What is a launch window? Instantaneous vs hours explained
A launch window is the time span on launch day when liftoff still reaches the planned orbit or target. An instantaneous window lasts about one second, so a late hold means a scrub; longer windows let the team hold or recycle and launch later the same day.
Instantaneous vs hours: quick comparison
| Window | How long | Typical missions | After a late hold |
|---|---|---|---|
| Instantaneous | About one second | Meeting a space station or another spacecraft | Usually a scrub; try another day |
| Minutes to hours | Several minutes up to a few hours | Flexible orbits, such as many satellite deployments | Hold or recycle and launch later the same day |
Why rendezvous windows are so short
A space station travels in a fixed orbital plane while Earth turns underneath it. The launch pad passes under that plane only briefly each day, and the rocket also has to arrive at the right point to catch up with the station. Leaving even a little late would cost extra propellant or miss the target, so these missions often get a single exact liftoff time.
Launch period vs launch window
A launch period is the range of days a mission can launch, which matters most for flights to other planets that depend on where the planets are. A launch window is the time of day, on one of those days, when liftoff works (NASA, Basics of Space Flight; Canadian Space Agency).
Holds, scrubs and windows
With a long window, a hold or a recycle can still end in a launch the same day as long as time remains. With an instantaneous window, any hold past T-0 means a scrub. Weather, the range, collision-avoidance cut-outs and technical issues can close parts of a window too. More: launch scrubbed? Scrub vs hold vs abort.
How to read the window on T-ZERO
Launch pages and countdowns show the planned liftoff time (T-0), plus the window open and close when Launch Library 2 provides them. Window length comes from the provider or range when it is published. See rocket launches today, the tracker, Florida schedule or watch a launch live.
FAQ
What is a launch window?
The span of time on a launch day when a rocket can lift off and still reach its planned orbit or meet its target. Outside it, the mission would miss its goal or need too much extra propellant.
What is an instantaneous launch window?
A window about one second long, common for missions meeting a space station. If liftoff slips past that moment, the attempt usually moves to another day.
Why are some launch windows hours long?
When the target orbit is flexible, as with many satellite deployments, a range of liftoff times works. Operators can then open a window of minutes to hours and hold or recycle the countdown.
What is the difference between a launch period and a launch window?
The launch period is the set of days a mission can launch; the launch window is the time of day on one of those days.
What happens if a rocket misses its launch window?
With an instantaneous window the launch is scrubbed for the day. With a longer window the team can hold or recycle and try a later time inside the same window.
How does T-ZERO show launch windows?
Each launch page shows the planned liftoff time and, when Launch Library 2 provides them, the window open and close times.
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