A launch window is the stretch of time in which a rocket can lift off and still reach its target orbit, planet or spacecraft safely.
A rocket cannot simply leave whenever it is ready. Its target is moving. An orbit is a fixed plane in space while Earth turns underneath it. A planet is moving around the Sun. The launch window is the span of time when a launch can still meet the mission's goals and safety limits.
For a satellite going into a particular orbit, the launch site has to pass under the right orbital plane. Earth's turning carries Sriharikota through that position at certain times each day. NASA notes that a launch window may be limited to a number of hours each day, to make best use of Earth's rotation.
Earth's spin gives a free push to rockets that launch towards the east. At the equator the ground moves at about 1,670 km/h, and NASA notes that a rocket launched there needs less propellant than one launched far from the equator.
Missions to other planets have much tighter windows. Earth and Mars must be lined up so that a spacecraft can travel between them efficiently. NASA's Mars fact sheet gives the time between these line-ups, the synodic period, as 779.94 days — about 26 months.
India's Mars Orbiter Mission lifted off on PSLV-C25 from the First Launch Pad at Sriharikota on 5 November 2013, at 14:36 IST. Its launch opportunity stretched only to 19 November. PSLV was to place it in an orbit of 250 × 23,500 km, inclined at 19.2°. Missing the window would have meant waiting about two years for the next one.
This is why ISRO announces launches at precise times — PSLV-C61, for example, was scheduled for 05:59 IST on 18 May 2025. NASA defines the window as the time in which a launch can take place while still meeting the mission's safety and objectives — so a delay past its end means waiting for the next one.
Check the lift-off time listed for each upcoming launch — most are set to the minute because of the launch window. See upcoming ISRO launches
Synodic period. Earth goes round the Sun in 365.256 days, Mars in 686.98 days. The time between repeat alignments S satisfies
1/S = 1/365.256 − 1/686.98 = 0.0027378 − 0.0014556 = 0.0012822 per day
so S ≈ 780 days, matching NASA's 779.94 days.
Free speed from Earth's spin. Equatorial speed = 2πR / T = 2π × 6,378 km / 23.9345 h ≈ 1,674 km/h ≈ 0.465 km/s. At latitude φ it is 0.465 × cos φ km/s. A rocket launched due east picks this up for free.
Launch azimuth and the lowest inclination. NASA's Ares I trajectory paper gives the first-guess launch direction (azimuth Az, measured from north) for a target inclination i from a site at latitude φ:
sin Az = cos i / cos φ
Take a hypothetical site at 14° N and a target inclination of 20.8° (the transfer orbit NVS-02 reached from Sriharikota in 2025): sin Az = cos 20.8° / cos 14° = 0.9348 / 0.9703 = 0.963, so Az ≈ 74°, a little north of east.
Since sin Az cannot exceed 1, cos i must be ≤ cos φ, which means i ≥ φ. A rocket launched directly cannot reach an orbit inclined less than its site's latitude. Lower inclinations need a costly turn in flight or a later plane change.
Daily windows. Because an orbital plane is fixed in space while Earth rotates once every 23.93 hours, a given site passes through a given plane at most twice a day. Mission planners pick the pass that suits the orbit and lighting, which fixes the lift-off time.
Earth and Mars must be in the right relative positions for an efficient transfer. That alignment repeats every synodic period, about 780 days.
sin Az cannot exceed 1, so cos i ≤ cos φ, which requires i ≥ φ.
On 5 November 2013 at 14:36 IST, on PSLV-C25 from the First Launch Pad at Sriharikota.