Glossary of Space Terms and Acronyms
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
Acceleration -- Rate of change of velocity.
Å -- Angstrom (0.0001 micrometre, 0.1 nanometre, 10-10metre).
ACS -- Attitude Control System onboard a spacecraft, providing the basis for navigation and proper solar panel and antenna positioning.
AC -- Alternating Current.
ALT -- Altitude.
AM -- Ante meridiem - Latin: before midday, morning.
Amor -- A class of Earth-crossing asteroids with Mars-crossing orbits.
Angular momentum -- Angular momentum intuitively measures how much the linear momentum is directed around a certain point called the origin.
Aphelion -- Apoapsis in solar orbit.
Apoapsis -- The farthest point in an elliptical orbit from the body being orbited.
Apogee -- Apoapsis in Earth orbit.
Apojove -- Apoapsis in Jupiter orbit.
Apollo -- A class of Earth-crossing asteroids.
Apolune -- Apoapsis in lunar orbit.
Apselene -- Apoapsis in lunar orbit.
Arc second -- A unit of angular measurement which comprises 1/3600 of a degree of arc.
Ascending node -- The point at which an orbit crosses the ecliptic plane going north.
Asteroids -- Small bodies composed of rock and metal in orbit about the Sun.
Aten -- A class of Earth-crossing asteroids.
Atmospheric refraction -- The direction of light as it passes through the Earth's atmosphere is changed due to the varying density of the atmosphere. This is atmospheric refraction.
AU -- Astronomical Unit, the mean Earth-to-Sun distance, 149,597,870 km. Used to indicate distances within the solar system.
Barycentre -- The common centre of mass about which two or more bodies revolve. The barycentre of the Earth and Moon is well within the radius of the Earth.
Beacon -- Downlink from a spacecraft that immediately indicates the state of the spacecraft.
Billion -- 109, i.e. a thousand million.
BPS -- Bits Per Second.
c -- The speed of light, 299 792 km per second.
C-band -- A portion of electromagnetic spectrum in the microwave range of frequencies ranging from 4 to 6 GHz. C band is primarily used for satellite communications.
Carrier -- The main frequency of a radio signal generated by a transmitter prior to application of any modulation.
Cassegrain -- Reflecting scheme in antennas and telescopes. The primary mirror contains a hole, light from the primary mirror is reflected by the secondary mirror through the hole in the primary to reach the eyepiece, placed behind the telescope.
CCD -- Charge Coupled Device, a solid-state imaging detector. Used in digital photography and astronomy.
C&DH -- Command and Data Handling subsystem on board a spacecraft.
Celestial equator -- Projection of the Earth's equator onto the celestial sphere.
Celestial sphere -- An imaginary rotating sphere of "very large radius", concentric with the Earth. All objects in the sky can be thought of (it is simply a convenient fiction) as lying on the sphere.
Centrifugal force -- The outward-tending apparent force of a body revolving around another body.
Centripetal force -- A force pulling an object toward the centre of a circular path as the object goes around the circle. An object can travel in a circle only if there is a centripetal force acting on it.
Cherenkov radiation -- Electromagnetic radiation emitted by a charged particle when it passes through a medium at a speed greater than that of light in the medium.
CNES -- Centre National dÉtudes Spatiales, the space agency of France. Its headquarters are located in central Paris.
Conjunction -- A configuration in which two celestial bodies have their least apparent separation on the sky.
Coma -- The cloud of diffuse material surrounding the nucleus of a comet.
Comet -- One of a family of small bodies composed of ice and rock in various orbits about the sun.
dB -- See Decibel.
DC -- Direct Current.
DEC -- See Declination.
Decibel -- A dimensionless unit that is a logarithmic measure of power ratio.
Declination -- One of two coordinates in the equatorial coordinate system used in astronomy. Measured Southwards in degrees from the South Celestial Pole.
Descending node -- The point at which an orbit crosses the ecliptic plane going south.
Doppler effect -- The effect on frequency imposed by relative motion between transmitter and receiver.
Downlink -- Signal received from a spacecraft.
DSN -- Deep Space Network, NASA's worldwide spacecraft tracking facility managed and operated by JPL.
DSS -- Deep Space Station, one of the stations of the DSN. The Hartebeesthoek Radio Observatory had its origin as Deep Space Station 51.
Eccentricity -- The distance between the foci of an ellipse divided by the major axis.
Ecliptic -- The plane in which Earth orbits the Sun and in which solar and lunar eclipses occur.
Electromagnetic radiation -- A combination of electric and magnetic fields oscillating perpendicular to each other, moving through space as a wave, effectively transporting energy and momentum.
