What is a launch window?
A launch window is a defined time interval during which a rocket can lift off from its launch site and reach the target orbit with an acceptable trajectory. The window is determined by orbital mechanics: Earth rotates beneath the target orbit, and the launch site passes through the correct geometric alignment only during specific time intervals each day — sometimes for just seconds (instantaneous window), sometimes for hours.
What constrains the window?
Orbital inclination: The latitude of the launch site limits the orbital inclinations directly achievable. Launching from Kennedy Space Center (28.5°N) to a 28.5° inclination orbit is most efficient; reaching a 90° polar orbit from Kennedy requires a dog-leg manoeuvre costing propellant. Phase matching: For rendezvous missions (ISS resupply, crewed docking), the target station must be in a specific phase of its orbit at the moment of launch for the trajectory to be feasible with minimum fuel expenditure. This may constrain the window to literally seconds. Range safety: Rockets must not overfly populated areas or shipping lanes with their spent stages; acceptable ground tracks define time windows relative to Earth's rotation. Lighting conditions: Some missions require dayside or specific shadow conditions at orbit insertion.
Implications for operators
Missing a launch window — due to weather, technical issues, or range conflicts — typically delays launch by one or more days (for non-time-critical missions) or requires a trajectory redesign (for rendezvous missions). Rideshare launches are particularly constrained by the primary mission's window requirements. Planning for launch window opportunities is a critical part of constellation deployment scheduling.