light falling on certain substances (e.g. a silicon cell) causes an electric current to flow.
Solar constant The electromagnetic radiation from the Sun that falls on a unit area of surface normal to the line from the Sun, per unit time, outside the atmosphere, at one astronomical unit.
Solar flare A sudden brightening in some part of the Sun, followed by the emission of jets of gas and a flood of ultra-violet radiation. The gale of protons which accompanies a flare can be very dangerous to astronauts.
Solar nebula The large cloud of gas and dust from which the Sun and planets condensed 4.6 billion years ago.
Solar panel An array of light-sensitive cells attached to a spacecraft and used to generate electrical power for the vehicle in space. Also called solar array.
Solar sensors Light-sensitive diodes which indicate the direction of the Sun.
Solar wind A current of charged particles that streams outward from the Sun.
Solid propellant A rocket propellant in solid form; usually consisting of a mixture of fuel and oxidizer.
Solid rocket booster A rocket, powered by solid propellants, used to launch spacecraft into orbit.
Sounding rocket A research rocket used to obtain data from the upper atmosphere.
Space The universe beyond Earth's atmosphere. The boundary at which the atmosphere ends and space begins is not sharp but starts at approximately 100 miles above Earth's surface.
Space colony Hypothetical extra-terrestrial habitat, for hundreds, thousands or even millions of people, perhaps established on a moon or planet or as an artificial construction in free space.
Space debris Man-made objects or parts thereof in space which do not serve any useful purpose.
Spacecraft A piloted or unpiloted vehicle designed for travel in space.
Spacecraft clock A counter maintained by the command & data subsystem. It meters the passing of time during the life of the spacecraft, and regulates nearly all activity within the spacecraft systems.
Space platform A large artificial satellite conceived as a habitable base in space with scientific, exploratory or military applications. A space station.
Space station An orbiting spacecraft designed to support human activity for an extended time.
Space weather The popular name for energy-releasing phenomena in the magnetosphere, associated with magnetic storms, substorms and shocks.
SPADATS Space Detection and Tracking System (USA).
SPASUR Space Surveillance System (USA).
Specific impulse Parameter for rating the performance of a rocket engine. Indicates how many pounds or kilograms of thrust are obtained by consumption of a pound or kilogram of propellant in one second.
Spectrometer An optical instrument that splits the light received from an object into its component wavelengths by means of a diffraction grating; then measuring the amplitudes of the individual wavelengths.
Spectroscopy The study of the production, measurement and interpretation of electromagnetic spectra.
Spectrum A particular distribution of wavelengths and frequencies.
Spin stabilization Spacecraft stabilization accomplished by rotating the spacecraft mass, thus using gyroscopic action as the stabilizing mechanism.
SRB Solid Rocket Booster.
SRB propellant Composite propellant used in the Space Shuttle Solid Rocket Boosters. Consists mainly of ammonium perchlorate as the oxidizer, powdered aluminum as the metallic fuel, and PBAN, polybutadiene-acrylic acid-acrylonitrile terpolymer, as the polymeric fuel binder. A small amount of iron oxide is added to increase the burning rate. The final product is a rubbery material not unlike a typewriter eraser.
SRC Science Research Council (UK).
SSO Sun-Synchronous Orbit.
SSPO Sun-Synchronous Polar Orbit.
Stage An independently powered section of a rocket or spacecraft, often combined with others to form multistage vehicles.
Star A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.
Static firing The firing of a rocket on a special test stand to measure thrust, etc.
Stratosphere A division of the Earth's atmosphere extending from altitudes ranging 5-10 miles to 18-30 miles.
Subatomic particles Fundamental components of matter such as electrons or protons.
Subcarrier Modulation applied to a carrier which is itself modulated with information-carrying variations.
Sublimator An exposed metal plate, located on the outside of a spacesuit, that functions as a cooling coil to control suit temperatures.
Sub-orbital Not attaining orbit, i.e. a ballistic space shot.
Sub-satellite A secondary object released from a parent satellite in orbit, e.g. an electronic "ferret" released by a reconnaissance satellite.
Sunspot cycle The recurring, eleven-year rise and fall in the number of sunspots.
Sunspots Dark regions on the Sun which are the centers of large vortices and possess powerful magnetic fields. Maximum sunspot activity occurs in cycles with a period of about 11 years.
Sun sychronous orbit A walking orbit whose orbital plan precesses with the same period as the planet's solar orbital period. In such an orbit, a satellite crosses periapsis at about the same local time every orbit.
Superior conjunction Alignment between Earth and a planet on the far side of the Sun.
Superior planets Planets whose orbits are farther from the Sun than Earth's, i.e. Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Also called outer planets.
Supernova A large dying star, the final collapse of which is a cataclysmic explosion, hurling its substance into space.
Surface penetrator A probe designed to penetrate the surface of a body, surviving an impact of hundreds of g's, measuring and telemetering the properties of the penetrated surface.
Surface rover A semi-autonomous roving vehicle deployed on the surface of a planet or other body, taking images and soil analyses for telemetering back to Earth.
Sustainer engine An engine that maintains propulsion of a launch vehicle once it has discarded its boosters.
Synthetic aperture radar A radar imaging instrument which provides a penetrating illumination of radio waves, and is capable of imaging surfaces covered by clouds and haze. SAR images are constructed of a matrix where lines of constant distance or range intersect with lines of constant Doppler shift.

