On December 6, 2020, Japan\u2019s 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\u2019t explain.<\/p>\n
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\u2019s not an asteroid but rather a dark comet or even an alien artifact (a theory we\u2019re not going to entertain, at least for the time being). Now, a new paper<\/a> challenges the idea that it\u2019s a comet, finding that the space rock is too warm to hold on to buried ice.<\/p>\n The findings, published in the journal Icarus, reignite the mystery surrounding 1998 KY26 as the Japanese spacecraft races toward the unknown.<\/p>\n Playing catch-up <\/p>\n Hayabusa2 is set to rendezvous with the space rock in 2031 as part of its extended mission to collect more dust and rock straight from the source. 1998 KY26 is a small, lumpy near-Earth object that rotates so quickly that a day on the rock ends almost as soon as it begins.<\/p>\n Things have not been looking good for the mission for a while now. In a previous paper<\/a> published in 2025, astronomers revealed that Hayabusa2\u2019s new target is about a third the size<\/a> previously estimated and spinning much faster than originally thought. Hayabusa2\u2019s first target measures nearly 3,000 feet (900 meters) wide. The spacecraft first touched down on asteroid Ryugu<\/a> on February 22, 2019. That April, it shot a copper impactor into the asteroid to form an artificial crater and then returned in July for a second touch-and-go encounter to collect material from beneath the surface.<\/p>\n Touching down on 1998 KY26, however, may prove far more challenging, as the rock is just 36 feet (11 meters) wide\u2014barely twice the span of the spacecraft\u2019s solar panels\u2014and completes a full spin roughly every 5 minutes. To make things more complicated, the asteroid\u2019s trajectory shows unusual, non-gravitational accelerations, according to a paper<\/a> published in The Planetary Science Journal in 2023.<\/p>\n The magnitudes of the accelerations are greater than those typically induced by the Yarkovsky effect, a tiny force imparted by sunlight that can gradually alter an asteroid\u2019s orbit, according to the paper, and \u201cthere is no radiation-based, non-radial effect that can be so large,\u201d the authors wrote.<\/p>\n Asteroid, comet, or something else entirely? <\/p>\n