GRACE mission which was launched on March 17th, 2002 had a 10 minutes launch window. The mission was to launch a couple of earth observation satellites.

And interestingly, NASA's GRACE Launch Press Kit, March 2002 mentions that the launch window keeps moving earlier by 2mins 12s on every subsequent day.

Launch timing

On March 16, 2002, the 10 minute launch window opens at 12:23 p.m. local time in Plesetsk (4:23 a.m. EST). If launch does not take place on March 16, the window on subsequent days falls earlier by 2 minutes, 12 seconds each day

Given satellites were launched from earth to observe earth, why would it require such a narrow launch window per day?

  • 2
    $\begingroup$ The -2:14 is about how fast sunrise is changing there and then. Perhaps they wanted a constant solar angle during the early part of the flight. Unfortunately, the fact they launched near noon argues against this. $\endgroup$ Jun 30, 2019 at 21:28
  • 1
    $\begingroup$ GRACE is supposed to track the movement of water on earth so maybe they wanted to synchronize it with some kind of meteorological process in the atmosphere. (e.g.: tracking specific could formation over a day?) $\endgroup$
    – GittingGud
    Jul 1, 2019 at 6:41
  • $\begingroup$ I'm guessing the mission's trajectory was influenced significantly enough by the Moon and Sun pertubations that there is an ideal condition that needs to be optimised with that in mind. These are two spacecraft flying in formation, so it would not be surprising for there to be an obscure trajectory, lighting, communications, or other requirement on launch. $\endgroup$ May 20, 2020 at 8:23

1 Answer 1


There is an extremely detailed paper on this, where a specifically designed tool, the GRACE Orbital Analysis Tool (GOAT) was developed to look at the 5 year orbital stability of a number of potential candidates. GRACE requires two spacecraft to stay close to each other for an extended period of time with minimal input, making the orbital design quite tricky.

The short version is, a long lifetime was desired, and each of the 6 Keplarian elements was varied to find a version where that time was maximized. Specifically, a phase angle of 40 degrees with respect to the Sun was desired. That basically requires launching at the same time of solar day. The mission was launched in the Spring, which has a quickly varying time of day. The small window ensured it would have an optimal lifetime.

  • $\begingroup$ I suspected something like this, but this paper (more of a thesis) never popped up in my searches. Great find! $\endgroup$
    – Ludo
    Dec 18, 2020 at 15:48
  • 1
    $\begingroup$ @OrganicMarble Some people are the best with coming up with tool names... $\endgroup$
    – PearsonArtPhoto
    Dec 18, 2020 at 15:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.