Space glossary

105 spaceflight terms in plain language, from apogee to the Van Allen belts. Terms link into the wiki where a full article exists.

A

Aerobraking: Using repeated passes through the upper atmosphere of a planet to slow a spacecraft and shrink its orbit without burning propellant. Magellan pioneered it at Venus in 1993, and Mars orbiters now aerobrake for months to reach their working orbits. Aerocapture, the single-pass version that captures a spacecraft straight out of an arrival trajectory, has never been flown. source
Apogee: The point in an orbit farthest from Earth. The general term for any central body is apoapsis; it is aphelion around the Sun and apolune around the Moon. A spacecraft moves slowest at apogee, which is where circularization burns are made.
Astronaut: A person trained to fly in space. Russia uses cosmonaut and China uses taikonaut for the same role; the US and most partners use astronaut.
Attitude control: Keeping a spacecraft pointed the right way using reaction wheels, small thrusters, or magnetic torquers. Loss of attitude control usually means antennas, solar arrays, and instruments stop pointing where they must.

B

Boostback burn: An engine burn a returning booster performs against its direction of travel to reverse course toward its landing site, the first of three recovery burns, followed by the entry burn and the landing burn. Falcon 9 and Starship boosters both use one when returning to the launch site.
Booster: The first stage of a rocket, or an auxiliary rocket strapped to the side of one. Boosters do the heavy lifting off the pad and are the easiest stage to recover, since they never reach orbital speed.
Burn: Any planned firing of a rocket engine, from a fraction of a second to several minutes. Missions are choreographed as sequences of burns separated by coasting.

C

CAPCOM: The capsule communicator: the single person in mission control who speaks to the crew, traditionally an astronaut. The name survives from the capsule era of Mercury, Gemini, and Apollo.
Cislunar space: The region between Earth and the Moon's orbit, including lunar orbits. The Artemis era has made cislunar operations a focus for both space agencies and militaries.
Commercial crew: NASA's model of buying astronaut transportation as a service from private companies rather than owning the spacecraft. See the Commercial Crew Program.
Cryogenic propellant: A propellant that has to be held liquid far below room temperature: oxygen below -183 C, methane below -162 C, hydrogen below -253 C. The cold is why a fueled rocket frosts over and vents, why the countdown tightens once loading starts, and why keeping propellant in orbit for weeks is still an unsolved problem. source
CubeSat: A small satellite built from standard 10 cm cube units, so a 1U is a cube the size of a large mug and a 3U about the size of a loaf of bread; 6U and 12U are also common. Standardization made them cheap enough for universities and startups, and rideshare launches carry them by the dozen. source

D

Dawn-dusk orbit: A sun-synchronous orbit aligned near the line between day and night, giving a spacecraft high solar availability and a stable lighting geometry. It can still pass through eclipses depending on altitude, season, and orbital geometry, so batteries or workload throttling may be required. Some proposed orbital data centers target this regime.
Delta-v: The change in velocity a maneuver demands, or that a vehicle can still produce with the propellant it has left, usually quoted in km/s. Delta-v is the currency of spaceflight: reaching low Earth orbit costs about 9.3 to 9.5 km/s once gravity and drag losses are counted, and every destination beyond has a price on top. See orbital mechanics.
Deorbit burn: A retrograde burn that lowers an orbit until it intersects the atmosphere, committing a spacecraft to reentry. Capsules time it precisely to hit a landing zone half a world away.
Docking and berthing: Two ways of joining spacecraft: docking is an active vehicle flying itself onto a port, berthing is a robotic arm grabbing a passive vehicle and attaching it. Crew Dragon docks; early cargo Dragons were berthed.
Downrange: Distance measured along a rocket's ground track from the launch pad. Drone ships park hundreds of kilometers downrange because boosters on high-energy missions cannot fly back to the coast.
Drone ship: An uncrewed, station-keeping barge that serves as a floating landing pad for returning boosters. SpaceX operates several with names taken from Iain M. Banks novels, such as Of Course I Still Love You.

