Laser fading seems to be a known issue with ring laser gyros. With their STEREO-B probe, NASA experienced a different, "unexpected" failure mode, giving false readings. The Honeywell MIMU's were well beyond the expected lifetime but still, there may be some lessons to be learned from this. Having a second IMU in hot standby increases redundancy in critical mission phases. During extended operations however, any spare unit that ages may better be sent into cold standby.
- Do ring laser gyros age (in deep space environment) with laser and dithering disabled?
- Were laser and dither disabled on the spare unit for the STEREO mission?
- Can the failure mode of STEREO-B's IMU's be attributed to the dithering mechanism?
Some context to the question:
NASA's STEREO-B is in uncontrolled spin (as of Oct 2016). Efforts to regain control are ongoing following a telemetry downlink in August 2016 after two years of communication break. Control of the spacecraft was lost on Oct. 1 2014 due to an unlucky double failure during a planned reset sequence in preparation to solar conjunction (star tracker timeout + IMU fail).
The sequence of events is analyzed in this presentation, prepared by mission specialists at John Hopkins Lab.