A new planetary hybrid has been found floating in the asteroid belt between Mars and Jupiter – one that is half comet, half asteroid.


Recently released images taken by NASA's Hubble Space Telescope of asteroid 2006 VW139/288P show that it is, in fact, two space rocks in orbit around each other.

Unusually, the pair were found to be shrouded in a coma and were followed by a long tail, like a comet.

Asteroid 288P was observed with Hubble as it passed through perihelion, its closest approach to the Sun as it travels through its elliptical orbit, in September 2016.

These images showed that the space rock was a binary pair, where each half was around the same size and mass, orbiting 100km apart.

However, during perihelion the rising temperatures caused 288P to exhibit some strange behaviour.

“We detected strong indications of the sublimation of water ice due to the increased solar heating – similar to how the tail of a comet is created,” says Jessica Agarwal from the Max Planck Institute for Solar System Research who led the team.

This dual behaviour means that 288P now defined as a main-belt comet.

These objects are thought to be vital in the process that brought water to Earth, but their rarity makes studying them difficult.

This particular example is even rarer – the wide separation of the two components, their near equal size and its highly eccentric orbit all combine to make this binary truly unique.

It appears that the pair are a relatively recent match up.

“Surface ice cannot survive in the asteroid belt for the age of the Solar System but can be protected for billions of years by a refractory dust mantle a few metres thick,” says Agarwal.

This leads the team to believe that 288P is only 5,000 years old.

The binary system was most likely created when a larger asteroid that was rapidly rotating spun itself apart. As it did so, it shed its surface layer, revealing the ice below.

Over time the two fragments drifted away from each other, creating the large separation.


The team will continue to hunt for similar objects to better understand the role asteroids and comets played in the creation of our Solar System.


Elizabeth Pearson
Ezzy PearsonScience journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.