The glossary

Abort modes The pre-planned ways a crewed launch can go wrong and still end with everyone alive. Launch & Flight Accretion disc The flat, ferociously hot spiral of matter falling onto a compact object. Black Holes Adaptive optics Bending a mirror hundreds of times a second to cancel out the atmosphere. Telescopes & Instruments Aerobraking Using a planet's atmosphere as a brake, over months, to save enormous amounts of fuel. The Solar System Aperture The diameter of the main mirror, which decides both how much light and how much detail. Telescopes & Instruments Apogee and perigee The high and low points of an orbit around Earth. Satellites & Orbit Asteroid and comet Rock that stays rock, and ice that grows a tail when it gets warm. Comets & Asteroids Astrometry Measuring exactly where things are, and how that position changes. Telescopes & Instruments Atmospheric escape Planets losing their air to space, sometimes visibly, sometimes entirely. Exoplanets & Life Binary star Two stars orbiting each other, which is at least as common as one on its own. Stars & Supernovae Biosignature A gas or feature in a planet's spectrum that life would explain and geology would struggle to. Exoplanets & Life Bolide An exceptionally bright fireball, usually one that explodes. Comets & Asteroids Booster recovery Flying the first stage back and landing it, so the expensive part is not thrown away. Launch & Flight Cepheid variable A star that pulses at a rate set by its true brightness, which is how the universe got measured. Stars & Supernovae Chandrasekhar limit 1.4 solar masses: the most a white dwarf can weigh before it cannot hold itself up. Stars & Supernovae Coma and tail A comet's atmosphere, and the two separate tails the Sun pushes off it. Comets & Asteroids Conjunction (satellites) Two objects predicted to pass close enough that somebody has to decide whether to move. Satellites & Orbit Coronagraph A mask inside the instrument that blocks a star so you can see what is next to it. Telescopes & Instruments Coronal hole An opening in the Sun's magnetic field that lets a fast stream of wind escape. The Sun & Space Weather Coronal mass ejection A billion tonnes of solar atmosphere thrown into space, and the cause of most serious aurorae. The Sun & Space Weather Cosmic microwave background The oldest light there is, released when the universe first became transparent. Galaxies & Cosmology Cryogenic propellant Fuel kept liquid only by being extremely cold, which starts a clock the moment you load it. Launch & Flight Cryovolcanism Volcanoes that erupt water and ammonia instead of molten rock. The Solar System CubeSat A satellite built to a 10-centimetre standard unit, which made orbit affordable to universities. Satellites & Orbit Dark energy The name for whatever is making the expansion of the universe speed up. Galaxies & Cosmology Dark matter Matter that has gravity but does not interact with light, and outweighs the visible sort five to one. Galaxies & Cosmology Delta-v The currency of spaceflight: how much you can change your velocity, in metres per second. Launch & Flight Deorbit burn The engine firing that lowers an orbit into the atmosphere on purpose. Satellites & Orbit Direct imaging Actually photographing a planet, by blocking out the star that drowns it. Exoplanets & Life Docking and berthing Two ways to attach a spacecraft to a station: it flies in, or it is caught and bolted on. Crewed Spaceflight Ergosphere The region around a spinning black hole where space itself is dragged round with it. Black Holes EVA (spacewalk) Working outside the spacecraft, in a suit that is itself a spacecraft. Crewed Spaceflight Event horizon The boundary around a black hole past which nothing gets out — not a surface, but a point of no return. Black Holes Expedition The numbered crew rotation aboard the space station, roughly six months at a time. Crewed Spaceflight Fast radio burst A millisecond flash of radio energy from another galaxy, cause still unsettled. Galaxies & Cosmology First light The night a new telescope is pointed at the sky and actually works. Telescopes & Instruments Galactic merger Two galaxies colliding, which takes a billion years and almost never breaks a star. Galaxies & Cosmology Geostationary orbit The one orbit where a satellite appears to hang motionless over a fixed point. Satellites & Orbit Gravitational lensing Mass bending light, turning galaxies into arcs and rings and acting as a free telescope. Black Holes Gravitational microlensing A planet betraying itself by briefly focusing the light of a star behind it. Exoplanets & Life Gravitational waves Ripples in spacetime itself, detected by measuring a distance change smaller than a proton. Galaxies & Cosmology Gravity assist Stealing a fraction of a planet's orbital momentum to go faster for free. The Solar System Habitable zone The orbital band where a planet could hold liquid water — no more than