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I could be forgetting some, but every space telescope I can think of that has died, has died because it ran out of spare gyroscopes.

The Saturn Orbiter ran out of fuel, but it wasn't a space telescope. And Voyager, new horizons, and the ion one still haven't run out of fuel.

But every single time,they have to balance another telescope on a single gyroscope like a kite in the solar wind. That exoplanet astrometry one was another.

Why don't they just include more gyroscopes to begin with? They're not very heavy, are they?

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It's worth noting that telescopes that have suffered from severe issues with gyroscopes or reaction wheels (e.g. Compton, Hubble, Kepler, Swift) have typically done so only after operating long past their nominal planned lifespans:

  • Hubble's planned lifespan was 15 years; it recently turned 34.
  • Compton was supposed to operate for about half a decade; it was deorbited after 9 years.
  • Swift's nominal mission lasted 2 years; it's now 20.
  • Kepler's initial mission was for 3.5 years; it had reaction wheel issues after 3 and was shut down after 9 because it ran out of fuel (not from further reaction wheel problems).

Now, it's reasonable to assume that a world-class telescope has a good chance of having its mission extended. All the same, it's not worth spending time, money and weight to deal with problems that would likely arise far beyond that lifetime. Hubble was completed well over its initial budget; given the difficulties in funding the dang thing, nobody could justify adding to the bottom line to throw in another backup part.

Compton, for instance, was given the hardware to last five years and hopefully then some; it's awesome that it made it to nine. A mission ending when gyroscopes or reaction wheels fail is usually a great problem to have.

Finally, while problems with these parts have led to the end of a number of missions, they're usually not the reason space telescopes have to be shut down. Funding cuts (GALEX), cooling issues (Herschel, Spitzer, Planck), fuel shortages (Kepler) and computer failures (CoRoT) have all taken out premier observatories -- again, typically at or well past the end of their nominal missions.

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    $\begingroup$ @ayr because when you design something to last for 2 years, guaranteed - that means it has to be super unlikely to stop working before that. And operational time is not some fixed value, it ends when something breaks (or runs out). So you design it with that in mind - and a computer that cannot break for two years will not magically break after 2 years and a day. Same for all the other parts. So it's not as much "errors in predicting life expectancy", it's that the life expectancy the devices are designed to are a minimum. $\endgroup$
    – Syndic
    Commented Jun 19 at 9:48
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    $\begingroup$ "will not magically break after 2 years and a day" No, that's the design-goal of consumer electronics! $\endgroup$
    – TripeHound
    Commented Jun 19 at 10:10
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    $\begingroup$ @ayr It's similar to how you can often eat food that is past its expiration date. The expiration date guarantees, with some degree of certainty, that food will be edible before the expiration date. After that, it's a guessing game. The same holds true for space telescopes, but microbes cause food to "fail" in the order of days, while engineered parts in space fail in the order of years. $\endgroup$
    – AccidentalTaylorExpansion
    Commented Jun 19 at 11:46
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    $\begingroup$ @ayr: If you design something to work for 2 years with as close to 100% probability as you can reasonably get (say, 99.999%), then it will almost automatically also work for for 4 years with 99.99% likelihood, 6 years with 99.9%, 8 years with 99%, 10 years with 90%, 20 years with 80%, and so on. (Numbers are completely made up, but the principle stands.) $\endgroup$
    – Jörg W Mittag
    Commented Jun 19 at 12:34
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    $\begingroup$ @TripeHound Good jab -- with some more serious background: There is a sweet spot for consumer electronics if, say, a low single-digit percentage of devices fail while the warranty is still on. The warranty cases are paid for by the profits from lower overall production cost. Also, because most parts will be off-the-shelf, their MTBF is better known (with lower error margins) than for one-offs. For a billion dollar device, a single digit premature failure rate (if nothing goes wrong!) is unacceptable, and the error margins will be larger, so the error will be on the safe side most of the times. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jun 19 at 14:45
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Exoplanet astrometry mission? Do you mean the exoplanet transit photometry mission, Kepler? Kepler's problem wasn't gyros: it was reaction wheels. They look a bit like large heavy gyros, but they aren't the same thing. A gyro measures rotation, a reaction wheel controls rotation.

A spacecraft has a limited capacity to accommodate large heavy subsystems, so you only fly enough wheels to have high confidence that you will meet your primary mission objectives. If they last into an extended mission, that's great, but you don't design for that.

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  • $\begingroup$ >Exoplanet astrometry mission? Do you mean the exoplanet transit photometry mission," ==[ yes. I thought it measured fine angular position (proper motion). Oops, sorry! $\endgroup$
    – Miss_Understands
    Commented Jun 19 at 2:17
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While not possible relevant to the current gyro issues there was in fact a design issue with the Gyros. The efforts to make flexible conductors flexible left them vulnerable to being dissolved by the damping fluid.

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More prosaically, SOMETHING will eventually end the life of a space mission, and it is far more likely to be something mechanical than software, and they do not come much more mechanical than gyros that must both spin incredibly fast and also achieve very precise measurement (see the Hubble failures where Gyros were mechanically fine, but damaged wiring prevented operation).

This was not unexpected, and why Hubble has had 22 Gyros in it's life while being designed to operate on three.

Not directly relevant, but the other spinning part on space craft, reaction wheels have also forced end of mission decisions after design choices lead to arcing damaging the bearings.

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    $\begingroup$ Worth noting that mechanical spinning gyroscopes are not mandatory, it's a systems design choice with the technology of the era. JWST uses hemispherical resonator gyroscopes that merely vibrate, the lower precision is made up by much more advanced star tracking. Another option is fiber-optic gyros which eliminate moving parts, but are subject to radiation damage. $\endgroup$
    – user71659
    Commented Jun 19 at 18:37

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