Glossary of Rocket and Space terminology

A ...

 * 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 that 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.

B ...

 * 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.

C ...

 * Calibration: Setting a 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 an 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.

D ...

 * Dark matter: A form of matter which has not been directly observed but whose existence has been deduced by its 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 current 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.

E ...

 * 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.
 * EELV: Evolved Expendable Launch Vehicle.
 * 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.

F ...

 * 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.
 * Fusion: The release of nuclear energy through the uniting of atoms.
 * FY: Fiscal Year.

G ...

 * g: The symbol for the acceleration of a freely moving body due to gravity at the surface of the Earth. 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 geosynchronous orbit with an inclination of zero degrees. A spacecraft in such an orbit appears to remain fixed above one particular point on the Earth's equator.
 * Geostationary transfer orbit: An elliptical orbit used to transfer a space vehicle from low earth orbit to geostationary orbit.
 * Geosynchronous orbit: A prograde, circular, low inclination orbit about Earth having a period of 23 hours 56 hours 4 seconds. A spacecraft in such an orbit appears to remain above Earth at a constant longitude, although it may seem to wander north and south.
 * 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 rubber-like 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: 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.

H ...

 * 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.
 * HEO: Highly Elliptical Orbit.
 * 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-terminator 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 MMH 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.

I ...
where the flux is constant over all directions.

Isp: Specific Impulse. Also abbreviated SI.
 * ISRO: Indian Space Research Organization. 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).
 * ICO: Intermediate Circular Orbit.
 * 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 extend from about 38 to 400 miles above the Earth's surface.
 * IR: Infrared.
 * IRBM: Intermediate Range Ballistic Missile (range 2,400-5,500 km).
 * IRFNA: Inhibited Red Fuming Nitric Acid; RFNA + 0.6% HF as corrosion inhibitor.
 * ISAS: Institute of Space and Aeronautical Science; University of Tokyo (Japan).
 * Isotropic: A property of a distribution of particles
 * IWFNA: Inhibited White Fuming Nitric Acid; WFNA + 0.6% HF as corrosion inhibitor.

J ...

 * Jansky: Unit used to express flux. 1 Jansky = 10-26 W m-2 Hz-1 Bandwidth.
 * JAXA: Japan Aerospace Exploration Agency (formed in 2003 by merger of ISAS, NAL and NASDA).
 * 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: Jet Propulsion Laboratory (Pasadena, California).
 * JSC: Johnson Space Center (Houston, Texas).
 * Jupiter: Fifth planet from the Sun, a gas giant or Jovian planet.

K ...

 * 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 (-459.69oF).
 * 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.

L ...

 * 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 neighborhood 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 Earth orbit: An orbit in the region of space extending from the Earth's surface to an altitude of 2,000 kilometers. Given the rapid orbital decay of objects close to Earth, the commonly accepted definition is between 160-2,000 km above the Earth's surface.
 * 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 descent 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.

M ...

 * 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 fraction: Mass of a component divided by the total mass of all components in the system.
 * 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 Earth Orbit: An orbit in the region of space above low Earth orbit (2,000 kilometers) and below geosynchronous orbit (35,786 kilometers). Sometimes called Intermediate 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.
 * MEO: Medium Earth Orbit.
 * 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 micrometeoroid 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, CH3NHNH2. 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 (range 800-2,400 km).
 * MSFC: 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.

N ...

 * N2O4: Nitrogen Tetroxide. Also abbreviated NTO.
 * Nadir: The direction from a spacecraft directly down toward the center of a planet. Opposite the zenith.
 * NAL: National Aerospace Laboratory of Japan.
 * NASA: National Aeronautics and Space Administration.
 * 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.

O ...

 * 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.

P ...

 * 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 polymeric 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 gas-like 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.

Q ...

 * 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.

R ...

 * 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.
 * RFNA: Red Fuming Nitric Acid; 85% HNO3 + < 5% H2O + 6-15% NOX
 * 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 it 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.

S ...

 * 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.
 * SRBM: Short Range Ballistic Missile (range <800 km).
 * 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 synchronous 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.

T ...

 * 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 an 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.

U ...

 * UDMH: Unsymmetrical Dimethyl Hydrazine, (CH3)2NNH2. A liquid hypergolic fuel.
 * UHF: Ultrahigh Frequency.
 * Ullage: The amount by which a container, such as a tank, falls short of being full.
 * Ullage maneuver: To supply positive acceleration to seat propellant in the bottom of its tanks.
 * 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.

V ...

 * 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.

W ...

 * 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.
 * WFNA: White Fuming Nitric Acid; 97.5% HNO3 + 2% H2O + < 0.5% NOX

X ...

 * X-axis: See roll.
 * X-band : A range of microwave radio frequencies in the neighborhood 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.

Y ...

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

Z ...

 * 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.