Electromagnetic spectrum -- The electromagnetic spectrum encompasses all possible wavelengths of electromagnetic radiation.
Ellipse -- A closed plane curve generated in such a way that the sums of its distances from the two fixed points (the foci) is constant.
ELV -- Expendable Launch Vehicle.
EM -- Electromagnetic.
EMR -- Electromagnetic Radiation.
Equator -- An imaginary circle around a body which is everywhere equidistant from the poles, defining the boundary between the Northern and Southern hemispheres.
Equinox -- The equinoxes are times at which the centre of the Sun is directly above the Earth's equator. The day and night would be of equal length at that time, if the Sun were a point and not a disc, and if there were no atmospheric refraction. Given the apparent disc of the Sun, and the Earth's atmospheric refraction, day and night actually become equal at a point within a few days of each equinox.
ESA -- European Space Agency.
eV -- Electron volt, a measure of the energy of subatomic particles.
Extra-Solar Planet -- A planet orbiting a star other than the Sun.
Fluorescence -- The phenomenon of emitting light at one wavelength upon absorbing radiation of another wavelength.
FM -- Frequency modulation.
Galaxy -- One of billions of systems, each composed of numerous stars, nebulae, and dust.
Galilean satellites -- The four large satellites of Jupiter so named because Galileo discovered them when he turned his telescope toward Jupiter: Io, Europa, Ganymede, and Callisto.
Gamma rays -- Very high energy electromagnetic radiation in the neighbourhood of 10-13m wavelength.
GEO -- Geosynchronous Earth Orbit.
Geostationary orbit -- A geosynchronous orbit in which the spacecraft is constrained to a constant latitude.
Geosynchronous orbit -- A direct, circular, low inclination orbit about the Earth having a period of 23 hours 56 minutes 4 seconds.
GeV -- Gigaelectronvolt, 109 electronvolts.
GHz -- Gigahertz (109 Hz).
GLONASS -- Glonass Global Orbiting Navigation Satellite System, the Russian counterpart to the United States' GPS system
GMT -- Greenwich Mean Time. Obsolete. UT, Universal Time is preferred.
GNSS -- Global Navigation Satellite Service.
GPS -- Global Positioning System, an American satellite navigation system.
Gravitation -- The mutual attraction of all masses in the Universe.
Gravity assist -- Technique whereby a spacecraft takes angular momentum from a planet's solar orbit (or a satellite's orbit) to accelerate the spacecraft, or the reverse.
Great circle -- An imaginary circle on the surface of a sphere whose centre is at the centre of the sphere. If the Earth is treated as a sphere then the lines of longitude as well as the equator are all great circles.
GTO -- Geostationary (or geosynchronous) Transfer Orbit.
HartRAO -- The Hartebeesthoek Radio Astronomy Observatory.
Heliocentric -- Sun-centred.
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.
H.E.S.S. -- High Energy Stereoscopic System, a system of Imaging Atmospheric Cherenkov Telescopes for the investigation of cosmic gammma rays in the 100GeV energy range.
HMO -- Hermanus Magnetic Observatory.
HGA -- High-Gain Antenna onboard a spacecraft.
Hohmann Transfer Orbit -- Interplanetary trajectory using the least amount of propulsive energy.
Hour Angle -- A measure of the amount of time before (-) or since (+) a star crosses the meridian. In effect, HA represents the right ascension for a particular location and time of day.
Hz -- Hertz, cycles per second.
Inclination -- The angular distance of the orbital plane from the plane of the planet's equator, stated in degrees.
Inferior planet -- Planet which orbits closer to the Sun than the Earth does.
Inferior conjunction -- Alignment of Earth, Sun, and an inferior planet on the same side of the Sun.
Ion -- A charged particle consisting of an atom which has been stripped of one or more of its electrons.
IR -- Infrared, meaning "below red" radiation. Electromagnetic radiation in the neighbourhood of 100 micrometers wavelength.
IRAS -- Infrared Astronomical Satellite.
ISM -- Interstellar Medium.
Isotropic -- Having uniform properties in all directions.
Jovian -- Jupiter-like planets, the gas giants Jupiter, Saturn, Uranus, and Neptune.
JPL -- Jet Propulsion Laboratory, operating division of the California Institute of Technology.
Jupiter -- Fifth planet from the sun, a gas giant or Jovian planet.
K-band -- A range of frequencies in the neighbourhood of 12 to 40 GHz.
kHz -- kilohertz, a thousand hertz.
Kuiper belt -- A disk-shaped region about 30 to 100 AU from the Sun considered to be the source of the short-period comets.
Lagrange points --Five points with respect to an orbit which a body can stably occupy. Designated L1 through L5.
