What Is A Launch Window, And Why Does NASA Use It?

Therefore, Mars missions need to be launched during a short time when the planets are closest, which is why you get groups of missions all launching around the same time — like in the summer of 2020, when the NASA Perseverance rover, the UAE Hope mission, and the Chinese Tianwen-1 mission all launched to Mars around the same time.

This allows spacecraft to use a Hohmann transfer orbit to travel to a target in the solar system efficiently. The launches need to happen at just the right time to leave Earth and travel to their destination just as the target reaches the same position.

Launch windows are tricky because if a mission misses its window, it has to be deferred until another window opens, causing delays that can be extremely expensive. NASA’s Psyche mission to visit a metal asteroid, for example, was supposed to launch around September or October 2022, in order to make the trip to the asteroid belt between the orbits of Mars and Jupiter. But software problems with the spacecraft caused it to miss this launch window. Now the spacecraft will have to wait until at least July 2023 before launching.

And don’t forget there are many other factors in the timing of rocket launches as well. The weather needs to be good, and scheduling needs to take into account any other launches that might be happening around the same time.