Astronomers recently spotted two space rocks behaving strangely. As the asteroids orbited each other, both of them appeared to be shedding material just like a comet does — observations revealed a coma around the objects as well as a long tail of material.

The group have identified this weird system, called 288P, as “the first known binary asteroid also classified as a comet.” There are a few select examples of single asteroids shedding water and dust and leaving behind a comet-like tail, but it’s the first time two of these objects were seen orbiting each other.

288P was first discovered in November 2006 by the astronomical survey Spacewatch. At the time, it was thought to be an asteroid and called Asteroid 300163 (2006VW139). In 2011, the Pan-STARRS telescope spotted cometary activity and the object was renamed 288P.

Astronomers didn’t know that 288P was two objects until the Hubble Space Telescope observed it in September 2016, though the team suggested it could be two objects in November 2015.

From the objects’ orbit, astronomers could see that 288P originally came from the asteroid belt. Astronomers expect that in the asteroid belt, water ice can only exist for billions of years if it is protected by a covering of dust, perhaps a few meters thick. But 288P sheds water ice as it approaches the sun, indicating that the dust layer is eroded or lost.

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