Rocket launch glossary: terms you'll hear on launch day

A rocket launch webcast is full of shorthand. T-minus is the time left on the countdown clock. NET means no earlier than: the posted time is the earliest try, not a promise. After liftoff you will hear max-Q, then MECO and stage separation, and on many flights the booster's trip back to a landing zone or a droneship.

Two timing ideas are easy to mix up. T-minus can pause during a hold, while L-minus keeps moving with the clock on the wall. A launch window is the span of time on launch day when liftoff still reaches the planned orbit or target. Some windows are a single exact moment; others stay open for minutes or hours. A hold pauses the count, a recycle resets it and runs it again, a scrub calls off the day's attempt, and an abort is a late stop.

This page is a short A-to-Z reference for those terms. Longer explainers already live on T-ZERO: launch windows, why launches get scrubbed, sonic booms, and landing sites. To watch a flight, see how to watch a rocket launch in person, watch a launch live, or add launches to your calendar.

How to use this glossary

Terms are listed A to Z. Each heading has a linkable anchor, so you can jump straight to a word you heard on a webcast. Definitions are short on purpose. Where T-ZERO already has a full explainer, the definition points there instead of repeating it.

Terms A to Z

A

Abort

An abort is a late stop, usually in the last seconds of the countdown and, on some rockets, just after the engines have started but before the vehicle leaves the pad. It normally ends that day's attempt. More on the scrub vs hold vs abort page.

Apogee

Apogee is the farthest point in an Earth orbit. The closest point is perigee. Source

ASDS / droneship

An Autonomous Spaceport Drone Ship (ASDS), often called a droneship, is a SpaceX landing platform stationed in the ocean so a booster can land far downrange. SpaceX lands Falcon boosters either on a droneship or at a landing zone near the launch pads, depending on how much performance the mission needs. Landing sites are listed on T-ZERO's landing sites page. Source

B

Boostback burn

A boostback burn is a firing of the booster's engines after stage separation that turns it around and sends it back toward a landing zone near the launch site. NASA Falcon flight timelines list a boostback burn, then an entry burn, then a landing burn. Source

Booster

The booster, or first stage, is the lower part of the rocket that fires at liftoff and does most of the work of leaving the pad. On SpaceX's Falcon 9, that first stage can then fly back and land for another flight; other rockets expend the booster or recover it in a different way. Source

E

Entry burn

An entry burn is a firing of the first-stage engines as the booster falls back into the atmosphere. On NASA Falcon flight timelines, it comes after the boostback burn and before the landing burn, and it slows the booster for reentry. Source

F

FAA launch license

In the United States, a company that launches a rocket commercially needs a vehicle operator license from the Federal Aviation Administration (FAA) before flight. The FAA reviews safety, environmental, policy, payload, and financial-responsibility requirements under 14 CFR Part 450. Source

Fairing separation

The payload fairing is the nose cone that protects the spacecraft from aerodynamic pressure, heating, and noise as the rocket climbs through the atmosphere. A few minutes into flight, once conditions are safe, the fairing splits away and the payload continues toward orbit. Source

G

Go/no-go poll

A go/no-go poll is a roll call near the end of the countdown in which the launch director asks each station whether its system is "go" for launch. If a required station is "no-go," the team holds, recycles, or scrubs, depending on the problem and the window. Source

GTO (geostationary transfer orbit)

A geostationary transfer orbit (GTO) is an elongated Earth orbit used as a stepping-stone to a geostationary orbit. Its high point, or apogee, is near geostationary altitude, about 36,000 kilometers from Earth's surface. The satellite later fires its own engine at apogee to circularize the orbit. Source

H

Hold

A hold pauses the countdown, and the launch can still happen that day if the hold ends while time remains in the window. Some holds are built into the count on purpose; others start when a problem appears. During a planned hold, the T-minus clock stops, while L-minus keeps advancing. More on the scrub vs hold vs abort page. Source

I

Inclination

Inclination is the tilt of an orbit relative to Earth's equator, measured in degrees. An orbit directly above the equator has 0 degrees of inclination; an orbit that passes over the poles has 90 degrees. Source

Instantaneous window

An instantaneous launch window is a single exact liftoff time. If the rocket cannot leave at that moment, the attempt usually moves to another day. Missions that must meet a spacecraft already in orbit, such as the International Space Station, often have this kind of window. More on the launch windows page. Source

L

L-minus

L-minus is the time remaining until the planned liftoff, tied to the clock on the wall. The "L" stands for launch. During a hold, L-minus keeps advancing even though the T-minus countdown clock has stopped. Source

Landing burn

The landing burn is the final firing of the booster's engines, used to slow it for touchdown on a landing zone or a droneship. On NASA Falcon flight timelines, it is the last of the recovery burns, after boostback and entry. Source

Landing zone (LZ)

A landing zone is a ground pad near the launch site where a booster can land when it returns to the launch area instead of flying out to a droneship. SpaceX lands Falcon boosters on a droneship or at a landing zone near its launch pads, depending on the mission. See T-ZERO's landing sites page. Source

