<p>Exoplanet astrometry mission? Do you mean the exoplanet transit <em>photometry</em> 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.</p>
<p>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.</p>

CC BY-SA 4.0
2024-06-19T02:17:16.477Z
>Exoplanet astrometry mission? Do you mean the exoplanet transit photometry mission," ==[ yes. I thought it measured fine angular position (proper motion). Oops, sorry!
Miss Understands
CC BY-SA 4.0
2024-06-19T01:53:11.123Z
2024-06-19T01:53:11.123Z
John Doty
<p>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.</p>
<p><a href="https://www.youtube.com/watch?v=Ra2IpumLMfs" rel="noreferrer">Video</a> <a href="https://science.nasa.gov/mission/hubble/observatory/design/hubble-one-gyro-mode/" rel="noreferrer">Words</a></p>
<p>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).</p>
<p>This was not unexpected, and why Hubble has had 22 Gyros in it's life while being designed to operate on three.</p>
<p>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 <a href="https://www.youtube.com/watch?v=KibT-PEMHUU" rel="noreferrer">arcing damaging the bearings</a>.</p>

CC BY-SA 4.0
2024-06-19T18:37:23.860Z
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.
user71659
CC BY-SA 4.0
2024-06-19T13:07:31.933Z
2024-06-19T13:07:31.933Z
GremlinWranger
<p>It's worth noting that telescopes that have suffered from severe issues with gyroscopes or reaction wheels (e.g. <em>Compton</em>, <em>Hubble</em>, <em>Kepler</em>, <em>Swift</em>) have typically done so only after operating long past their nominal planned lifespans:</p>
<ul>
<li>Hubble's planned lifespan <a href="https://science.nasa.gov/mission/hubble/overview/faqs/" rel="noreferrer">was 15 years</a>; it recently turned 34.</li>
<li>Compton was supposed to operate for about half a decade; it was deorbited after 9 years.</li>
<li>Swift's nominal mission lasted <a href="https://swift.gsfc.nasa.gov/swift.html" rel="noreferrer">2 years</a>; it's now 20.</li>
<li>Kepler's initial mission was <a href="https://science.nasa.gov/mission/kepler/" rel="noreferrer">for 3.5 years</a>; it had reaction wheel issues after 3 and was shut down after 9 because it ran out of fuel (<em>not</em> from further reaction wheel problems).</li>
</ul>
<p>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 <em>another</em> backup part.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2024-06-19T04:25:32.213Z
I wonder why there are such errors in predicting life expectancy?  When was the device designed for 2 years of operation, but it worked for 20?
ayr
CC BY-SA 4.0
2024-06-19T09:35:13.953Z
see also [comment](https://astronomy.stackexchange.com/questions/57934/why-dont-they-put-more-spare-gyros-in-expensive-space-telescopes#comment132810_57934)
uhoh
CC BY-SA 4.0
2024-06-19T09:48:40.910Z
@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.
Syndic
CC BY-SA 4.0
2024-06-19T10:10:43.010Z
"_will not magically break after 2 years and a day_" No, that's the design-goal of consumer electronics!
TripeHound
CC BY-SA 4.0
2024-06-19T11:46:50.213Z
@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.
AccidentalTaylorExpansion
CC BY-SA 4.0
2024-06-19T12:34:13.730Z
@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.)
Jörg W Mittag
CC BY-SA 4.0
2024-06-19T14:45:13.010Z
@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.
Peter - Reinstate Monica
CC BY-SA 4.0
2024-06-19T22:30:15.853Z
Another consideration for Hubble was that it was designed for maintenance -- no point in putting in extra gyroscopes when you can simply swap out the failed ones during the next servicing mission.
Mark
CC BY-SA 4.0
2024-06-19T23:53:08.657Z
Marked as winning answer. I like when my failures are to due to ignorance and not failure to understand.
Miss Understands
CC BY-SA 4.0
2024-06-20T09:34:24.473Z
@Peter-ReinstateMonica: But lots of time space hardware is so damn expensive because they try to make it 100% reliable at all cost. What if the JWST had been 1/10 the cost at a slightly decreased reliability? We could have launched 10 of them for the same cost and much earlier (and possibly finding unknown unknowns you wouldn’t have discovered in any other way). If the first 3 had failed it would still have been a win. Basically what SpaceX is doing with Starship.
Michael
CC BY-SA 4.0
2024-06-20T10:02:21.580Z
@Michael Not sure Hubble would fit a Falcon 9 fairing (it's close, I think), so may be these days one would consider multiple cheap launches of a number of cheap devices; but back then, Shuttle launches were much more expensive. I'm also not convinced that one could cut the Hubble cost to, say, 1/5th by using cheaper components because the mirror which is non-negotiable may be the most expensive part.
Peter - Reinstate Monica
CC BY-SA 4.0
2024-06-20T10:49:10.320Z
@Peter-ReinstateMonica: The Hubble mirror is actually a great example: It turned out to be manufactured incorrectly and required yet another expensive Shuttle launch to correct (and a 2 year delay if I recall correctly).
Michael
CC BY-SA 4.0
2024-06-20T22:34:01.097Z
@Michael NICER on the ISS is a good example: it's not a single telescope, it's 56. Once you set up a production process and make a few, the cost goes way down: 56 cost maybe 3x what one would have. And then there was a tradeoff between performance and reliability: we could have high performance at 90% reliability, or much lower performance at higher reliability. We went for high performance: a few telescopes failed, but the remaining ones are superb.
John Doty
CC BY-SA 4.0
2024-06-21T16:43:35.957Z
@Michael: The efforts at ensuring reliability often end up yielding increased longevity.  Having a twenty Voyager probes, most of which last ten or more years but which all fail within twenty years would be less useful than having two or even one which lasts forty plus.
supercat
CC BY-SA 4.0
2024-06-22T09:59:41.167Z
@JörgWMittag for many things this principle doesn't hold up. Aside from the obvious batteries etc., for example, rubbery items inevitably degrade to the point of becoming useless within twice-ish their design lifespan, despite performing very reliably before. It just so happens that such materials aren't much used in spacecraft, but this is for only loosely related reasons.
leftaroundabout
CC BY-SA 4.0
2024-08-06T00:16:15.350Z
@AccidentalTaylorExpansion I feel it should be emphasized that there are two different types of expiration dates on food.  One is "best before date", which essentially means exactly what it say, but is not a definite limit on when you can consume it safely.  The other is "use by" dates and they *are* intended to be a "only safe up to this date".
StephenG - Help Ukraine
CC BY-SA 4.0
2024-06-19T02:01:54.607Z
2024-06-19T02:19:07.680Z
2024-06-19T02:19:07.680Z
HDE 226868
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>Why don't they just include more gyroscopes to begin with? They're not very heavy, are they?</p>

