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Hubble telescope hit by mechanical failure
The Hubble Space Telescope is operating with only essential functions after it lost one of three gyroscopes needed to point the spacecraft. The observatory, described as one of the most important scientific instruments ever created, was placed in "safe mode" over the weekend, while scientists try to fix the problem. Hubble had been operating with four of its six gyroscopes when another failed on Friday.
[...] Operators will now try to revive a gyroscope that malfunctioned when flight controllers tried to bring it online to replace the failed one.
At any given time, Hubble needs three of its gyroscopes to work for optimal efficiency. If the "misbehaving" gyroscope turns out not to work, the orbiting observatory may have to operate on one. This would conserve the remaining gyros for as long as possible, but would restrict the telescope somewhat.
Dr Rachel Osten, deputy mission head for the Hubble Space Telescope, tweeted: "Very stressful weekend. Right now HST is in safe mode while we figure out what to do. Another gyro failed. First step is try to bring back the last gyro, which had been off, and is being problematic."
Also at Space.com.
(Score: 0) by Anonymous Coward on Tuesday October 09 2018, @08:33AM (2 children)
I've seen people saying that three gyros are the best (which intuitively makes sense if you have one for each axis), and that two gyros isn't much better than one, and that they are prepared for this because they knew that the gyros would eventually wear out anyway. But I haven't seen exactly what capabilities are lost as a result of going from three gyros to one.
(Score: 3, Informative) by VLM on Tuesday October 09 2018, @12:11PM (1 child)
Hubble takes a lot of long duration pix so its gotta stand still quite accurately. As a simplification you can either take qty 1000 one second photos or qty 1 one thousand second photos. On the earth urban astronomer dudes do the former and stack photos. Obviously it takes a very stable platform to take a 1000 second photo so use gyros. On the ground the bandwidth to xfer 1000 photos is irrelevant but in space its a big deal. I don't know if hubble is bandwidth constrained (data link always 100% busy) but certainly it'll be using more bandwidth to get the stackable short exposures.
As for the number of gyros, the stability of a gyro varies by angle and in the really old days of the manned space program they actually launched with three gyro platforms and went to some manual effort to avoid gyro lock. Things are a little more sane now a days.
A good engineering estimate is 100 kilo-hours is a mere one decade, so when you debate between spinning gyros where the bearings wear out or laser diodes for a laser ring gyro that only last 100K-hrs, its kinda a toss up. Supposedly as of "recently" old fashioned spinning gyros were still more accurate than laser ring gyros. If you could make a laser that would never wear out (how?) then you could make a LRG that would work forever, having no moving parts.
We're not losing "too much" in the sense that hubble is re-entering in about a decade because we have no way to boost it. Its been up there 28 years, its had a pretty good run. If all the gyros fail such that its out of control, I'm not sure a service mission would be possible if its just tumbling about, or at least it would be riskier.
(Score: 5, Informative) by VLM on Tuesday October 09 2018, @12:27PM
Or maybe rephrased with 3 to 6 gyros you can take as long of a photo as you want as long as the sun or earth don't pass thru the image LOL. But with 1 gyro everything is going to appear as a short arc, so you gotta take 100 short photos and stack them, which will burn a lot of bandwidth.
There are a lot of boring technical arguments about which photo strategy is better, but generally a single long duration photo will win, given they designed the CCDs on the hubble to optimize for that.