Brilliant work by NASA's Boffins
Really excellent how they manage to solve problems in craft far beyond their physical reach
NASA's rough month is improving somewhat: the American space agency is spinning up a spare gyroscope to bring the orbiting Chandra X-Ray Observatory back online by the end of the week, and it reckons it can wake the Hubble Space Telescope soon. Chandra automatically and unexpectedly entered “safe mode” late last week, sending …
"If the Hubble team can implement solutions from the ground to compensate for the problem, the space 'scope will return to three-gyro operations – if not, one of the gyros now in service will be parked and the telescope will be put into single-gyro mode."
So, three gyroscopes are good, one is ok, but two are not an option?
The optimal number is, of course, three because if one disagrees with the two others its results can be ignored in favor of the most common result.
Independantly of whatever argument there may be about using two, I think the point is that one of the three has gone bad, two are left and you need to have a spare. So they're shutting down the faulty one, keeping one as a spare and only using one gyro. When that one fails, the spare will pick up the job and Hubble will continue to be useful.
That's how I see it anyway.
So, three gyroscopes are good, one is ok, but two are not an option?
Pretty sure its down to a combination of:
* you can't do anything with none, so when Hubble is down to its last two gyros, the plan is to only use one of them at a time to preserve their lifetime further.
* the quality of results you get with two gyros is similar to with one gyro, 3 is much better.
You need three gyros ideally to sense movement on three different axis which is obviously the smallest number for accurate pointing. (There was six gyros originally in three pairs, the three older ones have failed).
If you'd like the full science on why a single gyro is almost as preferable as two-gyro, there's a whole paper on just that subject here.
Following gyro failures in April 2001 and April 2003, HST Pointing Control System engineers designed reduced-gyro control laws to extend the spacecraft science mission. The Two-Gyro Science (TGS) and One-Gyro Science (OGS) control laws were designed and implemented using magnetometers, star trackers, and Fine Guidance Sensors in succession to control vehicle rate about the missing gyro axes. Both TGS and OGS have demonstrated on-orbit pointing stability less than 7 milli-arcseconds, which depends upon the guide star magnitude used by the Fine Guidance Sensor. This paper describes the design, implementation, and on-orbit performance of the HST reduced-gyro control system.
Clapp, B.R. J of Astronaut Sci (2009) 57: 419. https://doi.org/10.1007/BF03321511
It's behind a paywall though :(
"I think I'll believe in them when NASA is put onto designing and programming the electronics."
That's the same NASA whose "Break not at all if possible, fix judiciously" policies resulted in launches with venting O-ring joints and bits of foam striking the wings, on the basis of "It hasn't caused a problem so far"
A little of A and a little of B. Some gets dropped (some experiments are time sensitive for instance) some get reallocated time asap, some have to fight over priority. Theres also a few blank spots in the schedule that can be assigned to "targets of opportunity" that can be used for cases like this
It would be a shame for NASA to not be thinking how they could do another servicing mission to Hubble using either SpaceX or Beoing's capsules to replace all six gyros with new ones, along with any other instrument upgrades that could be done easily. It's obvisouly not nearly as easy to do as it was with the Shuttle, but still... it would be a useful extension if possible.