If HST has been reduced to 2 fully functioning gyros, I read that they plan to turn off one gyro and run in single-gyro mode to save the second functioning gyro. The Compton Gamma Ray Observatory had three gyros and NASA decided to de-orbit the satellite when one gyro failed. Why did NASA not follow this current strategy for CGRO? Parts of CGRO survive re-entry, do they expect any of HST to survive re-entry? Is that the reason?

  • $\begingroup$ They are completely different spacecraft with different missions, so there is no reason to assume that one can do something useful in this mode when the other can. $\endgroup$ Commented Apr 30 at 21:53

2 Answers 2


I'll preface this by stating that I thought this was dumb at the time. I will explain why.

The official reason for deorbiting GRO when it was fully operational but one-failure-away from an uncontrolled reentry was that

  • Due to its large size and estimate of large fragments (1 ton) reaching the ground, "The risk of a public death or serious injury due to an uncontrolled entry from its 28.5° inclination was an estimated 1/1000. The much-improved risk associated with a controlled entry was an estimated 1/29,000,000."

NASA could not stomach the 1/1000 risk level and so decided to deorbit it after the single failure.

The official NASA allowable risk is 1/10,000 per NASA-STD 8719.14A, Requirement 4.7-2. So by that criterion, they made the correct decision.

I would also like to point out that GRO had a propulsion system and was therefore capable of purposefully deorbiting itself as long as it could control its attitude.


Now check this out: the risk estimate for an uncontrolled Hubble entry is 1/250. But Hubble is much more loved and takes prettier pictures in the public's eye than GRO did. Also, the original plan was for it to be retrieved by the shuttle and landed for display in a museum.

Now I would like to point out that Hubble never had a propulsion system and was therefore never capable of purposefully deorbiting itself whether it could control its attitude or not (although to be fair, if it lost attitude control, any of the disposal options that involve rendezvous and docking/grappling would be much more difficult.) Hence the original plan for shuttle retrieval.

There are still options on the table for Hubble disposal (detailed in the paper linked below) but as it says, the current baseline option is "do nothing".


Everything above this is factual, but I'll conclude with my opinion, which is that NASA is willing to wait and try to come up with something in panic mode when and if HST irretrievably loses attitude control because it's beloved by the public, but they were not willing to do that with GRO because it was largely unknown to the public.

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    $\begingroup$ Hubble is also in a much higher orbit - we likely have decades after loss-of-control to decide what to do, compared to a single year for GRO. $\endgroup$
    – asdfex
    Commented Apr 30 at 7:13
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    $\begingroup$ @asdfex HST 340 miles, GRO 317 miles; is that "much higher"? $\endgroup$ Commented Apr 30 at 11:13
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    $\begingroup$ Wikipedia has GRO with 360 km and HST with 540 km. The latter lost only 20km in 10 years while the first would deorbit in 1-2 years, like trash from the ISS. $\endgroup$
    – asdfex
    Commented Apr 30 at 11:22
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    $\begingroup$ @Joshua: The Polaris Program has offered to reboost or service Hubble at no charge multiple times. SpaceX, Polaris, and NASA have performed an unfunded study: nasa.gov/missions/hubble/… Not sure what the results are, though. So, it seems to me that there is interest in a boost or service mission by NASA, but no interest in spending a single cent on it. $\endgroup$ Commented May 1 at 9:26
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    $\begingroup$ @JörgWMittag agreed, NASA installed the soft capture device on HST and then cancelled the work to develop the active half of the mechanism. $\endgroup$ Commented May 1 at 10:46

Besides the safety arguments already discussed in another answer, there was probably also a political angle in play. This has to do with the desire of NASA to convince Russia to get rid of the Mir space station, on the one hand disposing of it safely but also freeing up resources that could then be redirected to ISS. By demonstrating the willingness and ability to do a controlled reentry of a working large NASA satellite, the likelihood of Russia doing the same with Mir might be increased.

By its nature, this kind of thing is unlikely to be widely discussed publicly, but it made it to news reports such as one by SpaceViews which references a CBS article. However, these articles focus on the safety aspects of deorbiting Mir, not the budget implications of its decommissioning for which I did not find references, unfortunately. I heard this discussed by colleagues involved with CGRO when I was at MPE as a graduate student, so this is third hand information at best. There is a collection of news articles related to the deorbiting of CGRO on the MPE web page where more discussion may be found.

CGRO was reboosted twice and the orbit was estimated to last until 2007:

  • 1993: 340 km → 450 km
  • 1997: 440 km → 515 km
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    $\begingroup$ Interesting take on it! I forgot to put in my answer that GRO was designed to be refueled and serviced by the shuttle, so they certainly could have fixed it if they wanted to. $\endgroup$ Commented May 1 at 18:44

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