that. Exoplanets & Life Hawking radiation The faint glow that means black holes are not quite black, and eventually evaporate. Black Holes Heliosphere and heliopause The bubble the solar wind blows in interstellar space, and the wall where it stops. The Sun & Space Weather Hertzsprung–Russell diagram The single plot that organises everything about stars. Stars & Supernovae Hot Jupiter A gas giant orbiting closer to its star than Mercury does to the Sun. Exoplanets & Life Hubble tension Two good measurements of the universe's expansion rate that stubbornly disagree. Galaxies & Cosmology Hypergolic propellant Fuel and oxidiser that ignite on contact, with no ignition system at all. Launch & Flight Interferometry Combining separate telescopes so they resolve detail like one enormous dish. Telescopes & Instruments Interstellar object Something passing through the solar system that was never bound to the Sun. Comets & Asteroids Kessler syndrome The runaway scenario where collisions in orbit generate debris that causes more collisions. Satellites & Orbit Kp index A 0–9 scale of how disturbed Earth's magnetic field is right now. The Sun & Space Weather Kuiper belt The flat ring of icy bodies beyond Neptune, where Pluto lives and short-period comets come from. Comets & Asteroids Lagrange point Five places where two bodies' gravity lets a third sit still relative to both. Telescopes & Instruments Launch window The span of time in which a launch can reach the orbit it is aiming for. Launch & Flight Libration The Moon's slight rocking, which lets us see 59% of it rather than half. The Solar System Life support (ECLSS) The machinery that keeps the air breathable and the water drinkable, indefinitely. Crewed Spaceflight Light-year and parsec Two units for the same job, one intuitive and one that falls out of the measurement. Galaxies & Cosmology Low Earth orbit Everything from about 160 to 2,000 km up, where almost all spacecraft and all people are. Satellites & Orbit Magnetar A neutron star with a magnetic field a thousand trillion times Earth's. Stars & Supernovae Martian dust storm season When Mars is closest to the Sun, the whole planet can disappear under dust for months. The Solar System Max-Q The moment of peak aerodynamic pressure on an ascending rocket. Launch & Flight Meteoroid, meteor, meteorite The same rock, named three times by where it is. Comets & Asteroids Molecular cloud The cold, dark gas that stars are made out of, and hide inside while forming. Stars & Supernovae Near-Earth object An asteroid or comet whose orbit brings it inside 1.3 astronomical units of the Sun. Comets & Asteroids NET (no earlier than) A launch date that is a lower bound, not a promise. Launch & Flight Neutron star A city-sized remnant so dense that a sugar cube of it would weigh as much as a mountain. Stars & Supernovae Oort cloud A shell of icy bodies at the edge of the Sun's influence, inferred rather than observed. Comets & Asteroids Opposition and conjunction The two alignments that decide when a planet is worth looking at and when it is unreachable. The Solar System Orbital inclination The angle between an orbit and the equator, which decides what ground the satellite ever sees. Satellites & Orbit Payload fairing The nose cone that protects the satellite through the atmosphere and is then discarded. Launch & Flight Perihelion and aphelion The closest and furthest points of an orbit around the Sun. Comets & Asteroids Photometry Measuring precisely how bright something is, repeatedly, and watching for change. Telescopes & Instruments Planetary protection The rules that stop us contaminating other worlds, and them contaminating us. The Solar System Protostar A star that is still assembling itself and has not yet started fusion. Stars & Supernovae Quasar and AGN A galaxy whose central black hole is feeding so hard it outshines everything around it. Black Holes Radial velocity Finding planets by the wobble they induce in their star's motion. Exoplanets & Life Radiation dose in space Without the atmosphere and outside most of the magnetic field, the sky is an X-ray source. Crewed Spaceflight Radio quiet zone A legally enforced area where transmitting is restricted so a telescope can hear. Telescopes & Instruments Red giant What a star becomes when the hydrogen in its core runs out. Stars & Supernovae Redshift The stretching of light from distant objects, and the universe's odometer. Galaxies & Cosmology Regolith The layer of broken rock and dust covering an airless world — and a serious engineering problem. The Solar System Reionization The era when the first stars lit up and burned the fog out of the universe. Galaxies & Cosmology Relativistic jet A beam of matter fired from near a black hole at very nearly the speed of light. Black Holes Rendezvous Catching another spacecraft, which requires slowing down to speed up. Crewed Spaceflight Sample return Bringing a piece of somewhere else