Ryugu as seen by Hayabusa-2 shortly before it grabbed
samples from the surface in 2019. Arrow indicates planned touchdown
site.

Though the last ion engine on the Japanese asteroid probe Hayabusa-2 has now failed, engineers think they can improvise a method to keep the spacecraft under control as it heads to a 2031 rendezvous with the tiny 32-foot-wide asteroid 1998 KY26, which some have suggested might instead be the failed Soviet Mars probe Phobos-1.

Of Hayabusa2’s four ion engines, A, C and D had degraded substantially, so engine B, which had been preserved during its journey to and from Ryugu, has been mainly used on the extended mission. After the B engine broke down in mid-August, JAXA unsuccessfully attempted to fix it.

The ion engine consists of an ion source and a neutralizer, and both must operate simultaneously for navigation. A close examination found that the B engine’s ion source has failed while its neutralizer is functioning. Meanwhile, the neutralizers of the C and D engines have broken down, but their ion sources are functioning. Therefore, JAXA decided to switch the circuit to combine the B engine’s neutralizer with the D engine’s ion source to resume the operation of the B engine.

This was the technique engineers used when a similar failure occurred on Japan’s first asteroid probe, Hayabusa-1. The difference is that with Hayabusa-1 the problems occurred early in the mission, when the spacecraft was young and still relatively healthy. Hayabusa-2 however has been in space for more than a dozen years, has returned samples from the rubble pile asteroid Ryugu, and flew past the asteroid Torifune. For it to last another five years under these conditions remains questionable.