A launch window is the period during which a rocket can launch and reach its intended orbit or trajectory within the vehicle’s delta-v budget. Outside the window, the required orbital plane change or phasing maneuver would cost more delta-v than the vehicle can provide.
What determines the window
Orbital plane alignment — Earth rotates, but orbital planes are fixed in inertial space. A launch site passes under the target orbital plane once or twice per day (once for polar orbits, twice for inclined orbits). The window opens when the launch site’s longitude aligns with the target plane. For the International Space Station (51.6° inclination), the launch window from Cape Canaveral is typically 5–10 minutes long, occurring roughly every 24 hours.
Interplanetary windows — reaching another planet requires the launch to coincide with the correct relative positions of Earth and the target. Earth-Mars Hohmann transfers open approximately every 26 months (the synodic period). Missing the window means waiting over two years.
Geostationary orbit — the window depends on the desired longitude slot. From Cape Canaveral, GEO launches have two daily windows (one ascending, one descending node crossing).
Sun-synchronous orbit — requires a specific local time of ascending node. The window is typically a few minutes long, occurring once per day.
Instantaneous vs. extended windows
An instantaneous window has zero duration — the vehicle must launch at one precise second to reach the target without additional delta-v. ISS rendezvous missions typically have instantaneous windows because the vehicle must match the station’s orbital phase precisely.
An extended window lasts minutes to hours. The vehicle can launch at any point within the window, accepting small delta-v penalties for non-optimal timing. Missions to GEO or high orbits often have windows of 1–4 hours.
Related terms
- Delta-v — the velocity budget that determines how far off-nominal a launch can be
- Orbital Mechanics — the physics governing window calculations
- Gravity Turn — the ascent trajectory executed within the window