back to a laboratory on Earth. The Solar System Satellite constellation Many satellites flown as one system, so that at least one is always overhead. Satellites & Orbit Schwarzschild radius How small you would have to squeeze something to make it a black hole. Black Holes Seeing How much the atmosphere is blurring the stars tonight, measured in arcseconds. Telescopes & Instruments Segmented mirror One giant mirror made of many small ones, held in alignment to a fraction of a wavelength. Telescopes & Instruments Singularity The point where our description of gravity stops producing meaningful answers. Black Holes Solar flare classes A, B, C, M, X — a logarithmic scale for how hard the Sun just flashed in X-rays. The Sun & Space Weather Solid rocket booster Enormous thrust from a tube of rubbery fuel that cannot be throttled or shut off. Launch & Flight Spectrograph The instrument that splits light into its wavelengths, which is where the physics is. Telescopes & Instruments Spiral, elliptical and irregular The three basic shapes galaxies come in, and roughly what they mean. Galaxies & Cosmology Splashdown Coming home under parachutes into the sea, which is softer than land and harder to reach. Crewed Spaceflight Staging Throwing away the empty part of the rocket so you stop accelerating it. Launch & Flight Standard candle An object whose true brightness is known, so how faint it looks gives you the distance. Galaxies & Cosmology Static fire Igniting the engines with the rocket bolted down, as a rehearsal. Launch & Flight Station-keeping The small, regular burns that stop an orbit drifting into uselessness. Satellites & Orbit Sub-Neptune The most common kind of planet in the galaxy, and there is not one in our solar system. Exoplanets & Life Subsurface ocean Liquid water under an ice shell, kept warm by tides rather than by sunlight. The Solar System Sun-synchronous orbit A near-polar orbit that crosses the equator at the same local time every day. Satellites & Orbit Sunspot A patch of the Sun where the magnetic field is strong enough to hold the heat back. The Sun & Space Weather Super-Earth A rocky planet heavier than Earth, up to about twice its size, with no local example. Exoplanets & Life Supermassive black hole Millions to billions of solar masses, one at the centre of essentially every large galaxy. Black Holes Supernova (Type Ia and Type II) Two unrelated ways for a star to explode, useful for two entirely different reasons. Stars & Supernovae Survey cadence How often a telescope revisits the same patch, which decides what it can possibly discover. Telescopes & Instruments Synodic period How long until two moving bodies return to the same alignment — the number mission planners live by. The Solar System Terminator zone The permanent twilight ring on a tidally locked planet, and the only part that might be habitable. Exoplanets & Life The corona The Sun's outer atmosphere, which is three hundred times hotter than the surface for reasons still argued over. The Sun & Space Weather The cosmic web The largest structure there is: galaxies strung along filaments around enormous voids. Galaxies & Cosmology The G, R and S scales Three separate one-to-five scales for the three separate ways the Sun causes trouble. The Sun & Space Weather The main sequence The long, stable middle age a star spends fusing hydrogen in its core. Stars & Supernovae The Parker spiral Why the Sun's magnetic field reaches the planets as a spiral rather than a straight line. The Sun & Space Weather The solar cycle An eleven-year rhythm in sunspots, flares and the Sun's entire magnetic field. The Sun & Space Weather Thrust-to-weight ratio Whether the rocket can lift itself, expressed as a number that must exceed one. Launch & Flight Tidal disruption event A star wandering too close and being pulled apart, producing a flare that lasts months. Black Holes Tidal locking When a body's spin has been dragged into matching its orbit, so one face never turns away. The Solar System Torino scale A 0-to-10 impact hazard rating designed to stop scary numbers being misread. Comets & Asteroids Transit method Finding planets by watching a star dim slightly as one passes in front. Exoplanets & Life Transit timing variation Finding an unseen planet by the way it makes a visible one run late. Exoplanets & Life Transmission spectroscopy Reading a planet's atmosphere in the starlight that passes through it during a transit. Exoplanets & Life Trojan asteroid Asteroids parked in the gravitational lay-bys 60° ahead of and behind a planet. Comets & Asteroids Two-line element set The compact, ancient text format that describes where a satellite is. Satellites & Orbit What weightlessness does to people Bone loss, fluid shift, weakened hearts and eyesight that does not always come back. Crewed Spaceflight White dwarf The exposed core a low-mass star leaves behind, Earth-sized and slowly cooling forever. Stars & Supernovae