Large Magellanic Cloud -- LMC, the larger of two small satellite galaxies orbiting nearby our Milky Way galaxy, which are visible from the Southern hemisphere.
Latitude -- Circles in parallel planes to that of the equator defining North-South measurements, also called parallels.
L-band -- A range of frequencies in the neighbourhood of 1 to 2 GHz.
LEO -- Low Equatorial Orbit.
Light speed -- 299,792 km per second, the constant c.
Light time -- The amount of time it takes light or radio signals to travel a certain distance at light speed.
Light year -- A measure of distance, the distance light travels in one year, about 63,197 AU.
LLR -- Lunar Laser Ranging.
LMC -- Large Magellanic Cloud.
Longitude -- Great circles that pass through both the North and South poles, also called meridians.
LOX -- Liquid Oxygen.
Major axis -- The maximum diameter of an ellipse. The line passing through the foci, centre, and vertices.
Mars -- Fourth planet from the Sun, a terrestrial planet.
Mean solar time -- Time based on an average of the variations caused by Earth's non-circular orbit. The 24-hour day is based on mean solar time. Mercury -- First planet from the Sun, a terrestrial planet. Meridians -- Great circles that pass through both the North and South poles,
also called lines of longitude. The great circle passing through the local zenith is called the local meridian. Meteor -- A meteoroid which is in the process of entering Earth's atmosphere. It is called a meteorite after landing. Meteorite -- Rocky or metallic material which has fallen to Earth or to another planet. Meteoroid -- Small bodies in orbit about the Sun which are candidates for falling to Earth or to another planet. MHz -- Megahertz (106 Hz), a million hertz. Microwave -- Electromagnetic waves with wavelengths longer than those of infrared radiation, but shorter
than those of radio waves. Milky Way -- The galaxy which includes the Sun and Earth. Modulation -- Change of an electric signal based on the change in another signal. Moon -- A small natural body which orbits a larger one. A natural satellite. Capitalised, the Earth's natural satellite. Nadir -- The, obviously invisible, point on the sky vertically downward. Opposite the zenith. NASA -- National Aeronautics and Space Administration, the space agency of the United States. NASSP -- National Astrophysics and Space Science Programme, an initiative to produce the next generation of astrophysicists in Africa. Nebula -- An interstellar cloud of dust and gas. Neptune -- Eighth planet from the Sun, a gas giant or Jovian planet. Nodes -- Points where an orbit crosses a plane. NRF -- National Research Foundation. Nucleus -- The central body of a comet. Nutation -- A small nodding motion in the axis of a rotating body.
Earth's nutation has a period of 18.6 years and an amplitude of 9.2 arc seconds. Oort cloud -- A large number of comets theorised to orbit the Sun in the neighbourhood of 50,000 AU. Opposition -- Configuration in which one celestial body is opposite another in the sky. A planet is in opposition when it is 180
degrees away from the Sun as viewed from another planet (such as Earth). For example, Saturn is at opposition when it is directly overhead at midnight on Earth. Periapsis -- The point in an orbit closest to the body being orbited. Perigee -- Periapsis in Earth orbit. Perihelion -- Periapsis in solar orbit. Perijove -- Periapsis in Jupiter orbit. Perilune -- Periapsis in lunar orbit. Periselene -- Periapsis in lunar orbit. Phase -- The phase of a waveform is the position of any peak or trough compared to the same feature on a second waveform. Phase -- The particular appearance of a body's state of illumination, such as the full or crescent phases of the Moon. Photoelectric effect -- Ejection of electrons from a surface (usually metallic) upon exposure to electromagnetic radiation that is
above a certain threshold frequency (particular to the type of surface from which the electrons are ejected). Photovoltaic -- Materials that use the photoelectric effect to convert light into electric current. Picometre -- 10-12 of a metre. Picosecond -- 10-12 of a second. Plasma -- Electrically conductive fourth state of matter (other than solid, liquid, or gas), consisting of ions and electrons. Pluto -- Ninth planet from the Sun, sometimes classified as a small terrestrial planet. PM -- Post meridiem (Latin: after midday), afternoon. Primary mirror -- The principle light-gathering surface of a reflecting telescope. Prograde -- Orbit in which the spacecraft moves in the same direction as the planet rotates. See retrograde. Quasar -- Quasi-stellar object observed mainly in radio waves. Quasars are extragalactic objects believed to be the very distant
centers of active galaxies. Radian -- Unit of angular measurement equal to the angle at the centre of a circle subtended by an arc equal in length to the radius. Equals about 57.296 degrees. Radio waves -- The longer wavelength portion of the electromagnetic spectrum in which
electromagnetic waves can be generated by alternating current fed to an antenna. Red dwarf -- A small star, on the order of 100 times the mass of Jupiter. Reflection -- The deflection or bouncing of electromagnetic waves when they encounter a surface. Refraction -- The deflection or bending of electromagnetic waves when they pass from one kind of transparent medium into another. Retrograde -- Orbit in which the spacecraft moves in the opposite direction from the planet's rotatation. See prograde. RF -- Radio Frequency. RFI -- Radio Frequency Interference. Right Ascension -- The angular distance of a celestial object measured in hours, minutes, and seconds along the celestial equator
Eastward from the vernal equinox. RTG -- Radioisotope Thermo-Electric Generator onboard a spacecraft. SAAO -- South African Astronomical Observatory. SALT -- Southern African Large Telescope, the largest single telescope in the Southern hemisphere. SAR -- Synthetic Aperture Radar Satellite -- A small body which orbits a larger one. A natural or an artificial moon. Earth-orbiting spacecraft are called satellites.
