Comets are instantly recognizable by their tails of gas and dust. Most comets originate in the far, frozen reaches of our Solar System, and only visit the inner Solar System occasionally. But some are in the Main Asteroid Belt, mixed in with the debris left over after the Solar System formed.
Astronomers just found water vapour coming from one of them.
NASA’s OSIRIS-REx spacecraft arrived at asteroid Bennu in December 2018, and just one week later, it discovered something unusual about Bennu: the asteroid was ejecting particles into space.
The spacecraft’s navigation camera first spotted the particles, but scientists initially thought they were just stars in the background. After closer scrutiny, the OSIRIS-REx team realized they were particles of rock, and were concerned that they might pose a hazard.