Core Vocabulary

These are the terms you need to read anything in this field — grouped by theme, because the grouping itself teaches structure. The ten marked ★ appear constantly in practitioner conversation; internalize those first, skim the rest, and come back to this page whenever you need it.

Orbits and motion

• Orbit — the path of continuous free fall around a body, sustained by sideways speed. No engine required once you’re in one.
• Delta-v ★ — the total change in velocity a maneuver or mission requires, in meters per second. The universal currency of spaceflight: fuel, vehicles, and missions are all sized from it.
• LEO (low Earth orbit) ★ — orbits up to roughly 2,000 km altitude, circling Earth in about 90 minutes. Home to the ISS, imaging satellites, and the big constellations.
• MEO (medium Earth orbit) — the region between LEO and GEO, around 20,000 km up. Mostly navigation satellites like GPS.
• GEO (geostationary orbit) ★ — the circular orbit 35,786 km up where one lap takes exactly one day, so a satellite appears fixed over one spot on the equator. Prime real estate for broadcast and weather satellites.
• GTO (geostationary transfer orbit) — the elliptical stepping-stone orbit a rocket delivers to; the satellite itself climbs the rest of the way to GEO. Unlike GEO, it’s a transfer path, not a destination.
• SSO (sun-synchronous orbit) — a near-polar LEO that drifts to pass over any given place at the same local time every day, giving imaging satellites consistent lighting.
• Apogee / perigee ★ — the highest and lowest points of an elliptical orbit. A satellite moves slowest at apogee, fastest at perigee.
• Inclination — the tilt of an orbit relative to the equator, in degrees. Expensive to change once you’re up; largely fixed by where and which direction you launch.
• Hohmann transfer — the standard fuel-cheap way to move between two orbits: one burn to stretch your orbit out, a second to circularize at the destination.
• Station-keeping — the small periodic burns that hold a satellite in its assigned orbit against drift and drag. When the station-keeping fuel runs out, the mission is over.
• Deorbit — deliberately lowering an orbit until the atmosphere destroys or lands the spacecraft; the responsible way to dispose of a dead satellite in LEO.

Propulsion

• Thrust — the raw pushing force of an engine, in newtons. Determines whether you can lift off; unlike specific impulse, says nothing about efficiency.
• Specific impulse (Isp) ★ — engine efficiency: how much push you get per unit of propellant, quoted in seconds. Chemical engines reach ~450 s; electric thrusters reach thousands, at the price of tiny thrust.
• Propellant — everything thrown out the back: fuel plus oxidizer. Rockets carry their own oxidizer, which is why they work in vacuum where jet engines can’t.
• Cryogenic propellant — propellant that’s only liquid when extremely cold, like liquid oxygen or hydrogen. High performance, but it boils away and can’t be stored long — yet.
• Hypergolic propellants — fuel-oxidizer pairs that ignite on contact, no igniter needed. Storable for years and ultra-reliable, which is why capsules and satellites use them despite their toxicity.
• Solid rocket motor — propellant cast as a solid block: simple, storable, powerful, and impossible to throttle or shut off once lit.
• Electric propulsion — thrusters (ion, Hall-effect) that accelerate propellant electrically. Ten times the efficiency of chemical engines, thrust measured in millinewtons: months of gentle pushing instead of minutes of violence.

Launch

• Payload ★ — the thing being delivered to orbit. Confusingly, on a satellite the same word means the revenue-earning instrument (the camera, the transponders) as opposed to the bus that supports it.
• Stage / staging ★ — a self-contained rocket section with its own engines and tanks, dropped when empty so the rest flies on lighter. The standard trick for beating the rocket equation.
• Booster — the first stage (or strap-on stages) doing the heavy lifting off the pad; the part Falcon 9 famously lands and reflies.
• Fairing — the nose-cone shell protecting the payload through the atmosphere, jettisoned once the air is gone. Its internal volume limits how big a satellite can be, independent of mass.
• Max-q — the moment of maximum aerodynamic pressure on the climbing rocket, when speed is high but the air hasn’t thinned yet; engines often throttle down to survive it.
• Launch window — the time slot when a launch can reach its target orbit — set by orbital geometry, since your destination plane passes over the pad only briefly each day.
• Expendable / reusable — whether hardware is discarded after one flight or recovered and flown again. The central economic axis of the current era.
• Rideshare — many small payloads splitting one rocket, like a bus. Cheap per kilogram; you accept the bus’s schedule and destination.

Spacecraft

• Bus ★ — the satellite’s chassis and support systems: power, thermal, pointing, computing, communications. Everything that isn’t the payload.
• ADCS (attitude determination and control system) — the sensors and actuators that measure and set a spacecraft’s orientation. “Attitude” means which way it points, not its mood.
• Reaction wheel — a spinning flywheel used to turn the spacecraft without spending propellant: spin the wheel one way, the craft turns the other.
• Solar array — the panels generating a spacecraft’s electricity, sized for the worst case: end of life, in eclipse-shortened daylight, after years of radiation degradation.
• Telemetry — the housekeeping data a spacecraft radios home — temperatures, voltages, statuses. The only window operators have into its health.
• Uplink / downlink — communication to and from the spacecraft. Downlink capacity is a chronic bottleneck: imaging satellites routinely collect more data than they can send home.
• Ground segment — everything terrestrial that makes a satellite useful: antennas, mission control, data processing. Perpetually underestimated in cost.
• Radiation hardening — designing electronics to survive space radiation. Rad-hard chips trail commercial ones by years in performance, which is why some operators fly cheap commercial parts and accept failures instead.

Industry

• Constellation ★ — many satellites operated as one coordinated system, like Starlink or GPS. The shift from single exquisite satellites to mass-produced fleets is the defining industry trend.
• CubeSat — a standardized miniature satellite built in 10 cm cubes. The standard slashed the cost of getting flight heritage and opened space to universities and startups.
• Space debris ★ — defunct satellites, spent stages, and fragments, moving at orbital speed. Tens of thousands of tracked objects and far more too small to track; the commons problem of the field.
• Kessler syndrome — the runaway scenario where collisions create debris that causes more collisions, cascading until an orbital band is unusable. A slow-motion risk, not a Hollywood explosion.
• Conjunction — a predicted close approach between two orbiting objects, prompting screening and sometimes an avoidance maneuver. Operators of large constellations handle these daily.
• Earth observation (EO) — the business of imaging Earth from orbit — optical or radar — for agriculture, insurance, defense, climate. Its bottleneck is turning pictures into answers.
• GNSS — global navigation satellite systems: GPS and its siblings. They broadcast one-way timing signals; your phone only listens, computing position from the time differences.