What once appeared a fairly ordinary inhabitant of the asteroid belt has turned out to be anything but. For one thing, the object known as 288P is actually two asteroids in orbit around each other. Moreover, the pair have an occasional tail, referred to as “transient comet-like activity”. The object could eventually help us understand the source of Earth’s water.

Binary asteroids are not unusual – a high proportion of asteroids are accompanied by their own tiny moons – although the fact that 288P’s components are both around a kilometer (0.6 miles) across is less common, as is their wide separation. However, the real interest comes from combining these observations with confirmation of last year’s discovery that 288P releases a cloud of water vapor when its orbit brings it closest to the Sun.

“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,” said Dr Jessica Agarwal of the Max Plank Institute for Solar System Research in a statement. Objects orbiting in the asteroid belt with surface ice that vaporizes when they get close to the Sun are common enough to have a name: main-belt comets.

Twelve main-belt comets were known before 288P, but none had been identified as being binary. The speed at which objects orbit each other can be used to reveal their mass, which, along with size measurements, could give us an idea of 288P’s composition.

Over many approaches to the Sun, a comet’s surface ice vaporizes away. Consequently, most surviving comets either spend the bulk of their time in the safety of the outer Solar System or have only recently moved to a new orbit as a result of a close approach with a planet. Main-belt comets clearly don’t fit the first category and have unusually round orbits for the second, leading to speculation their ice was buried in the interior until exposed by a recent collision.

Agarwal and fellow authors of a paper in Nature think 288P was once a single object that rotated so fast that some time in the last 5,000 years it spun apart. This created the twin asteroids and exposed interior ices to sunlight, restarting cometary behavior.

The question of whether the Earth’s water comes from comets goes back at least as far as Newton, and main-belt comets may hold the answer. The question is considered so important that a team of astronomers recently produced a detailed proposal for a mission, dubbed Castalia, to explore one of these rare objects. Their selected target was 133P/Elst Pizarro, the first of the class to be discovered, but perhaps 288P would be better, providing two objects to explore for the price of one.

This set of images from the ESA/NASA Hubble Space Telescope reveals two asteroids with comet-like features orbiting each other. These include a bright halo of material, called a coma, and a long tail of dust. The asteroid pair, called 288P, was observed in September 2016 just before the asteroid made its closest approach to the Sun. These images reveal ongoing activity in the binary system. The apparent movement of the tail is a projection effect due to the relative alignment between the Sun, Earth, and 288P changing between observations. The tail orientation is also affected by a change in the particle size. Initially, the tail was pointing towards the direction where comparatively large dust particles (about 1 millimetre in size) were emitted in late July. However, from 20 September 2016 onwards, the tail began to point in the opposite direction from the Sun where small particles (about 10 microns in size) are blown away from the nucleus by radiation pressure.