E

Entry burn: A braking burn a returning booster makes just before it hits the thick atmosphere, keeping reentry heating and aerodynamic loads within what the stage can take. Falcon 9 relights three of its nine engines for roughly 20 seconds, between the boostback burn and the landing burn.
Escape velocity: The speed at which an object's kinetic energy exactly balances the gravitational energy binding it, so it coasts away instead of falling back: about 11.2 km/s at Earth's surface. Leaving from orbit is far cheaper than that suggests, since a spacecraft already circling at 400 km is doing 7.7 km/s and needs about 10.9, a gap of roughly 3.2 km/s. source
EVA (spacewalk): Extravehicular activity: any time an astronaut works outside the spacecraft in a suit. Alexei Leonov performed the first in 1965; the Polaris Dawn crew performed the first commercial EVA in 2024. source
Expendable launch vehicle: A rocket discarded after one flight, the industry default until reusable rockets proved out. Many current rockets, including Ariane 6 and SLS, remain fully expendable.

F

Fairing: The nose cone shell that protects a satellite during ascent and splits away once the rocket leaves the atmosphere. Fairing diameter limits how big a payload can be, which is why New Glenn's 7 m fairing is a selling point. source
Flight-proven: SpaceX's term for a stage that has already flown and been recovered, now common industry usage. Most Falcon 9 launches use flight-proven boosters, and the fleet leader had flown 36 times by July 2026. See reusable rockets. source
Flyby: Passing a body without stopping, gathering data during the encounter. Flybys are the cheapest way to visit a world; New Horizons at Pluto is the classic example.
Free-return trajectory: A path that loops around the Moon and returns to Earth on lunar and terrestrial gravity alone, needing no further burns, which makes it a safety net for crewed lunar flights. Apollo 13 came home on one after its oxygen tank ruptured, and Artemis II flew one in April 2026. source

G

G-force: Acceleration expressed in multiples of Earth gravity. Crews feel about 3g on a Falcon 9 ascent and 4-7g on steep capsule reentries.
Geostationary orbit (GEO): A circular orbit 35,786 km directly above the equator, where one lap takes a sidereal day (23 h 56 min 4 s) and the satellite therefore appears fixed in the sky. Communications and weather satellites cluster there. A geosynchronous orbit has the same period but any tilt, so it traces a figure eight rather than holding still. source
Geostationary transfer orbit (GTO): An elliptical hand-off orbit with a low perigee and an apogee near GEO altitude. Rockets drop satellites there and the satellite's own engine spends about 1.5 km/s circularizing over days or months, which is why launch providers quote separate GTO and LEO payload figures. source
Graveyard orbit: A disposal orbit a few hundred kilometers above the geostationary belt, used because there is no atmosphere that high to pull retired satellites down. International guidelines put the target at least 235 km above the ring, plus a margin sized to how strongly sunlight pushes on the particular spacecraft. source
Gravity assist: Flying past a planet to trade momentum with it, changing a spacecraft's speed and direction without spending propellant. Relative to the planet the spacecraft leaves as fast as it arrived and only turns; relative to the Sun it gains or loses part of the planet's own orbital motion. The Voyager grand tour used assists to speed up, while Europa Clipper used Mars in 2025 to slow down. source
Gravity turn: The gentle pitchover a launcher flies shortly after liftoff, letting gravity bend the trajectory downrange so that thrust goes into building sideways speed instead of fighting weight. Flying it well is how a vehicle holds down gravity losses, the largest reason orbit costs more than the 7.8 km/s of orbital speed itself. See how rockets work.
Grid fin: A lattice-shaped fin that folds flat against a booster during ascent and deploys on the way down to steer it aerodynamically. Falcon 9 carries four, cast from titanium so they survive reentry heating without refurbishment between flights.

