This answer implies that it's due to budgetary constraints, but given that Kepler, Dawn and Hayabusa all had reaction wheel problems with lubrication before the end of the planned lifetime, it sounds more complex than that...

Is it just that it's untested in space so far?

  • $\begingroup$ Not at all a duplicate, but there is mention of magnetic bearings in the context of turbo pumps in this answer. $\endgroup$
    – uhoh
    Commented Jul 30, 2018 at 3:19
  • $\begingroup$ @uhoh Magnetic bearings are brittle and can loose flux under stress causing magnetic entrapment failure over time compared to ceramic but fare better at higher rotational speeds. $\endgroup$
    – Muze
    Commented Aug 22, 2018 at 4:45

1 Answer 1


There’s a very good answer to why all of those reaction wheels have been failing in a new video below.

Basically, most of the spacecraft you’re referring to use the same kinds of reaction wheels from the same manufacturer. There’s some new studies that indicate solar activity has been causing damage to the wheels.

Ok the plus side, more recent missions use ceramic reaction wheels which seem to be mitigating the problem.

A Newly Discovered Branch of the Fault Tree Explaining Systemic Reaction Wheel Failures and Anomalies

The notes on Scott Manley's video link to the recent paper A Newly Discovered Branch of the Fault Tree Explaining Systemic Reaction Wheel Failures and Anomalies by William Bialke and Eric Hansell of United Technologies Aerospace Systems, which was published in Proc. ‘ESMATS 2017’, Univ. of Hertfordshire, Hatfield, U.K., 20–22 September 2017


After a series of unresolved bearing failures and friction anomalies on spacecraft utilizing ITHACO reaction wheels, a Relentless Root Cause Analysis completed by United Technologies Corporation led to unexpected conclusions which implicated the space charging environment as a likely root cause, and which had not been considered in many previous failure investigations. A strong correlation of the space charging environment with a statistically significant number of friction events observed on-orbit was supported by the results of laboratory tests which successfully duplicated the friction event signatures. Countermeasures were developed to minimize the occurrence of friction events and to increase the probability of successful recovery from anomalous friction increases. This phenomenology likely has applications beyond reaction wheels and should be considered for all past and future mechanisms using ball bearings. (emphasis added)



The most significant breakthrough of the RRCA was when a strong correlation was made between RWA bearing failures/anomalies and space weather, specifically Coronal Mass Ejections (CME) from the Sun, and the resulting geomagnetic storms. In addition, there were a number of failures or anomalies on different spacecraft which occurred during the same geomagnetic storm. While the correlation is strong on all reviewed failures and anomalies, the statistical arguments for this paper will only take into account the failures and anomalies on two NASA spacecraft, namely FUSE and Kepler.

[...] From this it is concluded that that it is not only possible, but likely, that the CME shock front impacting Kepler was simultaneous with the abrupt friction increase observed on RW2. At a CME velocity of 65 km/sec, the 4.7 meter spacecraft would have been enveloped by the CME shock wave-front in 7 µsec, which would result in a very high rate of charging of the spacecraft chassis, and which strongly supports the space charging hypothesis. (emphasis added)

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  • 2
    $\begingroup$ Great find! I hope you don't mind the edits. $\endgroup$
    – uhoh
    Commented Aug 22, 2018 at 4:37
  • $\begingroup$ Please answer more! $\endgroup$
    – Muze
    Commented Aug 22, 2018 at 4:48
  • $\begingroup$ @Geuis the problem is that this is not an answer to the question as asked, so it should really only be a comment here. I would recommend asking why they fail so often as a new question, and then moving your answer to that question and then deleting it here. $\endgroup$
    – uhoh
    Commented Aug 22, 2018 at 4:54
  • $\begingroup$ I'll ask and you answer if you wish? $\endgroup$
    – Muze
    Commented Aug 22, 2018 at 5:08
  • 1
    $\begingroup$ @uhoh - Interesting, I think I may have been slightly wrong. It does seem that the energetic particles, not thermal, are the problem. However, I think I was wrong about the type of damage they are doing. Apparently, this was suspected after Cassini entered the Jovian radiation belts spaceacademy.net.au/spacelab/models/ddd.htm. $\endgroup$ Commented Jun 11, 2020 at 16:14

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