A quiet asteroid grew a tail: a 12-kilometre rock between Mars and Jupiter has started shedding dust into space

Asteroid 6993 (1995 BJ4) has been watched for years, and it has never done anything like this. The roughly 12-kilometre-wide rock, orbiting in the main belt between Mars and Jupiter, has grown a narrow tail of escaping material, visible in telescope images since the summer of 2026. The activity was flagged on August 5 by the Zwicky Transient Facility's survey at Palomar Observatory in California, and the ATLAS survey independently shows the same trail; the earliest confirmed image, from June 25, catches the asteroid some 435 million kilometres from the Sun with the tail already at least 32 arcseconds long — a streamer of dust being shaped by sunlight and the asteroid's own motion. A study led by Erin M. Clark of Northern Arizona University, accepted for publication in The Astrophysical Journal Letters, lays out the two suspects: an unseen collision that knocked dust loose, or buried ice now warming and releasing gas and dust. If it is ice, this quiet rock would become the largest known main-belt comet — a snowball hiding in the asteroid belt all along.