When is a comet not a comet?

When the peculiar object P/2010 A2 was discovered in January 2010, complete with a tail, it was designated as a comet. But its ‘headless’ appearance and its orbit in the inner reaches of the main asteroid belt were most un-comet-like, prompting suggestions that it was an asteroid with a tail. Two papers in this issue confirm the status of P/2010 A2 as an asteroid, rather than as a member of the recently recognized class of main-belt comets. Snodgrass et al. observed P/2010 A2 in March using the Rosetta spacecraft, which was approaching the asteroid belt for its 10 July flyby of the asteroid Lutetia. They conclude that the object’s tail is made up of debris from an asteroid collision — and computer modelling identifies the event in question as a collision that occurred in February 2009. Jewitt et al. took high-resolution images of P/2010 A2 with the Hubble Space Telescope between January and May 2010, and estimate a 120-metre diameter for the objects ‘nucleus’, with millimetre-sized dust particles forming the tail. They too trace the collision back to early 2009.