Had this been a conventional metal nozzle, it would likely have partly burned through, perhaps seeing some sideways thrust, and would have some thrust drop. Eventually the result might have been the same, but the degradation would probably have been more gradual, whereas the ceramic material cracked suddenly with the thermal stress, giving a very sharp change in the thrust and torque.

As the angular rate climbed to 12 degrees per second, the spacecraft’s on-board fault protection recognized that something was badly wrong, and shut down the engine. Akatsuki sailed past Venus without stopping.

JAXA controllers quickly diagnosed the status of the vehicle, and managed to snap a few pictures of the planet before the spacecraft receded to a distance where Venus shrank to a point, at least demonstrating that several of the cameras worked. The telemetry recording the propulsion system behavior pointed to the culprit. After the engine burn, the fuel-side pressure slowly climbed back to the intended level, implying that some helium was getting through to the tank, just not quickly enough to keep up with fuel usage during the main engine burn. It seems the non-return valve had been (mostly) held shut, perhaps by salts formed by corrosion of the valve seat by fuel vapor. That couldn’t be fixed, now, but perhaps the main engine could be used in short bursts.

Tests on the ground replicated the failure of the engine under low fuel flow conditions, and indeed the nozzle fractured. How much nozzle was left in flight? If part of it was present to cause asymmetric thrust, perhaps the spacecraft could be spun for stability during engine burns? And if engine burns were kept short (as had been the first burn, before orbit insertion), the helium pressure drop could be limited to safe levels. But an in-flight test, firing the engine for a couple of seconds, found that only about a tenth of the nominal thrust was generated, suggesting that probably the whole nozzle had come off, fractured near the throat. The engine was useless.