A binary main belt comet

The asteroids are primitive solar system bodies which evolve both collisionally and through disruptions due to rapid rotation [1]. These processes can lead to the formation of binary asteroids [2-4] and to the release of dust [5], both directly and, in some cases, through uncovering frozen volatiles. In a sub-set of the asteroids called main-belt comets (MBCs), the sublimation of excavated volatiles causes transient comet-like activity [6-8]. Torques exerted by sublimation measurably influence the spin rates of active comets [9] and might lead to the splitting of bilobate comet nuclei [10]. The kilometer-sized main-belt asteroid 288P (300163) showed activity for several months around its perihelion 2011 [11], suspected to be sustained by the sublimation of water ice [12] and supported by rapid rotation [13], while at least one component rotates slowly with a period of 16 hours [14]. 288P is part of a young family of at least 11 asteroids that formed from a ~10km diameter precursor during a shattering collision 7.5 million years ago [15]. Here we report that 288P is a binary main-belt comet. It is different from the known asteroid binaries for its combination of wide separation, near-equal component size, high eccentricity, and comet-like activity. The observations also provide strong support for sublimation as the driver of activity in 288P and show that sublimation torques may play a significant role in binary orbit evolution.