Space Terms & Definitions
A comprehensive glossary with over 100 space exploration terms, rocket science jargon, and astronomical definitions to help you understand the language of spaceflight
A
Abort
An emergency procedure to terminate a launch and separate the crew capsule from a failing rocket.
Apogee
The point in an elliptical orbit farthest from Earth. Opposite of perigee.
Asteroid
A rocky object smaller than a planet that orbits the Sun, mostly found in the asteroid belt between Mars and Jupiter.
Astronaut
A person trained to travel and work in space. Russian space travelers are called cosmonauts; Chinese are called taikonauts.
Astronomical Unit (AU)
The average distance from Earth to the Sun, approximately 93 million miles (150 million km). Used to measure distances within solar systems.
Attitude
The orientation of a spacecraft in space, typically described in terms of pitch, yaw, and roll.
B
Booster
The first stage of a rocket that provides initial thrust during launch. Can also refer to auxiliary solid rocket motors.
Burn
A period during which a rocket engine fires. "Insertion burn" places a spacecraft into orbit; "deorbit burn" brings it back to Earth.
C
Cislunar
The region of space between Earth and the Moon.
Constellation
A group of satellites working together, or a pattern of stars in the night sky.
Cosmonaut
A Russian astronaut. The term comes from "cosmos" (universe) and "nautes" (sailor).
Crewed/Crew
Refers to spacecraft or missions carrying human occupants. Preferred modern term instead of "manned."
Cryogenic
Extremely cold. Cryogenic propellants like liquid hydrogen and liquid oxygen must be kept at very low temperatures (below -150°C).
D
Delta-V
Change in velocity. A measure of the propulsion capability needed to perform a maneuver or reach a destination.
Deorbit
The process of leaving orbit and returning to Earth's atmosphere.
Docking
The joining of two spacecraft in orbit. Different from "berthing," where a robotic arm captures one spacecraft and connects it.
Downrange
In the direction of a rocket's flight path away from the launch site.
E
Elliptical Orbit
An oval-shaped orbit with varying distance from the central body, contrasted with a circular orbit.
Escape Velocity
The minimum speed needed to break free from a celestial body's gravitational pull. For Earth, approximately 25,000 mph (40,000 km/h).
EVA (Extravehicular Activity)
A spacewalk. Any activity performed by an astronaut outside a spacecraft in the vacuum of space.
Expendable
A rocket or component designed for single use, discarded after launch. Opposite of reusable.
Exoplanet
A planet orbiting a star outside our solar system. Over 5,000 exoplanets have been discovered, ranging from gas giants to rocky worlds in habitable zones.
F
Fairing
The aerodynamic nose cone that protects satellites and payloads during launch through the atmosphere. Jettisoned once outside the atmosphere to reduce weight.
Free Return Trajectory
An orbital path around the Moon that automatically returns a spacecraft to Earth without requiring additional propulsion. Used as a safety feature on Apollo missions.
Fuel Cell
An electrochemical device that combines hydrogen and oxygen to produce electricity and water. Used on spacecraft like the Space Shuttle and Apollo for power and drinking water.
G
Geostationary Orbit (GEO)
An orbit 22,236 miles (35,786 km) above Earth's equator where a satellite's orbital period matches Earth's rotation, making it appear stationary above a fixed point.
Geosynchronous Orbit
An orbit with a period equal to Earth's rotation (24 hours) but not necessarily above the equator.
Gravity Assist
Using a planet's gravity to alter a spacecraft's speed and trajectory without using fuel. Also called a gravitational slingshot.
Grid Fins
Lattice-like control surfaces used by some rockets (like Falcon 9) to steer during atmospheric descent.
G-Force
A measure of acceleration felt as weight. 1g equals Earth's surface gravity. Astronauts experience 3-4g during launch and reentry.
H
Habitat Module
A pressurized compartment where astronauts live and work in space. Can be part of a space station or standalone on long-duration missions.
Hohmann Transfer
An energy-efficient elliptical orbit used to transfer between two circular orbits. Named after Walter Hohmann who described it in 1925.
Hypergolic Propellant
Rocket fuels that ignite spontaneously when mixed together, requiring no ignition system. Common in spacecraft maneuvering systems. Example: hydrazine and nitrogen tetroxide.
I
Inclination
The tilt of an orbit relative to Earth's equator, measured in degrees. A 0° orbit is equatorial; 90° is polar.
Insertion Burn
A rocket burn that places a spacecraft into a specific orbit.
Interplanetary
Between planets; refers to space travel or trajectories beyond Earth orbit but within our solar system.
Interstellar
Between stars; the vast space between stellar systems.
Ion Engine
An electric propulsion system that accelerates ions to very high speeds. Provides low thrust but excellent fuel efficiency for long-duration missions. Used on deep space probes.
ISRU
In-Situ Resource Utilization—producing fuel, water, oxygen, or building materials from local resources on other worlds rather than transporting everything from Earth.
J
Jettison
To discard or eject something from a spacecraft, such as spent rocket stages, fairings, or excess propellant.
K
Kármán Line
The boundary between Earth's atmosphere and outer space, conventionally set at 100 km (62 miles) altitude.
Kepler's Laws
Three fundamental laws describing planetary motion and orbital mechanics discovered by Johannes Kepler.
Kick Stage
A small upper stage used to move a payload from a transfer orbit to its final orbit.
L
Lagrange Points
Five positions in space where gravitational forces and orbital motion balance, allowing objects to maintain a stable position. The James Webb Space Telescope orbits the L2 point.
Launch Escape System (LES)
A system designed to quickly separate a crew capsule from a failing rocket during launch.