CC BY-SA 4.0
2024-06-19T09:34:07.567Z
"I could be forgetting some ..." Here's some: [On 29 April 2013, ESA announced that Herschel's supply of liquid helium, used to cool the instruments and detectors on board, had been depleted, thus ending its mission.](https://en.wikipedia.org/wiki/Herschel_Space_Observatory#End_of_mission) and [Infrared Space Observatory; End of mission](https://en.wikipedia.org/wiki/Infrared_Space_Observatory#End_of_mission) and [Spitzer Warm mission and end of mission](https://en.wikipedia.org/wiki/Spitzer_Space_Telescope#Warm_mission_and_end_of_mission)
uhoh
CC BY-SA 4.0
2024-06-19T09:34:27.400Z
Also see answers to [Which (if any) space telescope would have worked longer if it hadn't simply run out of helium?](https://astronomy.stackexchange.com/q/50359/7982)
uhoh
CC BY-SA 4.0
2024-06-19T23:15:35.630Z
It is worth mentioning that gyro and reaction wheel failures could have been due to  coronal mass ejections (CMEs) from the Sun causing arcing between the metal bearings and races. Space missions have changed from metal ball bearings to ceramic bearings in their reaction wheels, greatly reducing the chance of arcing across the bearing races.
https://hackaday.com/2018/09/11/do-space-probes-fail-because-of-space-weather/
Xavier
CC BY-SA 4.0
2024-06-20T14:09:05.697Z
Hubble had six gyros and got them all replaced during one of the service missions.  That's 12 gyros. How many more do you think it needed?
Darth Pseudonym
CC BY-SA 4.0
2024-06-20T16:57:41.063Z
You load the vehicle with 5 gyros and 1 unit of fuel. The fuel runs and for the next mission you put 2 units on board. This next mission eventually all the gyros fail and for the next one, you put 8. But this next mission all the fuel runs out, so next time, you put 3 units on board, and so the loop continues. At what point do you say enough is enough?
Bob Goddard
CC BY-SA 4.0
2024-06-20T21:57:39.673Z
If the gyros ALWAYS fail first, you always add a gyro. Obviously. If that reduces fuel, it's just an optimization problem. God!
Miss Understands
<space-telescope>
Why don't they put more spare gyroscopes in expensive space telescopes?
Miss Understands
# Why don't they put more spare gyroscopes in expensive space telescopes?

(Asked by: username_0 on 2024-06-19)

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?