H

Halo orbit: A looping path around an unstable Lagrange point rather than a fixed station at the point itself, which keeps a spacecraft continuously in sunlight and out of Earth's shadow. The James Webb Space Telescope circles Sun-Earth L2 once every 168 days and trims its orbit about every three weeks. source
Heat shield: The protective barrier that absorbs or deflects reentry heating, either by charring away (ablation) or by insulating behind tiles. Coming back from low Earth orbit means surviving surface temperatures around 1,650 C, and returning from the Moon is hotter still, which is why Apollo capsules carried a thick ablative shield. source
Hohmann transfer: A two-burn ellipse that just touches both a starting and a destination circular orbit, and the cheapest route for most orbit changes: low Earth orbit to geostationary costs about 3.9 km/s in two burns. Above a radius ratio of roughly 12 a three-burn bi-elliptic transfer beats it. Most Mars missions fly close to a Hohmann path, which is why launch windows recur about every 26 months. source
Hot staging: Igniting the upper stage while it is still attached to the booster, gaining performance by never coasting and never having to settle propellant first. Starship adopted it in 2023; Soviet and Russian rockets have separated upper stages this way for decades. source
Hydrolox: Liquid hydrogen fuel with liquid oxygen oxidizer, the highest-efficiency common chemical combination (about 450 s specific impulse). The trade-offs are deeply cryogenic handling and bulky tanks; SLS and Ariane 6 cores use it. source
Hypergolic propellant: Fuel and oxidizer that ignite on contact, no ignition system required. Their reliability makes them standard for spacecraft thrusters and abort engines, despite being toxic to handle.

I

Inclination: The tilt of an orbit relative to the equator, in degrees. The ISS flies at 51.6 degrees so Russian rockets can reach it; polar satellites fly near 90 degrees to see the whole planet. source
Ion thruster: An electric engine that ionizes a gas and accelerates it electrostatically to tens of km/s, giving thrust measured in millinewtons at efficiencies chemical engines cannot approach. Gridded ion engines pull the ions through charged grids, as on Dawn; Hall-effect thrusters use a magnetic field instead, and are what Starlink satellites and NASA's Psyche fly. Either way the burns last months rather than minutes. source
ISRU (in-situ resource utilization): Living off the land in space: making propellant, water, or oxygen from local materials instead of hauling them from Earth. Lunar polar ice and Mars atmospheric CO2 are the headline targets; Perseverance's MOXIE experiment made oxygen on Mars in 2021. source

K

Kármán line: The 100 km altitude used as the boundary of space by the FAI, the body that ratifies aviation and spaceflight records, and named for the aerodynamicist Theodore von Kármán. The US government instead recognizes 50 miles, about 80 km, so a flight peaking at 90 km counts as spaceflight under one definition and not the other. Physically there is no line; the atmosphere just thins out. source
Kerolox: Refined kerosene (RP-1) with liquid oxygen, the dense, practical propellant of the Saturn V first stage, Soyuz, and Falcon 9. Less efficient than hydrogen but far easier to handle.
Kessler syndrome: A runaway scenario where collisions in a crowded orbit create debris that causes more collisions, potentially rendering the orbit unusable. Avoiding it drives modern space debris rules.
Kick stage: A small final stage that delivers payloads to precise orbits after the main rocket finishes. Rocket Lab's Electron popularized the term with its Curie-powered kick stage.

L

Lagrange points: Five positions in a two-body system where gravity and orbital motion balance, letting a spacecraft keep station with the pair. L1, L2, and L3 are unstable and need correction burns every few weeks; L4 and L5 are stable, which is why Trojan asteroids collect there. The James Webb Space Telescope loops around Sun-Earth L2, 1.5 million km from Earth. See orbital mechanics. source
Landing burn: The final burn that stops a returning stage just above its pad, ship, or catch tower. Speed and altitude have to run out together, because a nearly empty booster produces more thrust at minimum throttle than its own weight and cannot hover: Falcon 9 uses a single center engine for about half a minute.
Launch window: The time span when a launch can reach its target, set by orbital geometry. Station missions get minutes per day; Mars missions get weeks every 26 months. See transfer window.
Low Earth orbit (LEO): Orbits below roughly 2,000 km, where the ISS, Starlink, and most satellites fly. Near the bottom of the band a spacecraft circles Earth in about 90 minutes at 7.8 km/s; at the top of it a lap takes over two hours. source
LOX: Liquid oxygen, the oxidizer in nearly every large rocket, kept below -183 C. The white frost and venting seen on a fueled rocket is LOX boiling off. source

