This answer to How long was the HST initially supposed to work? in Astronomy SE quotes Assessment of Options for Extending the Life of the Hubble Space Telescope: Final Report as follows:

Hubble was designed with an anticipated 15-year lifetime based on the expected integrity of the main mirror. It was believed that over HST’s 15-year life the space environment in low Earth orbit would cause sufficient degradation of the mirror that the telescope’s light-gathering capabilities would be severely damaged by cosmic rays and orbital debris.

Question: How was the high rate of degradation of Hubble's main mirror initially predicted? Why did it turn out to be lower? Were cosmic rays a realistic concern as a meaningful contribution?

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    $\begingroup$ Many years ago I attended a lecture from one of the Hubble engineers. I was flabbergasted to learn they would allow it to point along the velocity vector direction. He said they were not worried about this. Years later I came to learn that orbital debris strikes can be as likely in nearly every direction, especially as debris accumulates along that orbital altitude. I am surprised that Hubble seemed never to suffer a strike big enough to make the news. Having made small mirrors, I know it would not take much, especially if secondary optics were hit by ricochet instead of the primary directly. $\endgroup$
    – Chris Ison
    Jun 5 at 19:31
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    $\begingroup$ It's best to think of those initial lifespan estimates as minimum guarantees. Those estimates are, by their nature, rather pessimistic. Cassini's prime mission was only 3 years. It orbited Saturn for over 13 years. The Sojourner, Spirit, and Opportunity Mars rovers all far exceeded their "planned" lifetimes before they ultimately failed. NASA and/or Congress will hold hearings / inquests / inquisitions should a vehicle fail to meet that minimum guarantee. There's no problem if a vehicle lasts a lot longer (60 times longer in the case of Opportunity) than the minimum guarantee. $\endgroup$ Jun 8 at 12:29
  • $\begingroup$ @DavidHammen yes that makes sense, you space folks are a generally a conservative bunch when it comes to mission design (except for some notable historical exceptions) For the purposes of this question it will be interesting to see how the estimate was done and how much cosmic rays really contributed to the estimate. $\endgroup$
    – uhoh
    Jun 8 at 12:32
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    $\begingroup$ I suspect that this is one of those things for which the documentation either no longer exists or is buried in some federal archive deeper than the Lost Ark of the Covenant (no longer lost; it was discovered by Dr. Indiana Jones). NASA has retention policies; dry technical memos are typically destroyed after they are no longer needed. The memo you want was probably written in the late 1970s, on a typewriter. A physical copy might have been retained until 2005. That was when the HST reached that 15 year milestone, and there would have been no reason to retain that paper after that. $\endgroup$ Jun 8 at 13:25
  • $\begingroup$ @DavidHammen that's right, I forget just how long ago planning started for the HST, there probably wasn't DOS or PCs... $\endgroup$
    – uhoh
    Jun 8 at 13:33

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