Space glossary

80 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. Mars orbiters routinely aerobrake for months to reach their working orbits.
Apogee: The highest point of an orbit around Earth. For orbits around other bodies the general terms are apoapsis, or apolune for the Moon.
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. Falcon 9 and Starship boosters both use one when returning to land.
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.
CubeSat: A small satellite built in standard 10 cm cube units, from 1U shoeboxes to 12U briefcases. Standardization made them cheap enough for universities and startups, and rideshare launches carry them by the dozen.

D

Delta-v: The total change in velocity a spacecraft can achieve with its remaining propellant, measured in km/s. Delta-v is the currency of spaceflight: reaching low Earth orbit costs about 9.4 km/s including losses, 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

Escape velocity: The speed at which an object's kinetic energy equals the gravitational pull holding it, about 11.2 km/s at Earth's surface. Probes leaving for other planets must exceed it, usually with an upper-stage burn from low orbit.
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.
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.
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 with no further burns, used as a safety net for crewed lunar flights. Apollo 13 survived on one, and Artemis II flew one in 2026.

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 above the equator where a satellite circles Earth in exactly one day, appearing fixed in the sky. Communications and weather satellites cluster there.
Geostationary transfer orbit (GTO): An elliptical hand-off orbit with its high point near GEO altitude. Rockets drop satellites there, and the satellite's own engine circularizes the orbit over days or months.
Gravity assist: Stealing a little of a planet's orbital momentum by flying past it, changing a spacecraft's speed and direction for free. The Voyager grand tour and Europa Clipper's Mars and Earth flybys both depend on it.

H

Heat shield: The protective barrier that absorbs or deflects the several-thousand-degree heating of reentry, either by charring away (ablation) or by insulating tiles. It is routinely the deciding factor in whether a reentry vehicle survives.
Hohmann transfer: The minimum-energy two-burn path between two circular orbits, an ellipse touching both. Most Mars missions fly close to one, which is why launch windows recur about every 26 months.
Hot staging: Igniting the upper stage while still attached to the burning booster, gaining performance by never coasting. Starship adopted it in 2023; Soyuz has always staged this way.
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.
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.
Ion thruster: An electric engine that accelerates ionized gas to tens of km/s, giving tiny thrust with extraordinary efficiency. Starlink satellites and deep-space probes like Psyche maneuver on ion propulsion for months at a time.
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.

K

Kármán line: The 100 km altitude used by record-keepers as the boundary of space. The US recognizes 50 miles (about 80 km); physically the atmosphere just thins gradually.
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 where the gravity of two bodies and orbital motion balance, letting spacecraft hold station cheaply. The James Webb Space Telescope orbits Sun-Earth L2, 1.5 million km from Earth.
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. Objects there circle Earth in about 90 minutes at 7.8 km/s.
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.

M

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.
Methalox: Liquid methane with liquid oxygen, the propellant of choice for new rockets like Starship, New Glenn, and Vulcan. It balances performance, clean-burning reusability, and the possibility of manufacture on Mars.
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.

N

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

O

Orbit: A path where an object falls around a body fast enough to keep missing it. Reaching orbit is about sideways speed, not altitude, which is why rockets pitch over almost immediately. See how rockets work.
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.
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. Cryogenic transfer at scale has not yet been demonstrated and sits high on the test agenda.

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 lowest point of an orbit around Earth (generally, periapsis; for the Sun, perihelion). 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.
Propellant: The reaction mass a rocket throws out the back: fuel plus oxidizer for chemical rockets, often 85-95 percent of a launcher's liftoff mass. Propellant, not engines, dominates what a rocket can do.

R

Reaction wheel: A motor-spun flywheel that turns a spacecraft by conservation of angular momentum, no propellant needed. Telescopes depend on them for fine pointing; Hubble's failing gyros and wheels have driven its reduced operating modes.
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 delicate dance where speeding up raises your orbit and slows your approach. Buzz Aldrin's MIT thesis helped work out the techniques Gemini proved in 1966.
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.
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.

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; Mars sample return remains unfunded as of 2026.
Satellite: Any object orbiting another; artificial satellites number over ten thousand active spacecraft today, more than half 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.
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: Rocket fuel efficiency, in seconds: how long one unit of propellant weight can produce one unit of thrust. Kerosene engines reach about 300-350 s, hydrogen about 450 s, ion thrusters thousands.
Staging: Dropping empty tanks and spent engines mid-flight so the rocket stops hauling dead weight. The rocket equation makes multi-stage designs all but mandatory for reaching orbit. 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.
Suborbital: Reaching space without reaching orbital speed, arcing up and back down in minutes. New Shepard's tourist flights and early Mercury missions are suborbital; the difference from orbit is the roughly 7.8 km/s of sideways velocity.
Sun-synchronous orbit: A near-polar orbit precessing exactly once per year, so a satellite crosses every location at the same local solar time. Earth-imaging satellites use it for consistent lighting.

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 or tonnes-force. A rocket lifts off only when thrust exceeds its weight; the ratio between them (TWR) must exceed 1 at liftoff.
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 to Artemis II began with one.
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.

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.