Jaxa's litany of errors spun Hitomi to pieces

Agency releases analysis of why the satellite broke up

Japan's space agency Jaxa has detailed the litany of errors that ended with the failure of its Hitomi (Astro-H) spacecraft.

The agency has published a 90-page discussion of what caused the break-up.

Their conclusions are pretty damning for the agency, centring around a lack of protocols to manage a major change in the craft's thruster, and the disabling of safety systems.

The sequence of events before the break-up, as far as the analysis can determine, started with a problem in the attitude control system (ACS). This reported that the satellite was rotating when it wasn't, and to break the mistakenly-reported spin, mission controllers started the reaction wheel spinning.

The next failure was that the magnetic torquer operated by the ACS didn't work, and that caused the reaction wheel to keep accumulating angular momentum (in other words, speeding up).

This was exacerbated by a loss of Star Tracker (STT) data, which would have told the controllers the reported spin was incorrect; and even when the data was available, a misconfiguration meant it was ignored.

The ACS decided the satellite was in a critical situation, switched it to a safe mode, and tried to use the thrusters to correct things – but it used “inappropriate thruster control parameters”, and sped up the rotation. That resulted in the solar array paddles, the “extensible optical bench”, and other components breaking away.

There was yet another failsafe that wasn't operating. The satellite had a Coarse Sun Aspect Sensor, which again would have provided data contradicting that from the ACS. The designers had decided not to use this data, because the limited 20° field of view of the sensors risked it producing false positives.

The result, as Spaceflight 101 notes, was: “designers allowed one attitude measuring device overruling all other systems, introducing a single point of failure”.

Other failings detailed in the report include:

  • Thruster parameters that were inappropriate for the configuration after the 6.3 metre Extensible Optical Bench (EOB) was deployed;
  • A lack of documentation or operational plans to change the thruster parameters in the presence of the EOB;
  • Instructions were given to a third party company to change the thruster parameters, but as Spaceflight 101 puts it, “technical details on how the parameters were changed were not shared between that company and JAXA.”

The software used to generate thruster parameters, the RCS Drive Matrix Generation Tool, also lacked a user manual or operational training.

There's another analysis in Japanese here, which Google Translate almost makes sense of. ®

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