Hitomi Experience Report: Investigation of Anomalies Affecting the X-ray Astronomy Satellite “Hitomi” (ASTRO-H)
May 24, 2016
That's quite a mishap report. As with most mishaps, there's a chain of events leading up to the loss of spacecraft.
1. February 28, 2016: Operator fails to remove minus signs in copying data from one tool to another. Verification of the dataset is missed twice. Root cause is lack of documentation. Result is that Thruster Safe Hold mode is rendered unsafe and will instead spin spacecraft up until catastrophic structural failure occurs. (Mechanism 3)
2. Design decision: Interaction of Star Tracker and Inertial Reference Unit logic in attitude control system. Star tracker mode switch causes glitch in IRU bias estimate, which the Star tracker corrects over time, but if the Star tracker suffers a glitch as well, the IRU can end up far enough off that the AOCS rejects the Star tracker's correction when the Star tracker comes back on line. This happens March 26. (Mechanism 1)
3. Design decision: Reaction Wheel Safe Hold is triggered by Attitude Control System, not by Sun Sensors. The failure above means that the AOCS has the wrong state, so it never triggers RW Safe Hold. Reason for this decision is that Sun Sensor field of view is smaller than the off-pointing in normal operation. (Mechanism 2)
4. Catastrophic structural failure. Solar arrays break off. (Mechanism 4)