Launch period

A launch period is the set of days on which a mission can launch and still reach its target. The launch window is the time of day, on one of those days, when liftoff works. For a trip to Mars, the Canadian Space Agency notes the launch period may be only two to three weeks every 26 months. More on the launch windows page. Source

Launch window

A launch window is the span of time on a given day when a rocket can lift off and still reach its planned orbit or meet its target. Some windows are a single exact moment; others stay open for minutes or hours. More on the launch windows page. Source

LEO (low Earth orbit)

Low Earth orbit (LEO) is an Earth-centered orbit at an altitude of 1,200 miles (2,000 kilometers) or less. The International Space Station flies in LEO. Source

M

Max-Q

Max-Q is the moment of peak mechanical stress on the rocket, when aerodynamic pressure on the vehicle is at its highest. Source

MECO

MECO stands for main engine cutoff. On most webcasts, it is the moment the first-stage engines shut down, just before the first and second stages separate. Source

N

NET (no earlier than)

NET means "no earlier than." A NET time is the earliest a launch may occur, not a guarantee that it will lift off then. Weather, technical checks, or range issues can still push it later. NASA often posts a launch as NET followed by a date or time. Source

O

Orbit insertion

Orbit insertion is the point at which the rocket, usually with an upper-stage burn, has placed the payload on the intended path around Earth or toward its target. After the final cutoff, the spacecraft can separate and continue on its own. Source

P

Payload

The payload is the cargo the rocket is carrying, such as a satellite, a spacecraft, or a crew capsule. The rest of the vehicle exists to get that payload off the pad and onto the right path. Source

Perigee

Perigee is the closest point in an Earth orbit. The farthest point is apogee. Source

Plume / twilight phenomenon (jellyfish)

When a rocket launches around dawn or dusk, the high-altitude exhaust plume can catch sunlight while the ground is still dark, so the cloud looks bright and sometimes like a jellyfish. NASA notes that this mix of light and shadow is possible at dawn or dusk, and describes a related effect called the twilight phenomenon. Source

R

Range

The range is the launch range: the safety team and the air and sea space downrange of the pad. A boat or aircraft in the hazard area, or a range system that is not ready, can delay a launch. On Florida's Space Coast, that work is done by the U.S. Space Force's Eastern Range. Source

Recycle

A recycle resets the countdown to an earlier point and runs it again, usually after a hold, while time remains in the launch window. NASA's Artemis I wet dress rehearsal, for example, practiced recycling the clock back to the T-10 minute point. More on the scrub vs hold vs abort page. Source

RTLS (return to launch site)

RTLS means return to launch site: the booster flies back and lands at a landing zone near the pad, instead of on a droneship downrange. After engine cutoff and separation, returning boosters can fire a boostback burn to head home. See landing sites and sonic booms. Source

S

Scrub

A scrub calls off the launch attempt for that day, and a new date or time follows. Common causes are weather, a technical problem, a range conflict, or poor conditions where the booster is supposed to land. More on the scrub vs hold vs abort page. Source

SECO

SECO stands for second-stage engine cutoff, the moment the second-stage engine shuts down. Some missions have more than one second-stage burn, labeled SECO-1, SECO-2, and so on. Source

Sonic boom

A sonic boom is the sound produced when something travels faster than sound. Returning boosters can create one as they come back to Earth; whether people on shore hear it depends on where the booster lands. T-ZERO's sonic boom page explains when a launch may produce a boom on land. Source

SSO (sun-synchronous orbit)

A sun-synchronous orbit (SSO) is a nearly polar orbit in which a satellite passes over the same place at the same local time of day, which keeps lighting consistent for Earth observations. The European Space Agency notes that satellites in SSO are usually at an altitude of 600 to 800 kilometers. Source

Stage separation

Stage separation is the moment the spent lower stage is released and the next stage continues the flight. On a two-stage rocket it usually comes just after MECO, and the upper-stage engine starts a few seconds later. Source

Static fire

A static fire is a prelaunch test in which the rocket's engines are fired on the pad while the vehicle stays on the ground. Before a crew flight, SpaceX has fired Falcon 9's nine first-stage Merlin engines in a routine static fire test. Source

T

T-minus

T-minus is the time remaining on the official countdown clock. The "T" stands for time. During a hold, the T-minus clock stops, even though L-minus, the wall-clock time to the planned liftoff, keeps moving. Source

U

Upper stage

The upper stage is the upper part of the rocket that fires after the booster separates and places the payload onto the intended path. On Falcon 9, the second-stage engine can be restarted more than once to place payloads into different orbits. Source

W

Wet dress rehearsal

A wet dress rehearsal is a practice countdown that loads propellant into the rocket, runs the clock, and then stops before liftoff and drains the tanks. The engines are not ignited for launch. NASA used a wet dress rehearsal to practice tanking, a recycle, and a scrub before Artemis I. Source

Sources

Numbers and specific claims in this glossary come from NASA, the FAA, the European Space Agency, SpaceX, NOAA, and the Canadian Space Agency. A source link is given with each sourced term.