Glossary of Key Terms

Space Terminology

As you are researching this topic you are likely to come across words that are new to you.  Below we have defined key terms you may come across as you begin to read about this topic.

Term Definition
Antimatter 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.
Astronomical Unit The mean distance of Earth from the Sun, i.e. 92,955,807 miles (149,597,870 km).
Cislunar Relating to the space between the Earth and the orbit of the Moon.
Cosmic year The time it takes the Sun to revolve around the center of the galaxy, approximately 225 million years.
Dark Matter A form of matter which has not been directly observed but whose existence has been deduced by it’s gravitational effects.
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.
Fission The release of energy through splitting atoms.
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.
Geocentric Earth centered.
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).
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.
Heliocentric Centered on the Sun. 
Heliosphere The space within the boundary of the heliopause, containing the Sun and solar system.
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.
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.
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).
Hydrosphere The water on or around the surface of a planet.
Inferior conjuction 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.
Jovian Planet Any of the four biggest planets: Jupiter, Saturn, Uranus, and Neptune.
Kelvin Scale of temperature named after the English physicist Lord Kelvin, based on the average kinetic energy per molecule of a perfect gas. Absolute zero is equivalent to -273.16oC (-459.4oF).
Kinetic Energy An object’s energy of motion; for example, the force of a falling body.
Lander Spacecraft A spacecraft designed to reach the surface of a planet or moon and survive long enough to telemeter data back to Earth.
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).
Lithosphere The crust of a planet.
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-Gain Antenna An omni-directional spacecraft antenna that provides relatively low data rates at close range, several AU for example.
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.
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 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.
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.
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.
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.
Meridian Great circle that passes through both the north and south poles, also called line of longitude.
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 Protection Shielding used to protect spacecraft components from micrometeroid impacts. Interplanetary spacecraft typically use tough blankets of Kevlar or other strong fabrics to absorb the energy from high-velocity particles.
Minor Planet An asteroid.
Moon A small natural body which orbits a larger one. A natural satellite.
Multistage Rocket A rocket having two or more stages which operate in succession each being discarded as its job is done.
Omni directional Capable of transmitting or receiving signals in all directions, as an antenna.
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.
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.
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.
Pegasus A rocket-vehicle concept for transportation of commercial high-priority freight or 172 passengers.
Photosphere The visible surface of the sun.
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.
Plasma A gas-like association of ionized particles that responds collectively to electric and magnetic fields.
Plasmasphere The region of the atmosphere consisting of cold dense plasma originating in the ionosphere and trapped by the Earth’s magnetic field.
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.
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.
Satellite A body, natural or artificial, in orbit around a planet. The term is used most often to describe moons and spacecraft.
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.
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.
Stratosphere A division of the Earth’s atmosphere extending from altitudes ranging 5-10 miles to 18-30 miles.
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.
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.
Trotopause 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.
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.
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.
Vector A quantity that is specified by magnitude, direction and sense.
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.


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