Space glossary
Clear, correct definitions of the orbits, launch, tracking, space-weather and sky terms behind every satellite tracker — each linked straight to the live 3D view.
Orbits & orbital mechanics
- LEOLow Earth orbit (LEO) is the region of space from about 160 to 2,000 km above Earth's surface, where satellites circle the planet in roughly 90 minutes.
- MEOMedium Earth orbit (MEO) is the band from about 2,000 km up to the geostationary altitude of 35,786 km, best known as the home of navigation satellites.
- GEOGeostationary orbit (GEO) is the circular orbit 35,786 km above the equator where a satellite's period exactly matches Earth's rotation, so it appears fixed over one spot.
- HEOA highly elliptical orbit (HEO) is a stretched, egg-shaped orbit with a low perigee and a very high apogee, letting a satellite linger for hours over one hemisphere.
- Sun-synchronous orbitA sun-synchronous orbit (SSO) is a near-polar low Earth orbit tuned so the satellite passes over every location at the same local solar time on each visit.
- Geostationary satelliteA geostationary satellite orbits 35,786 km above the equator in step with Earth's rotation, so from the ground it appears to hang motionless in the sky.
- ApogeeApogee is the point in an orbit around Earth where the satellite is farthest from the planet.
- PerigeePerigee is the point in an orbit around Earth where the satellite is closest to the planet.
- Orbital inclinationOrbital inclination is the angle between a satellite's orbital plane and Earth's equator, measured in degrees.
- RAAN & the ascending nodeThe ascending node is where a satellite crosses the equator heading north; RAAN (right ascension of the ascending node) is the angle that fixes that crossing point in space.
- Lagrange pointA Lagrange point is one of five positions in a two-body system, such as the Sun and Earth, where gravity and orbital motion balance so a spacecraft can hold station with little fuel.
- Orbital periodThe orbital period is the time a satellite takes to complete one full orbit, set almost entirely by its altitude.
- Ground trackA ground track is the path traced on Earth's surface directly beneath a satellite as it orbits.
- Satellite constellationA satellite constellation is a group of satellites working together as one system to give continuous or global coverage that a single satellite cannot.
Launch & reentry
- Delta-vDelta-v (Δv) is the change in velocity a spacecraft must produce to perform a manoeuvre — launching, changing orbit or landing — and it is the basic currency of orbital mechanics.
- Max-QMax-Q is the moment during a rocket launch when aerodynamic pressure on the vehicle is at its greatest, usually about a minute after liftoff.
- Atmospheric reentryAtmospheric reentry is the return of a spacecraft or piece of debris into Earth's atmosphere, where friction and compression heat it to thousands of degrees.
Tracking & data
- GNSSGNSS is the umbrella term for the satellite constellations that provide positioning, navigation and timing — GPS, Galileo, GLONASS and BeiDou.
- Ground sample distanceGround sample distance (GSD) is the real-world distance on the ground covered by one pixel of a satellite or aerial image — the core measure of spatial resolution.
- TLEA TLE (two-line element set) is a compact, standardized text format that encodes a satellite's orbit at a specific moment in two 69-character lines.
- SGP4SGP4 is the standard mathematical model that turns a satellite's TLE into a predicted position and velocity at any given time.
- Kessler syndromeKessler syndrome is a scenario in which collisions between orbiting objects create debris that triggers more collisions, cascading until parts of low Earth orbit become hazardous to use.
- ConjunctionIn satellite tracking, a conjunction is a close approach between two orbiting objects — a predicted near-miss operators watch in case a collision-avoidance manoeuvre is needed.
Space weather
- Kp indexThe Kp index is a 0–9 scale of global geomagnetic activity over three-hour periods, and it is the number aurora watchers check to judge tonight's odds.
- Geomagnetic stormA geomagnetic storm is a temporary disturbance of Earth's magnetic field driven by bursts of solar wind and coronal mass ejections from the Sun.
Sky & observing
- AzimuthAzimuth is the compass direction to an object in the sky, measured in degrees clockwise from true north — 0° is north, 90° east, 180° south, 270° west.
- TwilightTwilight is the period before sunrise and after sunset when the sky is partly lit by the Sun below the horizon, split into civil, nautical and astronomical stages.
- AphelionAphelion is the point in a body's orbit around the Sun where it is farthest from the Sun; perihelion is the closest point.