Index

Tape recorder A mechanical device for recording digital information on magnetic tape and for playing back the recorded material.
TCM Trajectory Corrective Maneuver.
TEI Trans-Earth Injection.
Telecommunication Any process of communication over considerable distance.
Telemetry The system for radioing information, including instrument readings and recordings, from a space vehicle to the ground.
Terrestrial Of or pertaining to the Earth.
Terrestrial planet Any of the four planets closest to the Sun: Mercury, Venus, Earth, Mars
Thermal energy Energy in the form of heat.
Thermal tile Silica fiber insulation used to protect 70% of the exterior of the Space Shuttle orbiter against reentry temperatures of up to 1430oC. Surface heat dissipates so rapidly that an uncoated tile can be held by its edges with the bare hand while its interior glows red hot.
Thermosphere The Earth atmosphere between 120 and 250 to 400 km (depending on the solar and geomagnetic activity levels), where temperature has an exponential increase up to a limiting value Texo at the thermopause. The temperature Texo is called the exospheric temperature.
Three-axis stabilization Stabilization accomplished by nudging a spacecraft back and forth within a deadband of allowed attitude error, using small thrusters or reaction wheels.
Three-way Coherent communications mode wherein a DSS receives a downlink whose frequency is based upon the frequency of an uplink provided by another DSS.
Throat That part of a rocket engine between the combustion chamber and nozzle.
Throttle To decrease the supply of propellant to an engine, reducing thrust. Liquid propellant rocket engines can be throttled; solid rocket motors cannot.
Thrust The force that propels a rocket or spacecraft measured in pounds, kilograms or Newtons. Thrust is generated by a high-speed jet of gases discharging through a nozzle.
Thrust chamber The area in a propulsion rocket in which force accumulates before ejection, e.g. the reaction chamber.
Thruster Rocket engines used for maneuvering spacecraft in space.
Thrust vector control Control of the thrust vector direction to steer a rocket or spacecraft during powered flight. Thrust vector control is most often achieved by hydraulically gimbaled engines.
Time of periapsis passage The time in which a planet or satellite moves through its point of periapsis.
TLI Trans-Lunar Injection.
TNT Trinitrotoluene, a high explosive.
Tonne Metric ton, a unit of mass equal to 1,000 kilograms (2,205 pounds).
Torus Solid geometrical figure with the shape of a doughnut or innertube.
Tracking The science of monitoring satellite locations by means of radio antennas at ground stations or by using other satellite systems in space.
Tracking station A station set up to track an object through the atmosphere or space, usually by means of radar or radio.
Trailing side For a satellite that keeps the same face toward the planet, the hemisphere that faces backwards, away from the direction of motion.
Trajectory The flight path of a projectile, missile, rocket or satellite.
Transducer Device for changing one kind of energy into another, typically from heat, position, or pressure into a varying electrical voltage or vice-versa, such as a microphone or speaker.
Trans-Earth injection The firing of a spacecraft's engines to increase speed and break out of a orbit around the Moon, or another planet, and begin it on a trajectory to Earth.
Trans-Lunar injection The firing of a spacecraft's engines to increase speed and break out of a parking orbit around Earth and begin it on a trajectory to the Moon.
Trans-Neptunian object A small body orbiting the Sun beyond Neptune in a region known as the Kuiper belt.
Transmitter An electronic device that generates and amplifies a carrier wave, modulates it with a meaningful signal, and radiates the resulting signal from an antenna.
Transponder A device that transmits a response signal automatically when activated by an incoming signal.
Trojan relay system A method of ensuring uninterrupted radio contact with the surface of any planet in the Solar System at any time first proposed by James Strong in 1967. Two radio satellites, keeping station along the Earth orbit, 60o ahead and 60o behind the Earth, transmit/receive signals from a similar pair of relay satellites at the Trojan equilaterals of another planet. Radio communications via these satellite links, from surface to surface, then becomes possible day and night, despite planetary rotation or orbital displacement. It could be used, for example, in steering a remotely-controlled vehicle on the surface of Mars.
Tropopause The level separating the troposphere and the stratosphere, occurring at an altitude of 5-10 miles.
Troposphere A division of the Earth's atmosphere extending from ground level to altitudes ranging 5-10 miles.
True anomaly The angular distance of a point in an orbit past the point of periapsis, measured in degrees.
TT&C Tracking, Telemetry and Command.
Turbopump A pump driven by a gas turbine, generally used to pump propellant into a combustion chamber.
TVC Thrust Vector Control.
Two-way Communications mode consisting of downlink received from a spacecraft while uplink is being received at the spacecraft.