M

Mass ratio: A rocket stage's fueled mass divided by its empty mass, the quantity the rocket equation takes the logarithm of. Reaching orbit on kerosene and oxygen takes a ratio near 18, which means roughly 17 kilograms of propellant burned for every kilogram that arrives.
Max q: The moment of maximum aerodynamic pressure during ascent, when speed is high but the air is still thick. Rockets often throttle down through it; passing max q is a standard callout.
MECO: Main engine cutoff: the moment the first stage stops burning, just before stage separation. On Falcon 9 it happens about two and a half minutes after liftoff.
Medium Earth orbit (MEO): The band between LEO and GEO, from about 2,000 to 35,786 km. GPS and other navigation constellations live there, circling Earth roughly twice a day. source
Megaconstellation: A satellite network numbering in the thousands rather than the dozens, made practical by mass-produced spacecraft and reusable rockets. Starlink is the first and by far the largest, at more than 10,000 satellites; Amazon Leo, Guowang, and Qianfan are each still under 400. source
Methalox: Liquid methane with liquid oxygen, the propellant of choice for new rockets like Starship, New Glenn, and Vulcan Centaur. It sits between kerosene and hydrogen for efficiency, burns cleanly enough to suit engines meant to fly again, and could in principle be made on Mars. LandSpace's Zhuque-2 was the first methalox rocket to reach orbit, in July 2023. source
Microgravity: The apparent weightlessness of free fall. Orbiting astronauts still feel about 90 percent of Earth's gravity; they float because the spacecraft falls with them. source
Molniya orbit: A 12-hour, highly elliptical orbit inclined 63.4 degrees, with apogee parked high over the northern hemisphere so the satellite loiters there for about two-thirds of each lap. Soviet engineers adopted it because geostationary satellites sit too low in the sky to be useful at high latitudes. source

N

Nominal: Aerospace speak for exactly as planned. A nominal trajectory is good news, not average news.

O

Optical intersatellite link: A laser link that carries data directly between satellites instead of routing it through a ground station, sidestepping the crowded radio spectrum. Starlink satellites hand traffic to each other this way, and proposed orbital data centers would use the same technique to connect compute nodes.
Orbit: A path where an object falls around a body fast enough to keep missing it. Reaching orbit is about sideways speed, not altitude: crossing the Kármán line takes about 1 km/s, while staying up takes nearly 8. See how rockets work and orbital mechanics.
Orbital data center: A computing system in orbit, ranging from an edge processor that reduces its own spacecraft's data to a proposed network of shared server satellites. Solar availability, radiative cooling, radiation, links, reliability, launch mass, and disposal constrain the architecture. See orbital data centers.
Orbital decay: The gradual lowering of an orbit by thin-atmosphere drag. It cleans up low orbits naturally: a dead satellite at 400 km reenters within a few years, while one at 1,000 km can persist for centuries. source
Orbital elements: The six numbers that pin an orbit down completely. Semi-major axis and eccentricity give the ellipse its size and shape, inclination and longitude of the ascending node and argument of periapsis orient it in space, and a time of periapsis passage says where the spacecraft is along it. See orbital mechanics.
Orbital period: The time for one full orbit: about 90 minutes for the ISS, 24 hours at GEO, 27.3 days for the Moon.
Orbital refueling: Transferring propellant between spacecraft in orbit, the enabling technology for Starship's Moon and Mars missions, which need a series of tanker flights to fill one vehicle. SpaceX moved liquid oxygen between tanks inside a single ship in 2024, but a transfer between two docked vehicles has not been demonstrated and remains a test objective. source

P

Payload: Whatever the rocket exists to deliver: satellites, cargo, crew. Payload mass to a given orbit is the standard measure of a launcher's capability.
Perigee: The point in an orbit closest to Earth. The general term is periapsis; it is perihelion around the Sun and perilune around the Moon. A spacecraft moves fastest at perigee, so departure burns are made there, and reentry planning is largely the art of placing perigee inside the atmosphere at the right spot.
Planetary protection: Rules for not contaminating other worlds with Earth microbes, and Earth with returned samples. They constrain where landers may go and how sample-return hardware is sealed, a live issue for Mars missions.
Polar orbit: An orbit inclined close to 90 degrees, passing near both poles while the planet turns underneath, so the spacecraft eventually sees all of it. Mapping, weather, and reconnaissance satellites fly one, and it costs payload, because a polar launch collects none of the free speed of Earth's rotation.
Propellant: The reaction mass a rocket throws out the back: fuel plus oxidizer for chemical rockets, typically 85 to 90 percent of a launcher's liftoff mass. Propellant, not engines, dominates what a rocket can do. See how rockets work.