**Tags:** space-telescope
> **Comments:**
> **username_1**: "I could be forgetting some ..." Here's some: [On 29 April 2013, ESA announced that Herschel's supply of liquid helium, used to cool the instruments and detectors on board, had been depleted, thus ending its mission.](https://en.wikipedia.org/wiki/Herschel_Space_Observatory#End_of_mission) and [Infrared Space Observatory; End of mission](https://en.wikipedia.org/wiki/Infrared_Space_Observatory#End_of_mission) and [Spitzer Warm mission and end of mission](https://en.wikipedia.org/wiki/Spitzer_Space_Telescope#Warm_mission_and_end_of_mission)  
> **username_1**: Also see answers to [Which (if any) space telescope would have worked longer if it hadn't simply run out of helium?](https://astronomy.stackexchange.com/q/50359/7982)  
> **username_2**: It is worth mentioning that gyro and reaction wheel failures could have been due to  coronal mass ejections (CMEs) from the Sun causing arcing between the metal bearings and races. Space missions have changed from metal ball bearings to ceramic bearings in their reaction wheels, greatly reducing the chance of arcing across the bearing races. https://hackaday.com/2018/09/11/do-space-probes-fail-because-of-space-weather/  
> **username_3**: Hubble had six gyros and got them all replaced during one of the service missions.  That's 12 gyros. How many more do you think it needed?  
> **username_4**: You load the vehicle with 5 gyros and 1 unit of fuel. The fuel runs and for the next mission you put 2 units on board. This next mission eventually all the gyros fail and for the next one, you put 8. But this next mission all the fuel runs out, so next time, you put 3 units on board, and so the loop continues. At what point do you say enough is enough?  
> **username_0**: If the gyros ALWAYS fail first, you always add a gyro. Obviously. If that reduces fuel, it's just an optimization problem. God!  

---

## 3 answers

### Answer by username_8 on 2024-06-19. Score: 53 | Accepted answer

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](https://science.nasa.gov/mission/hubble/overview/faqs/); 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](https://swift.gsfc.nasa.gov/swift.html); it's now 20.
* Kepler's initial mission was [for 3.5 years](https://science.nasa.gov/mission/kepler/); 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.

> **Comments:**
> **username_9**: I wonder why there are such errors in predicting life expectancy?  When was the device designed for 2 years of operation, but it worked for 20?  
> **username_1**: see also [comment](https://astronomy.stackexchange.com/questions/57934/why-dont-they-put-more-spare-gyros-in-expensive-space-telescopes#comment132810_57934)  
> **username_10**: @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.  
> **username_11**: "_will not magically break after 2 years and a day_" No, that's the design-goal of consumer electronics!  
> **username_12**: @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.  
> **username_13**: @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.)  
> **username_14**: @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.  
> **username_15**: Another consideration for Hubble was that it was designed for maintenance -- no point in putting in extra gyroscopes when you can simply swap out the failed ones during the next servicing mission.  
> **username_0**: Marked as winning answer. I like when my failures are to due to ignorance and not failure to understand.  
> **username_16**: @Peter-ReinstateMonica: But lots of time space hardware is so damn expensive because they try to make it 100% reliable at all cost. What if the JWST had been 1/10 the cost at a slightly decreased reliability? We could have launched 10 of them for the same cost and much earlier (and possibly finding unknown unknowns you wouldn’t have discovered in any other way). If the first 3 had failed it would still have been a win. Basically what SpaceX is doing with Starship.  
> **username_14**: @Michael Not sure Hubble would fit a Falcon 9 fairing (it's close, I think), so may be these days one would consider multiple cheap launches of a number of cheap devices; but back then, Shuttle launches were much more expensive. I'm also not convinced that one could cut the Hubble cost to, say, 1/5th by using cheaper components because the mirror which is non-negotiable may be the most expensive part.  
> **username_16**: @Peter-ReinstateMonica: The Hubble mirror is actually a great example: It turned out to be manufactured incorrectly and required yet another expensive Shuttle launch to correct (and a 2 year delay if I recall correctly).  
> **username_5**: @Michael NICER on the ISS is a good example: it's not a single telescope, it's 56. Once you set up a production process and make a few, the cost goes way down: 56 cost maybe 3x what one would have. And then there was a tradeoff between performance and reliability: we could have high performance at 90% reliability, or much lower performance at higher reliability. We went for high performance: a few telescopes failed, but the remaining ones are superb.  
> **username_17**: @Michael: The efforts at ensuring reliability often end up yielding increased longevity.  Having a twenty Voyager probes, most of which last ten or more years but which all fail within twenty years would be less useful than having two or even one which lasts forty plus.  
> **username_18**: @JörgWMittag for many things this principle doesn't hold up. Aside from the obvious batteries etc., for example, rubbery items inevitably degrade to the point of becoming useless within twice-ish their design lifespan, despite performing very reliably before. It just so happens that such materials aren't much used in spacecraft, but this is for only loosely related reasons.  
> **username_19**: @AccidentalTaylorExpansion I feel it should be emphasized that there are two different types of expiration dates on food.  One is "best before date", which essentially means exactly what it say, but is not a definite limit on when you can consume it safely.  The other is "use by" dates and they *are* intended to be a "only safe up to this date".  

---

### Answer by username_5 on 2024-06-19. Score: 12

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.

> **Comments:**
> **username_0**: >Exoplanet astrometry mission? Do you mean the exoplanet transit photometry mission," ==[ yes. I thought it measured fine angular position (proper motion). Oops, sorry!  

---

### Answer by username_6 on 2024-06-19. Score: 12

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.

[Video](https://www.youtube.com/watch?v=Ra2IpumLMfs) [Words](https://science.nasa.gov/mission/hubble/observatory/design/hubble-one-gyro-mode/)

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](https://www.youtube.com/watch?v=KibT-PEMHUU).

> **Comments:**
> **username_7**: 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.  

---

