Launch glossary

Following a launch means reading two languages at once: the schedule's vocabulary of dates and statuses, and the broadcast's vocabulary of flight events. This glossary covers the terms you will meet in both, written for someone planning to watch — not for an engineer. Where providers use a term slightly differently, the operator's own usage wins.

Scheduling and the countdown

NET (No Earlier Than) — The earliest date a mission is currently targeting. It is a planning marker, not a promise: missions routinely move later than their NET date, and a NET date can sit alongside a confident-looking countdown. Treat it as "not before this day, and quite possibly after."

T-0 — The planned moment of liftoff, the zero point of the countdown. Everything in the final minutes is measured against it.

T-minus / T-plus — Time before and after T-0. "T-minus 10 minutes" means ten minutes remain in the count; "T-plus 2 minutes" means the flight has been underway for two minutes. A hold freezes the T-minus clock; a scrub ends the attempt.

Launch window — The interval during which a flight can depart and still reach its destination. Some missions have a window of hours; others have an instantaneous window — a single second — dictated by the geometry of the target orbit. Missing the window usually means waiting for another day, and tomorrow's window often opens at a different time.

Hold — A pause in the countdown. Holds can be built into the timeline on purpose, giving the team time for checks and to align the count with the window. An unplanned hold means something needs attention. A hold does not end the attempt by itself.

Scrub — The launch attempt is called off for the day. Unlike a hold, a scrub ends that try; the team stands down and works toward a new target, often a day or more later. Weather, a technical issue, or a range problem are common causes.

Recycle — Resetting the countdown to an earlier point to try again within the same window, after a hold is resolved. If you hear "recycling," the attempt is still alive.

Launch commit criteria — The set of conditions that must all be met for the flight to proceed: weather limits, range readiness, vehicle health, and more. Any single violated criterion can hold or scrub the count, which is why a perfect-looking forecast at the pad is never the whole story.

Static fire — A test in which the rocket's engines are briefly ignited while the vehicle stays held down on the pad. It verifies the engines and ground systems before launch day. A successful static fire is a milestone, not a guarantee.

Wet dress rehearsal — A full practice countdown with propellants loaded, stopped before ignition. It tests the vehicle, the ground equipment, and the team together. "Wet" refers to the cryogenic propellants on board.

Flight events

Liftoff — The moment the rocket leaves the pad. It is the beginning of the flight, not its conclusion: nothing about the payload's fate is settled at liftoff.

Max-Q — The point of maximum aerodynamic pressure, usually about a minute into flight, when the combination of speed and thick lower atmosphere stresses the vehicle most. Throttling down through Max-Q is routine; commentators mention it because it is a milestone the vehicle must pass, not because it is dangerous by itself.

MECO (Main Engine Cutoff) — Shutdown of the first stage's engines. What happens next depends on the mission: the stage separates, and it may head back for recovery or fall away.

Stage separation — The moment the spent first stage detaches from the upper stage carrying the payload. On the broadcast, watch the upper stage from here on: it is the part still flying toward orbit.

SECO (Second Engine Cutoff) — Shutdown of the upper stage's engine(s) after the planned burns. A coast phase may follow before another burn or before deployment.

Fairing separation — Jettison of the payload fairing, the protective nose cone shielding the satellite during ascent through the atmosphere. It typically happens once the air is thin enough that protection is no longer needed — and it often gives viewers the first actual glimpse of the payload.

Payload deployment — Release of the satellite or spacecraft from the upper stage into its orbit. This is the result the mission was built for. Until the operator confirms separation and a healthy signal, the mission is not complete, no matter how good the ascent looked.

Recovery

Boostback burn — An engine burn that reverses a returning booster's downrange motion and sends it back toward the landing site. Not every recovery profile uses one.

Entry burn — A burn during atmospheric reentry that slows and protects the booster as it meets the atmosphere at high speed.

Landing burn — The final burn that slows the booster to a touchdown. Watching it succeed is spectacular; it is also entirely separate from the payload's mission.

RTLS (Return to Launch Site) — A recovery in which the booster flies back and lands near where it launched. It requires the mission to reserve enough propellant for the return trip.

Droneship — An autonomous ocean platform used as a landing target when the booster cannot return to land. Landings at sea let missions with heavier payloads or more demanding orbits still recover the stage.

Orbits

LEO (Low Earth Orbit) — Orbit at a few hundred kilometres of altitude, where the International Space Station flies and where most Starlink-style constellations operate. It is the easiest orbit to reach and the most crowded.

SSO (Sun-Synchronous Orbit) — A near-polar orbit in which the satellite passes over any given point at roughly the same local solar time each day. Favoured by Earth-observation missions because lighting conditions stay consistent from pass to pass.

GTO (Geostationary Transfer Orbit) — An elliptical transfer orbit used as a stepping stone: a satellite is dropped in GTO and then raises itself into its final geostationary slot under its own power.

GEO (Geostationary Orbit) — An orbit about 35,786 km above the equator where a satellite circles Earth at the same rate the planet rotates, appearing fixed over one spot. Home to communications and weather satellites.

Polar orbit — An orbit passing over or near the poles, letting a satellite eventually see the entire planet as Earth rotates beneath it.

Apogee / perigee — The highest and lowest points of an orbit around Earth. Operators often quote both to describe how elliptical an orbit is.

Inclination — The tilt of an orbit relative to the equator, measured in degrees. It determines which parts of the planet a satellite can fly over.

Suborbital — A flight that reaches space (or near it) but does not complete an orbit, arcing back down instead. Test flights and some tourism flights are suborbital; they are a different category from orbital missions.

Mission and operations

Rideshare — A launch carrying multiple payloads for different customers on one rocket, sharing the cost. Dedicated rideshare missions, such as SpaceX's Transporter flights, exist specifically for small satellites.

Range — The organisation and infrastructure responsible for launch safety at a spaceport, including tracking, flight-termination systems, and clearance of the surrounding airspace and waters. The Eastern Range covers Florida launches; the Western Range covers Vandenberg. A "range violation" — a boat or aircraft in the wrong place — can scrub a launch.

Downrange — The area along the flight path beyond the launch site. Expended stages fall in downrange zones, and droneships station themselves downrange of the pad.

Operator — The organisation providing the launch service. It is not necessarily the organisation that owns the satellite: a SpaceX rocket can carry another company's payload, and the schedule lists SpaceX as the operator either way.