R

Radiation hardening: Designing electronics to survive charged particles and cosmic rays through shielding, fault-tolerant circuits, specialized manufacturing, error-correcting memory, redundancy, and recovery software. Proposed orbital data centers may use commercial accelerators, but software alone cannot protect every component or failure mode.
Radiator: The surface a spacecraft uses to shed waste heat. A vacuum offers no outside air or water to carry heat away, so a spacecraft ultimately radiates heat as infrared light. Required area depends strongly on temperature, emissivity, geometry, and absorbed sunlight, Earth infrared, and albedo; large orbital data centers would need substantial radiator systems.
Reaction wheel: A motor-driven flywheel that turns a spacecraft by conservation of angular momentum, spending electricity instead of propellant. Telescopes depend on them for fine pointing, and losing them ends missions: Kepler had failed two of its four wheels by 2013 and could no longer hold a target, which forced the improvised K2 survey. source
Reentry: Returning through the atmosphere, converting orbital speed into heat against a heat shield. It remains one of the most demanding phases of spaceflight, as Starship's early test flights kept demonstrating.
Regenerative cooling: Running cold propellant through channels in an engine's nozzle and chamber walls before burning it, so the engine cools itself. Nearly every large liquid engine uses it.
Rendezvous: Bringing two orbiting spacecraft together, a maneuver where thrusting toward a target ahead of you raises your orbit and makes you fall further behind. The chaser instead flies lower and faster to close the gap, then climbs to meet the target. Buzz Aldrin's MIT doctoral thesis helped work out the guidance, and Gemini VI-A proved it in December 1965. source
Retrograde: Motion against the usual direction: an orbit opposite a planet's spin, or a burn against the direction of travel. Deorbit and capture burns are retrograde burns.
Rideshare: Selling space on a single launch to many small payloads, so a CubeSat operator buys a slot rather than a rocket. SpaceX's Transporter missions carry dozens of spacecraft at a time, and Starcloud-1 reached orbit as a rideshare passenger.
Rocket equation: Konstantin Tsiolkovsky's 1903 result that a rocket's delta-v equals its exhaust velocity multiplied by the natural logarithm of its mass ratio. The logarithm is what makes launch hard: extra propellant has to be carried and accelerated before it burns, which is why orbital rockets are 85 to 90 percent propellant and why staging exists. See how rockets work.
RTG (radioisotope thermoelectric generator): A nuclear battery converting the heat of decaying plutonium-238 into electricity, no sunlight needed. Voyager, Cassini, New Horizons, and the Mars rovers Curiosity and Perseverance all run on RTGs. source