Launch Vehicle
A rocket used to carry payloads from Earth's surface to space.
Launch Window
A specific time period during which a launch must occur to reach the intended orbit or destination.
LEO (Low Earth Orbit)
Orbits between 160-2,000 km (100-1,200 miles) altitude. The ISS orbits at approximately 400 km.
LOX
Liquid oxygen, a cryogenic oxidizer used in many rocket engines.
M
Microgravity
The condition of apparent weightlessness experienced in orbit. Technically there is still gravity, but objects are in free fall.
Meteoroid
A small rocky or metallic body in space. When it enters Earth's atmosphere and burns up, it's called a meteor. If it reaches the ground, it's a meteorite.
Mission Elapsed Time (MET)
Time measured from the moment of launch, used to track events during a space mission.
N
Near-Earth Object (NEO)
An asteroid or comet whose orbit brings it close to Earth's orbital path.
NET (No Earlier Than)
Indicates the earliest possible date/time for a launch, though delays are possible.
Node
The point where an orbit crosses a reference plane, such as Earth's equator.
O
Orbit
The path a celestial object takes around another due to gravitational attraction. Spacecraft orbit Earth; planets orbit the Sun.
Orbital Mechanics
The study of the motions of artificial and natural bodies in space under the influence of gravity.
Oxidizer
A chemical that provides oxygen for rocket fuel combustion. Common oxidizers include liquid oxygen (LOX) and nitrogen tetroxide.
P
Payload
The cargo carried by a rocket—satellites, scientific instruments, supplies, or crew.
Payload Fairing
The protective nose cone that shields the payload during atmospheric ascent. Typically jettisoned once outside the atmosphere.
Perigee
The point in an elliptical orbit closest to Earth. Opposite of apogee.
Perihelion
The point in an orbit closest to the Sun.
Polar Orbit
An orbit passing over or near Earth's poles, with an inclination of approximately 90°.
Propellant
The fuel and oxidizer combination used to propel a rocket. Can be liquid (LOX/RP-1) or solid.
Propulsive Landing
Using rocket engines to slow a spacecraft for a controlled vertical landing. SpaceX Falcon 9 and Starship use this technique for booster recovery.
Q
Quarantine
Isolation procedures used to prevent biological contamination. Apollo astronauts were quarantined after Moon missions; future Mars samples will be quarantined to prevent potential Martian organisms from reaching Earth.
R
Reentry
The return of a spacecraft from orbit through Earth's atmosphere.
Retrograde
Motion in the opposite direction to normal orbital motion. Retrograde burns slow a spacecraft down.
Reusable
Designed to be recovered and flown again, reducing launch costs. SpaceX Falcon 9 boosters are reusable.
Rocket Equation
Tsiolkovsky's equation describing the relationship between rocket velocity, exhaust velocity, and propellant mass.
RP-1
Refined kerosene used as rocket fuel, commonly paired with liquid oxygen.
S
Scrub
Cancellation of a launch attempt, usually due to weather or technical issues. The mission will be rescheduled.
Solar Wind
A stream of charged particles (plasma) ejected from the Sun's upper atmosphere.
Spacecraft
A vehicle designed for travel or operation in outer space.
Specific Impulse (Isp)
A measure of rocket engine efficiency. Higher Isp means better fuel efficiency.
Stage
A section of a rocket containing its own engine(s) and propellant, designed to be jettisoned when empty.
Static Fire
A test where rocket engines are fired while the vehicle is held down, conducted before launch to verify systems.
Suborbital
A flight that reaches space but doesn't achieve orbital velocity, following a parabolic trajectory back to Earth.
Sun-Synchronous Orbit (SSO)
A polar orbit where the satellite passes over any given point on Earth at the same local solar time. Ideal for Earth observation satellites requiring consistent lighting conditions.
Supernova
The explosive death of a massive star, briefly outshining entire galaxies. These events create and spread heavy elements throughout the universe.
T
T-Minus
Countdown notation. "T-10 minutes" means 10 minutes before launch. After launch, it becomes "T-Plus."
TBD / TBC
To Be Determined or To Be Confirmed. Used when exact launch dates/times haven't been finalized.
Telemetry
Data transmitted from a spacecraft to ground stations, including position, velocity, and system status.
Thrust
The force produced by a rocket engine, typically measured in newtons or pounds-force.
Trajectory
The path followed by a spacecraft or projectile through space.
Trans-Lunar Injection (TLI)
A propulsion maneuver that sets a spacecraft on a trajectory toward the Moon.
Tether
A long cable connecting spacecraft or satellites. Can be used for artificial gravity, momentum exchange, or electrodynamic propulsion.
U
Ullage Motor
Small rocket motors that provide acceleration to settle propellant at the bottom of tanks before main engine ignition, especially important in microgravity.
Upper Stage
The topmost rocket stage that operates in vacuum to place payloads into their final orbits or send spacecraft beyond Earth orbit.
V
Velocity
Speed in a given direction. Orbital velocity is the speed needed to maintain an orbit.
Vernier Engines
Small thrusters used for fine adjustments to a spacecraft's trajectory or attitude.
Van Allen Belts
Regions of intense radiation trapped by Earth's magnetic field, discovered by Explorer 1 in 1958. Spacecraft must pass through them quickly or avoid them with orbital inclination.
W
Webcast
Live video broadcast of a rocket launch, typically streamed on YouTube or the agency's website.
Wet Mass
The total mass of a rocket stage including propellant. Compare to dry mass (empty stage). The ratio between them affects efficiency.
Z
Zero-G / Zero Gravity
Colloquial term for weightlessness or microgravity. Technically incorrect as gravity still exists in orbit, but objects are in continuous free fall.