A small dictionary
Space words
These are the calls you hear on a countdown. Tap one on a launch, a timeline, or launch day, and the card uses the kids line or the grown up line to match the switch in the corner. Words you open stay in this browser.
On this device
My space words
Tap a word on a launch, on the timeline, or on launch day. It stays here, and only in this browser.
The clock
T minus tells you how long until the rocket is supposed to leave.
T is the planned liftoff instant, so T minus ten minutes means ten minutes before that instant.
Liftoff means the rocket has left the ground.
Liftoff is the first motion that carries the rocket off the pad under its own power.
A hold means they paused the countdown to check something, and the rocket is still on the pad.
A hold pauses the countdown while the team checks something, and the clock stays put until they continue or call it off.
A scrub means they decided not to fly today, so the rocket stays on the pad.
A scrub calls off the launch attempt before liftoff, and the rocket stays on the ground for a later try.
A recycle means they set the clock back and get ready to try again.
A recycle sets the countdown back to an earlier step so the team can try again, sometimes inside the same window.
An abort means the launch stops before leaving, or the crew ship flies away to keep people safe.
An abort stops the launch before the rocket leaves, or fires a crew escape system to carry people away from a failing rocket.
Go for launch means the team said yes and the countdown can keep going.
Go for launch is the call from the launch director that the team is ready for the countdown to run through liftoff.
NET means the soonest they might launch, and it can still move later.
No earlier than is the soonest a launch will be tried, and the time can still slip later.
A static fire lights the engines while the rocket is still held to the pad, then turns them off.
A static fire lights the engines while the rocket is held down on the pad, then shuts them off, and it is not a launch.
The climb
Max Q is when the air pushes on the rocket as hard as it will.
Max Q is the moment in the climb when speed and air density together push harder on the vehicle than at any other point.
MECO means the big engines turn off on purpose while the rocket is still flying.
MECO is the planned shutdown of the first stage main engines, just before that stage lets go.
Stage separation means the bottom lets go of the top, and the top keeps flying.
Stage separation is the moment the used lower stage releases the upper stage so the upper stage can keep climbing.
The first stage is the big bottom of the rocket that lifts it off the ground.
The first stage is the bottom of the rocket, with the engines that do most of the work of leaving the pad.
The second stage is the smaller top of the rocket, and it keeps going after the bottom lets go.
The second stage is the smaller upper stage that takes over after the first stage drops away and finishes the climb toward orbit.
SES means the engine on the top part lights and keeps climbing.
SES is the lighting of the upper stage engine after the booster has dropped away, including a later restart.
SECO means the smaller engine on top turns off, and the cargo is on the path they wanted.
SECO is the shutdown of the upper stage engine when that burn has reached the planned speed and path.
The fairing is the cover that protects the cargo from the wind, and then it comes off.
The fairing is the clamshell cover that protects the payload from the air during the climb, then comes off once the air is thin.
Fairing separation means the nose cover opens and drops away so the cargo can come out.
Fairing separation is the split and release of that nose cover once the air is too thin to harm the satellite.
LOX is oxygen made so cold it turns into a liquid, and the engines mix it with fuel.
LOX is oxygen chilled until it is a liquid, and engines burn it with the fuel.
Propellant is what the engines burn, including the very cold oxygen.
Propellant is the fuel and the oxidizer the engines burn, and on many rockets the oxidizer is liquid oxygen.
Coming back
A booster landing means the bottom of the rocket comes back and tries to land standing up.
A booster landing is the first stage restarting its engines to set down upright, either near the pad or on a ship at sea, so it can fly again.
RTLS means the booster turns around and lands back near the launch pad.
RTLS means the booster turns around and lands on a pad near the site it launched from, which it can do only when enough propellant is left.
A droneship is a ship with nobody aboard that waits on the ocean so the booster can land on it.
A droneship is an uncrewed ship downrange with a landing deck, used when the booster cannot carry enough propellant to fly all the way back to shore.
ASDS is the ocean ship a booster lands on, and nobody is aboard for the landing.
ASDS is the formal name for a SpaceX droneship, an uncrewed barge positioned at sea for a booster landing.
The entry burn lights the engines on the booster on the way down so it does not fall too fast.
The entry burn is a short firing as the booster falls back into thicker air, to slow it and steer it before the landing burn.
The landing burn is the last burst of the engines so the booster can set down softly.
The landing burn is the final engine firing just above the pad or the ship that slows the booster for touchdown.
Expended means this part of the rocket is not coming home, because it flies only once.
An expended stage is not recovered. It falls into the ocean or burns up after this single flight.
A splashdown means coming down in the ocean.
A splashdown is a landing in the water, used by some crew capsules and by stages that are not trying to stand on a deck.
Downrange means farther ahead, the way the rocket is flying, usually over the ocean.
Downrange means farther along the flight path, away from the pad and usually out over the ocean.
Orbits
LEO is a lap around Earth that stays fairly close, and the space station flies there.
LEO is a path from roughly 160 to 2,000 kilometers up, where the space station and constellations such as Starlink fly.
GTO is a stretched path that carries a satellite most of the way to the high parking spot, and the satellite does the last push.
GTO is a stretched path with a high point near geostationary height, so the satellite can circularize that orbit with its own engine.
GEO is a very high path where a satellite can stay over one spot because it circles as fast as Earth turns.
GEO is a circular path about 35,786 kilometers above the equator, where a satellite circles once a day and stays over one longitude.
SSO is a path nearly over the poles that sees the ground in the same kind of daylight each day.
SSO is a near polar orbit whose plane drifts with the seasons so the satellite crosses places in about the same local sunlight each day.
A polar orbit goes over the top and bottom of Earth, so the satellite can see almost the whole world.
A polar orbit passes over or near both poles, so Earth turns underneath and the satellite can eventually see most of the planet.
Apogee is the farthest that path gets from Earth.
Apogee is the highest point of an orbit around Earth, where the satellite is farthest from the planet.
Perigee is the closest that path gets to Earth.
Perigee is the lowest point of an orbit around Earth, where the satellite is closest to the planet.