S

Sample return: Bringing extraterrestrial material back to Earth labs, where instruments outclass anything flyable. Apollo returned 382 kg from the Moon, Hayabusa2 and OSIRIS-REx returned asteroid samples, and China's Chang'e 6 brought back the first material from the lunar far side in June 2024. A Mars sample return has been paused and rescoped repeatedly, and no mission to collect Perseverance's sealed tubes has been approved. source
Satellite: Any object orbiting another. About 16,000 working spacecraft are in Earth orbit today by ESA's count, roughly two-thirds of them Starlink.
Scrub: Calling off a launch attempt before liftoff, for weather, hardware, or range issues. Scrubs are routine risk management, not failures.
Service module: The uncrewed section carrying a spacecraft's power, propulsion, and consumables, discarded before reentry. Orion's is built by ESA, which buys European astronauts seats to the Moon.
Single-event upset: A bit flipped in memory or logic when one energetic particle passes through a chip. Upsets corrupt data rather than destroy hardware, so spacecraft computers are built to detect and correct them rather than to prevent every one.
Sounding rocket: A small suborbital rocket that lobs experiments above the atmosphere for a few minutes of space conditions, far cheaper than reaching orbit.
Specific impulse: How efficiently an engine uses propellant, quoted in seconds: how long a given weight of propellant can produce a thrust equal to that weight. Multiply by 9.8 m/s2 to get exhaust velocity, so 450 s means gas leaving the nozzle at about 4.4 km/s. Kerosene engines reach about 300-350 s, hydrogen about 450 s, ion thrusters thousands. See how rockets work.
Staging: Dropping empty tanks and spent engines mid-flight so the rocket stops hauling dead weight, which resets the mass ratio for whatever is left. Stages can sit one atop another, as on the Saturn V, or strap on alongside a core, as on Falcon Heavy. The rocket equation is why no launcher in service reaches orbit in a single stage. See how rockets work.
Static fire: Test-firing a rocket's engines while it stays bolted down, the standard pre-flight checkout. Not without risk: a static-fire explosion destroyed a New Glenn in May 2026. source
Station-keeping: The small burns that hold a spacecraft in its assigned slot against drag, the pull of the Sun and Moon, and the pressure of sunlight. A geostationary satellite spends roughly 45 m/s a year on north-south corrections alone, and has to launch carrying a decade or more of that propellant.
Suborbital: Reaching space without reaching orbital speed, arcing up and back down in minutes. New Shepard's tourist flights and the first two Mercury missions were suborbital. The gap is not altitude but the roughly 7.8 km/s of sideways velocity, which is about sixty times the kinetic energy per kilogram.
Sun-synchronous orbit: A near-polar orbit, tilted about 98 degrees, whose plane is dragged around by Earth's equatorial bulge at almost exactly one degree per day, matching Earth's progress around the Sun. The satellite therefore passes over each place at the same local solar time on every orbit, which is what Earth-imaging missions want for consistent shadows. See also dawn-dusk orbit. source

T

Telemetry: The stream of measurements a vehicle radios to the ground: temperatures, pressures, positions, health. When commentators say a vehicle was lost, they often mean its telemetry stopped.
Thrust: The force a rocket engine produces, measured in newtons, tonnes-force, or pounds-force. A rocket lifts off only when thrust exceeds its weight, so the thrust-to-weight ratio must be above 1: Falcon 9 leaves the pad on about 7.6 million newtons from nine engines.
Thrust vector control: Steering a rocket by aiming its thrust rather than by pushing on the air, usually by swiveling (gimballing) the engine so the thrust line misses the center of gravity and torques the vehicle. The RS-25 gimbals through plus or minus 10.5 degrees; earlier rockets used vanes set in the exhaust or separate vernier engines.
Tower catch: Recovering a rocket stage with arms on the launch tower instead of landing legs, which saves the mass of the legs and sets the stage straight back on its mount. SpaceX first caught a Super Heavy booster in October 2024; catching the Starship upper stage the same way has not yet been attempted. source
Trans-lunar injection (TLI): The burn that stretches a parking orbit into a path reaching the Moon, adding roughly 3.1 km/s. Every crewed lunar mission from Apollo 8 in 1968 to Artemis II in April 2026 has begun with one. source
Transfer window: The recurring period when planetary alignment makes a journey affordable, set by both planets' orbits. Earth-Mars windows open for a few weeks about every 26 months, which is why Mars missions launch in clusters. source
Turnaround: The time between a recovered stage's flights, and the number that decides whether reuse actually saves money. The Space Shuttle needed months of inspection and refurbishment; a Falcon 9 booster flew twice in nine days in March 2025. See reusable rockets. source

U

Ullage: The gas space above liquid propellant in a tank, and the small settling burns that push floating propellant to the tank bottom before an engine lights in weightlessness.
Umbilical: The power, data, and propellant lines connecting a rocket to its tower until the moment of liftoff, retracting or tearing away as it climbs.
Upper stage: The stage that finishes the job after booster separation, providing most of the velocity needed for orbit. It is the hardest stage to reuse because it reenters from orbital speed, the central challenge of Starship's design.

V

Van Allen belts: Zones of charged particles trapped by Earth's magnetic field, discovered by Explorer 1 in 1958. Crewed missions minimize time inside them; hardy electronics are required to operate there. source