Index

UDMH Unsymmetrical Dimethyl Hydrazine. A liquid hypergolic fuel.
UHF Ultrahigh Frequency.
Ullage The amount by which a container, such as a tank, falls short of being full.
Ultrahigh frequency Short radio waves used for communicating with spacecraft.
Ultraviolet A band of electromagnetic radiation with a higher frequency and shorter wavelength than visible blue light. Ultraviolet astronomy is generally performed in space, since Earth's atmosphere absorbs most ultraviolet radiation.
Umbilical A cable conveying power to a rocket or spacecraft before liftoff. Also a tethering or supply line for an astronaut outside a spacecraft.
Universal time The mean solar time of the meridian of Greenwich, England. Formerly called Greenwich mean time.
Universal time coordinated The world-wide scientific standard of timekeeping; based upon carefully maintained atomic clocks and accurate to within microseconds. The addition or subtraction of leap seconds, as necessary, keeps it in step with Earth's rotation. Its reference point is Greenwich, England; when it is midnight there, it is midnight UTC.
Universe All matter and energy, including Earth, the galaxies and all therein, and the contents of intergalactic space, regarded as a whole.
Uplink The radio signal transmitted to a spacecraft from Earth.
Uranus Seventh planet from the Sun, a gas giant or Jovian planet.
UT Universal Time.
UTC United Technologies Corporation; Chemical Systems Division (USA).
UTC Universal Time Coordinated.
UV Ultraviolet.

Index

Van Allen radiation belts Two doughnut-shaped zones of radiation about the Earth, concentrated at altitudes of 3,000 and 10,000 miles; named after James A. Van Allen who instrumented the satellite Explorer I. The belts contain charged particles generated by solar flares and trapped by the Earth's magnetic field.
Vector A quantity that is specified by magnitude, direction and sense.
Velocity trim See orbit trim maneuver.
Velocity vector Magnitude of speed plus direction.
Venus Second planet from the Sun, a terrestrial planet.
Vernier Rocket engine of small thrust used for fine adjustments in velocity and trajectory.
VfR Verein fur Raumschiffahrt e. V. (Germany).
VHF Very High Frequency.
Vidicon An imaging device consisting of a vacuum tube, in which an electron beam is swept across a phosphor coating on the glass where the image is focused, and its electrical potential varies in proportion to the level of light it encounters. This varying potential becomes the basis of the video signal produced.
VLBI Very Long Baseline Interferometer.
Volcano An opening in a planet's crust that allows magma to reach the surface.
Volcanism Volcanic force or activity.

Index

Walking orbit An orbit in which gravitational influences are used to induce a precession in a satellite's orbital plane.
Wavelength The distance from crest to crest, or trough to trough, of an electromagnetic or other wave. Wavelengths are related to frequency: The longer the wavelength, the lower the frequency.
Weight The force acting on a body in a gravitational field, equal to the product of its mass and the acceleration of the body produced by the field.
Weightlessness A state experienced in a ballistic trajectory (i.e. in orbit or free fall) when, because the gravitational attraction is opposed by equal and opposite inertial forces, a body experiences no mechanical stress.

Index

X-axis See roll.
X-band A range of microwave radio frequencies in the neighbourhood of 8 to 12 GHz.
X-rays A band of electromagnetic radiation intermediate in wavelength between ultraviolet radiation and gamma rays. Because x-rays are absorbed by the atmosphere, x-ray astronomy is performed in space.

Index

Yaw The rotation of a vehicle about its vertical (Z) axis, i.e. movement in azimuth.
Y-axis See pitch.

Index

Z-axis See yaw.
Zenith The point on the celestial sphere directly above the observer. Opposite the nadir.
Zero gravity A condition in which gravity appears to be absent. Zero gravity occurs when gravitational forces are balanced by the acceleration of a body in orbit or free fall.
Zero lift trajectory A trajectory in which the control system acts to maintain a condition of no aerodynamic lift on the rocket.

Index


Compiled by Dave Culley.

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