Reference
Scientific glossary
Key terms from astronomy, astrophysics and spaceflight engineering, with links to the tools that bring them to life.
72 terms shown
- Albedo
- The fraction of incoming light a surface reflects. Bond albedo counts light reflected in all directions and at all wavelengths; Earth's is about 0.3.
- Aphelion
- The point in an orbit around the Sun that is farthest from the Sun. Earth reaches aphelion in early July.
- Apoapsis
- The farthest point of any orbit from the body being orbited (apogee for Earth, aphelion for the Sun).
- Asteroid
- A small rocky or metallic body orbiting the Sun, most of them in the main belt between Mars and Jupiter.
- Astronomical unit (AU)
- Exactly 149,597,870,700 m, approximately the mean Earth–Sun distance.
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- Attitude control
- The systems that orient a spacecraft, using reaction wheels, thrusters, magnetorquers, gyroscopes and star trackers.
- Black hole
- A region where gravity is so strong that nothing, not even light, can escape from inside its event horizon.
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- Burn
- A firing of a spacecraft's engine to change its velocity. Prograde burns add speed along the direction of motion; retrograde burns remove it.
- Comet
- An icy body that releases gas and dust as it approaches the Sun, forming a coma and often one or more tails.
- Cosmic microwave background
- Faint microwave radiation filling the universe, released about 380,000 years after the Big Bang when the universe became transparent.
- Dark energy
- The unknown cause of the accelerating expansion of the universe, making up roughly 68% of its energy content.
- Dark matter
- Matter that does not emit or absorb light but reveals itself through gravity, roughly five times more abundant than ordinary matter.
- Declination
- The celestial equivalent of latitude: an object's angle north or south of the celestial equator.
- Deep Space Network (DSN)
- NASA's worldwide network of large radio antennas in California, Spain and Australia used to communicate with distant spacecraft.
- Delta-v (Δv)
- The change in velocity a manoeuvre requires or a spacecraft can deliver; the currency of mission design.
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- Doppler shift
- The change in observed wavelength caused by motion along the line of sight: blueshift for approach, redshift for recession.
- Dwarf planet
- A body that orbits the Sun and is massive enough to be round but has not cleared its orbital neighbourhood, such as Pluto, Ceres and Eris.
- Eccentricity
- A number describing an orbit's shape: 0 is a circle, values between 0 and 1 are ellipses, 1 is a parabola and above 1 a hyperbola.
- Ecliptic
- The plane of Earth's orbit around the Sun, and the Sun's apparent yearly path across the sky.
- Equilibrium temperature
- The temperature a planet would have if it re-radiated all absorbed starlight as a blackbody, ignoring any greenhouse effect.
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- Escape velocity
- The minimum speed an unpowered object needs to escape a body's gravity: vₑ = √(2GM/r).
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- Event horizon
- The boundary around a black hole beyond which escape is impossible.
- Galaxy
- A gravitationally bound system of stars, gas, dust and dark matter. The Milky Way contains a few hundred billion stars.
- Geostationary orbit
- A circular equatorial orbit at 35,786 km altitude whose period equals one sidereal day, so a satellite appears fixed over one point.
- Gravity assist
- Using a flyby of a planet to change a spacecraft's speed and direction relative to the Sun without using propellant.
- Habitable zone
- The range of distances from a star where a rocky planet with a suitable atmosphere could keep liquid water on its surface.
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- Heliopause
- The boundary where the solar wind meets the interstellar medium. Voyager 1 crossed it in 2012 and Voyager 2 in 2018.
- Hertzsprung–Russell diagram
- A plot of stellar luminosity against temperature that reveals the main sequence, giants and white dwarfs.
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- Inclination
- The tilt of an orbit's plane relative to a reference plane, such as Earth's equator or the ecliptic.
- Ion engine
- An electric thruster that accelerates ions with electric fields, giving very high specific impulse but very low thrust.
- Kármán line
- An altitude of 100 km commonly used as the boundary of space.
- Kepler's laws
- Three laws of planetary motion: elliptical orbits, equal areas in equal times, and T² ∝ a³.
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- Kuiper Belt
- A region of icy bodies beyond Neptune, from about 30 to 50 AU, including Pluto and Arrokoth.
- Lagrange point
- One of five positions in a two-body system where a small object can stay in balance with the two large bodies. Webb orbits near Sun–Earth L2.
- Light curve
- A graph of an object's brightness over time, used to find transiting planets, variable stars and supernovae.
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- Limb darkening
- The darkening of a star's disk towards its edge, because we see cooler, higher layers there.
- Low Earth orbit (LEO)
- Orbits up to about 2,000 km altitude. The ISS orbits at about 400–420 km.
- Magnetosphere
- The region around a planet dominated by its magnetic field, which deflects much of the solar wind.
- Magnitude
- A logarithmic brightness scale where a difference of 5 magnitudes is a factor of 100 and smaller numbers are brighter.
- Main sequence
- The long, stable phase in which a star fuses hydrogen into helium in its core.
- Mass ratio
- A rocket stage's initial (wet) mass divided by its final (dry) mass.
- Microgravity
- The near-weightless condition in orbit, where spacecraft and occupants are in continuous free fall.
- Nebula
- A cloud of gas and dust in space — a stellar nursery, the remains of a dying star, or a supernova remnant.
- Neutron star
- The collapsed core of a massive star, more massive than the Sun yet only about 20 km across.
- Oort Cloud
- A hypothesised spherical shell of icy bodies far beyond the Kuiper Belt, thought to be the source of long-period comets.
- Parallax
- The apparent shift in a nearby star's position as Earth orbits the Sun, used to measure stellar distances.
- Periapsis
- The closest point of an orbit to the body being orbited (perigee for Earth, perihelion for the Sun).
- Pulsar
- A rotating neutron star that sweeps beams of radiation past Earth, appearing to pulse with clock-like regularity.
- Radial velocity method
- Detecting planets by the Doppler wobble they induce in their star's spectrum.
- Reaction wheel
- A spinning flywheel whose speed changes rotate a spacecraft without propellant.
- Redshift
- The stretching of light to longer wavelengths, by motion away from us or by the expansion of the universe.
- RTG
- Radioisotope thermoelectric generator: converts heat from decaying plutonium-238 into electricity for missions far from the Sun.
- Semi-major axis
- Half the longest diameter of an elliptical orbit; it sets the orbit's period and energy.
- Sidereal day
- Earth's rotation period relative to the stars: 23 h 56 min 4 s.
- Solar wind
- A stream of charged particles flowing outward from the Sun's corona at hundreds of kilometres per second.
- Specific impulse (Isp)
- A measure of rocket efficiency: effective exhaust velocity divided by standard gravity, in seconds.
- Spectroscopy
- Splitting light into its wavelengths to measure composition, temperature and motion.
- Supernova
- A stellar explosion that can briefly outshine a galaxy, from either a collapsing massive star or a white dwarf exceeding its mass limit.
- Synodic period
- The time for two orbiting bodies to return to the same relative arrangement — about 26 months for Earth and Mars.
- Telemetry
- Data transmitted from a spacecraft about its health, status and measurements.
- Thrust-to-weight ratio
- Engine thrust divided by vehicle weight; it must exceed 1 for vertical lift-off.
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- Tidal locking
- When a body's rotation period matches its orbital period, so it always shows the same face to its partner, as the Moon does to Earth.
- Vis-viva equation
- v² = μ(2/r − 1/a): the speed of an orbiting body at any point on its orbit.
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- White dwarf
- The hot, dense remnant core of a Sun-like star, about the size of Earth.