Failure of reaction wheels often causes premature mission ending or failure to achieve the goal because spacecrafts are no longer able to maintain orientation and have to rely on chemical propellants. The Hubble telescope, Hayabusa spacecraft, Kepler telescope, Dawn spacecraft etc. all faced reaction wheel failure.

Recently there was a study that “space weather” induces electrostatic discharge between steel ball bearings and causes surface defects which eventually compromise the whole mechanism. But for me it’s hard to believe that this is the only reason, because I doubt that spacecraft only use steel ball bearings when ceramic, especially silicon nitride, are superior and can even survive lubricant starvation. What about contactless magnetic bearings?

Can “space weather” have other effects on flywheels like damaging permanent magnets (demagnetization), electronics, lubrication instead of steel ball bearings?

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    $\begingroup$ Scott Manley's Scientists May Have Figured Out Why So Many Spacecraft Were Failing may or may not shed some light on this $\endgroup$
    – uhoh
    Commented May 17, 2020 at 11:57
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    $\begingroup$ Ceramic bearings are more fragile, more prone to manufacturing faults, more sensitive to thermal shock, etc. And magnetic bearings are more complex to design and build and have a whole set of pros and cons of their own. It's not a simple situation of "technology X is superior". $\endgroup$ Commented May 17, 2020 at 15:36
  • $\begingroup$ True for many ceramics, but not for silicon nitride, It's one of toughest materials. Bigger silicon nitride ball bearings can withstand bullet impacts without substantial damage. youtube.com/watch?v=i5RIFbk178E In particular, it was identified as one of the few monolithic ceramic materials capable of surviving the severe thermal shock and thermal gradients generated in hydrogen/oxygen rocket engines. en.wikipedia.org/wiki/Silicon_nitride#/media/… $\endgroup$
    – WOW 6EQUJ5
    Commented May 17, 2020 at 16:04
  • $\begingroup$ This excellent video by Scott Manley covers reaction wheels in appreciable detail explaining the effects of solar flares on bearings, based on the paper mentioned in @Christopher's answer. $\endgroup$
    – Krish
    Commented May 17, 2020 at 17:19
  • $\begingroup$ Searching for "Ithaco" from one of the answers I finally found the related but different question that I couldn't find earlier: Why aren't magnetic bearings used more frequently in reaction wheels? $\endgroup$
    – uhoh
    Commented May 17, 2020 at 22:21

3 Answers 3


Ceramic bearings are a relatively new concept. They were a hot research topic in the late 1970s and didn't become commercially available until the mid 1990s.

The Far Ultraviolet Spectroscopic Explorer (FUSE), the first spacecraft on which these anomalous bearing failures were seen, was launched in 1999. It takes several years to multiple decades to design and build and test a satellite. That means FUSE not only was designed before ceramic bearings became commercially available, it means that construction on FUSE started before ceramic bearings became commercially available. There was zero chance that either NASA or its contractor would have wanted to switch from a very well known product (steel ball bearings) to an unknown product (ceramic ball bearings) at that time.

Even a decade later, which was when the Kepler Space Telescope was launched, there remained zero chance that either NASA or its contractor would have wanted to switch from steel ball bearings to ceramic ball bearings. The problems with steel ball bearings would not be known and attributed to the use of steel for another seven years after the launch of Kepler.

The space industry is inherently conservative. For items related to space exploration, one does not switch from a known quantity to something new just because the new thing is shinier. A very good reason is needed, and shininess doesn't count. Drastically reducing costs, markedly improving lifespan, or markedly approving mission success -- those are factors that do count.

Now there are solid signs that the problems experienced with reaction wheels on FUSE, Kepler, and other satellites were a result of the use of steel ball bearings. This known knowledge is now driving a switch away from steel ball bearings. That knowledge was not known until a few years ago.

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    $\begingroup$ I know a couple optics and thermal folks that might disagree with you on "shininess doesn't count" :) $\endgroup$
    – aranedain
    Commented May 18, 2020 at 20:00

The study was specifically of ITHACO reaction-wheel assemblies that proved to be highly prone to failure in the space environment. These assemblies used steel bearings.

Interference with electronics is certainly possible, but would not be limited to reaction wheels. Demagnetization of permanent magnets would require such extreme conditions that it would be surprising if any of the electronics were left functional afterward, and would also affect things other than reaction wheels. I see no plausible way for the lubricant to be affected more directly than the mechanism proposed, and again, there are other mechanical components that depend on lubrication.

Reaction wheels are unique in that they spend much of their life running at relatively high speeds, and so can rapidly accumulate damage from the proposed mechanism, while slow and/or intermittently-moving mechanisms using similar bearings might avoid damage altogether or just not accumulate noticeable levels. Pretty much everything else about them...electronics, permanent magnets, lubrication, etc...is similar to other spacecraft components.

Finally, a lack of similar problems with a newer version that used ceramic bearings was specifically cited in the paper.


I think the Bialke and Hansell paper referred to in this post is interesting but its not the last word as it applies to a specific design. Also, unless one is at the epicenter of the design and anomaly reviews, it is very hard for anyone else to come to a definitive view of what caused any anomaly. Obviously the coincidence of a new root cause with announcements about the positive behaviour of a new product line doesn't help in remaining rationale. Overall one simply has to accept that parts of the anomaly and its analysis won't be visible.

As an example a wheel operating in reaction wheel mode has an average speed of zero and relatively low rms speed whereas wheels operated in momentum wheel mode operate at high speeds. This is relevant because there are speed dependencies to known problems on wheels in both modes and so the electro-static discharge explanation would have to explain the problems all wheel operating speeds to be universal. Of course it could remain a valid interpretation of the FUSE and Kepler problems.


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