On December 6, 2020, Japan’s Hayabusa2 probe dropped a capsule of precious samples from asteroid Ryugu into the Australian outback, then set off on an extended mission that will end at a much smaller space rock. Its next encounter may prove more challenging, however, as the new target is moving in ways scientists still can’t explain.

The object, designated 1998 KY26, is accelerating in a way that cannot be accounted for with gravity alone. This has led some scientists to suggest that it’s not an asteroid but rather a dark comet or even an alien artifact (a theory we’re not going to entertain, at least for the time being).…