Spaceflight Now
Launch window
The span of time in which a launch can reach the orbit it is aiming for.
A launch site is a moving point on a spinning planet and the target orbit is a fixed plane in space. The two line up for a limited period, which for a rendezvous with the space station is instantaneous — missed by a second is missed — and for a low orbit with no rendezvous can be hours. Interplanetary windows are the extreme case: the transfer to Mars opens for a few weeks every twenty-six months, which is why a slipped Mars launch slips by two years and not by two days.
Where it has come up lately
Universe Today
European Spaceflight
Related terms
Delta-v
The currency of spaceflight: how much you can change your velocity, in metres per second.
NET (no earlier than)
A launch date that is a lower bound, not a promise.
Max-Q
The moment of peak aerodynamic pressure on an ascending rocket.
Staging
Throwing away the empty part of the rocket so you stop accelerating it.
Questions
What is Launch window?
The span of time in which a launch can reach the orbit it is aiming for.
Why does it matter?
The two line up for a limited period, which for a rendezvous with the space station is instantaneous — missed by a second is missed — and for a low orbit with no rendezvous can be hours. Interplanetary windows are the extreme case: the transfer to Mars opens for a few weeks every twenty-six months, which is why a slipped Mars launch slips by two years and not by two days.