While deep-space vehicles are technically satellites of the Sun or of another planet, or of the galactic centre, they are generally called spacecraft
instead of satellites. Saturn -- Sixth planet from the Sun, a gas giant or Jovian planet. S-band -- Part of the electromagnetic spectrum ranging from 1.5 to 5 GHz. Used in communications satellites. Secondary mirror -- A second light gathering and focusing surface in a reflecting telescope. Semi-major axis -- Half the distance of an ellipse's maximum diameter, the distance from the centre of the ellipse to one end. Sidereal time -- Time relative to the stars other than the Sun. SKA -- Square Kilometre Array, an international radio telescope for the 21st century. SMC -- Small Magellanic Cloud, the smaller of two small galaxies orbiting nearby our Milky Way galaxy, which are visible from the
Southern hemisphere. SA -- Solar Array, photovoltaic panels onboard a spacecraft. SLR -- Satellite Laser Ranging. Solar system -- The Sun and all the objects that orbit around it, including asteroids, moons, comets, and planets. Solar wind -- Flow of lightweight ions and electrons (which together comprise plasma) thrown from the Sun. SCP -- South Celestial Pole, projection of the Earth's South pole onto the celestial sphere. Specific Impulse -- A measurement of a rocket's relative performance. Expressed in seconds, the number of which a rocket can produce one pound
of thrust from one pound of fuel. The higher the specific impulse, the less fuel required to produce a given amount of thrust. SunSpace -- A South African company providing high-performance satellite systems. SUNSAT -- First South African satellite, launched in 1999. Sun synchronous orbit -- A spacecraft orbit that precesses, wherein the location of periapsis changes with respect to the planet's surface
so as to keep the periapsis location near the same local time on the planet each orbit. Superior planet -- Planet which orbits farther from the Sun than Earth's orbit. Superior conjunction -- Alignment between Earth and a planet on the far side of the Sun. Terrestrial planet -- One of the four inner Earth-like planets. True anomaly -- The angular distance of a point in an orbit past the point of periapsis, measured in degrees. UHF -- Ultra-high frequency (around 300MHz). Uplink -- Signal sent to a spacecraft. Uranus -- Seventh planet from the sun, a gas giant or Jovian planet. UT -- Universal Time, also called Zulu (Z) time, previously Greenwich Mean Time. UT is based on the imaginary "mean sun", which averages
out the effects on the length of the solar day caused by Earth's slightly non-circular orbit about the sun. UT is not updated with leap seconds as is UTC. UTC -- Coordinated Universal Time, the world-wide scientific standard of timekeeping. It is
based upon carefully maintained atomic clocks and is highly stable. Its rate does not change by more than about 100 picoseconds per day. The addition or
subtraction of leap seconds, as necessary, at two opportunities every year adjusts UTC for irregularities in Earth's rotation. UV -- Ultraviolet (meaning "above violet") radiation. Electromagnetic radiation in the neighbourhood of 10-7m wavelength. Velocity -- A vector quantity whose magnitude is a body's speed and whose direction is the body's direction of motion. Venus -- Second planet from the Sun, a terrestrial planet. Vernal equinox -- The equinox at the beginning of Spring in the Northern hemisphere. VLBI -- Very Long Baseline Interferometry System. A technique used in radio astronomy to synthesize a large aperture. W -- Watt, a measure of electrical power equal to potential in volts times current in amps. Wavelength -- The distance between repeating units of a wave pattern. X-ray -- Electromagnetic radiation in the neighbourhood of 100 picometre wavelength. Zenith -- The point on the celestial sphere (i.e.the sky) directly above the observer. Opposite the nadir.
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