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The Wikipedia article on the Hubble telescope mentions that during the aftermath of the Challenger disaster, the telescope had to be stored for a few years until the Space Shuttle launches could be resumed. It is mentioned that:

The telescope had to be kept in a clean room, powered up and purged with nitrogen, until a launch could be rescheduled.

I understand the clean room part, and this question and answer addresses why the purging with nitrogen was necessary. But why did the telescope had to be powered on all the time? Does this also mean it had to be powered on during launch?

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  • $\begingroup$ When something is in storage for 4 years, you want to turn it on some times to make sure it functions properly. That might be the reason. $\endgroup$ – Star Man Aug 21 '19 at 22:22
  • $\begingroup$ I don't have any evidence to make a real answer but I believe that some of the instruments had to be temperature controlled. I is also likely that they wanted to keep the gyros running since there would be the chance of failure on startup. $\endgroup$ – ShadoCat Aug 22 '19 at 0:17
  • $\begingroup$ @ShadoCat according to nasa.gov/content/goddard/… the entire HST was powered off when the Power Control Unit was replaced, so the instruments could at least be powered off in space. Temperature control thus is a possibility. I don't think gyros need to stay running at all times to work. At least, I would assume they would be started up once in orbit... $\endgroup$ – Ludo Aug 23 '19 at 14:51
  • $\begingroup$ I read through the book The Universe in a Mirror by Zimmerman which is a detailed history of the telescope's origins. Sadly it does not describe the storage phase in any detail. $\endgroup$ – Organic Marble Aug 24 '19 at 15:26
  • $\begingroup$ @OrganicMarble do you happen to have "The Space Telescope: A Study of Nasa, Science, Technology, and Politics" by Robert W. Smith on your bookshelf? I'm tempted to buy it second hand, but I'll need to fill up my cart to make shipping worthwhile... $\endgroup$ – Ludo Aug 24 '19 at 16:08
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In a somewhat implicit way, Chapter 16 (By Joseph Tatarewicz) of NASA's SP-4219, "From Engineering to Big Science", provides an answer:

While the $6 million per month seems like a lot for storage, the spacecraft had to be kept in a mammoth clean room with active air conditioning and filtering systems operating constantly, some nitrogen purging of areas of the spacecraft, and some of its systems powered and operating. This was not "dead storage," but much more akin to a patient in intensive care. By the time the spacecraft was launched, it had spent almost a third of its intended design life suspended vertically in a gravity and atmospheric environment for which it was not designed, and with many of its systems powered up and running. Rothenberg, however, believes that this period provided opportunity for testing and debugging that ultimately was beneficial.

In other words, keeping Hubble powered on was a choice made at the moment it went into storage; Wikipedia is wrong in so far as that the Wiki article implies keeping Hubble powered up was necessary, which it wasn't. However, keeping the power on allowed for testing, debugging and upgrading.

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  • $\begingroup$ It seems unlikely they would have kept it powered without strong reason. Why not just power it on when you wanted to do one of those tasks and then power it back off? $\endgroup$ – Organic Marble Aug 24 '19 at 12:02
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    $\begingroup$ @OrganicMarble Because of the gyroscopes. Making a gyroscope go through an off-on-off cycle causes wear and tear; keeping it on (or off, for that matter) doesn't. So you have to choose: steady on, or steady off. They chose steady on, and got the benefit of testing, debugging and upgrading. $\endgroup$ – Jan van Oort Aug 24 '19 at 12:09
  • $\begingroup$ Can you confirm that the gyros were powered in storage? Note it says "many of the systems". $\endgroup$ – Organic Marble Aug 24 '19 at 12:23
  • $\begingroup$ @OrganicMarble I have no positive proof of written evidence of that. $\endgroup$ – Jan van Oort Aug 24 '19 at 12:51
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    $\begingroup$ That is actually the reference used on the Wikipedia page - I hadn't realised it was available online. I would suggest to include the preceding sentence in the quote: While the $6 million per month seems like a lot for storage, the spacecraft had to be kept in a mammoth clean room with active air conditioning and filtering systems operating constantly, some nitrogen purging of areas of the spacecraft, and some of its systems powered and operating. This was not a "dead storage"... $\endgroup$ – Ludo Aug 24 '19 at 13:16
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Jan van Oort's answer triggered me to buy Robert W. Smith's "The Space Telescope" that was referenced in his source. This book gives a bit more context.

According to Smith, the extra time was not just "testing and debugging". In particular, the thermal-vacuum tests performed on the fully assembled telescope conducted in May of 1986 revealed a number of issues. In particular, it was found that the spacecraft consumed 100 - 300 Watts more than was predicted, leading to the conclusion that after only a few years in orbit the power consumption would already exceed the production due to degradation of the batteries as they aged.

Other issues arose from the thermal-vacuum testing. None was major in itself, but when taken in combination with the power problem and the very immature state of the ground system, they, in the opinion of many participants, made in highly unlikely that the telescope could have come even close to meeting a launch date of late 1986.

(Smith, p. 370)

(The launch was first scheduled for August, 1986 and then rescheduled to October 27, 1986.)

So keeping the spacecraft in a clean room at Lockheed after the Challenger disaster, was not just storage: it was still being actively worked on. The power system issue had to be fixed and also the safing system was found to be unreliable and modified. In the end so many changes had been performed after the first thermal-vacuum test that at late as 1987 it was considered to do a second thermal-vacuum test. In 1988, they were still working on the ground support systems (Smith, p. 371).

Of course, after all issues had been resolved, presumably somewhere in late 1988, the spacecraft still had to remain in storage for another one or two years until its launch in 1990. It may have been powered on during that period also, but I couldn't find anything on that, nor could I find during that last period it still cost $6-7 million per month to keep it in storage and keep essential staff employed.

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