Astronomers have used the Hubble space telescope to observe what happens after asteroids crash together. This was the first time scientists have studied the aftermath of a collision, relying normally on models to make predictions about the frequency of such crashes and the amount of dust they produce. They say the findings will help explain where the dust in the Solar System comes from and may offer answers to other mysteries including how the dusty debris discs around other stars were produced. Last January astronomers thought they had witnessed a fresh collision between two asteroids when images from Hubble revealed an X-shaped object at the head of a comet-like trail of material. 'When I saw the image I knew it was something special,' said astronomer Jessica Agarwal from the European Space Agency (ESA) in the Netherlands. 'The nucleus seemed almost detached from the dust cloud and there were intricate structures within the dust.' The astronomers presumed the crash had just occurred, but after using Hubble to track the oddball body for five months, they were surprised to find they had missed the suspected smash-up by a year. 'We expected the debris field to expand dramatically, like shrapnel flying from a hand grenade,' said David Jewitt from the University of California in Los Angeles (UCLA) in the US. 'But...we found that the object is expanding very, very slowly.' The Hubble images, taken from January to May 2010, reveal a point-like object about 120 metres wide, with a long, flowing dust tail behind an X-shaped pattern. The observations also show that the object retained its X-shape even as the debris field slowly expanded. Particle sizes in the tail are estimated to vary from about 1 millimetre to 2.5 centimetres in diameter. The object in the Hubble image is the remnant of a slightly larger precursor body, and the astronomers think a smaller rock slammed into the larger one. The pair probably collided at high speed, at about 18,000 kilometres an hour, smashing and vaporising the small asteroid and stripping material from the larger one. Professor Jewitt estimated that the violent encounter was as powerful as the detonation of a small atomic bomb. Radiation pressure from the Sun then swept the debris behind the remnant asteroid, forming a comet-like tail. The tail contains enough dust to make a ball 20 metres wide, most of it blown out of the bigger body by the explosion that followed the impact. 'These observations are important because we need to know where the dust in the Solar System comes from, and how much of it comes from colliding asteroids as opposed to "outgassing" comets,' Professor Jewitt explained. 'We can also apply this knowledge to the dusty debris discs around other stars, because these are thought to be produced by collisions between unseen bodies in the discs. Knowing how the dust was produced will yield clues about those invisible bodies.' He said this study was particularly important because 'catching colliding asteroids on camera is difficult because large impacts are rare, while small ones, such as [this] one...are exceedingly faint'. The UCLA professor explained that 'the two asteroids whose remains make up [the object] were unknown before the smash-up because they were too faint to be noticed', while 'the collision was unobservable because it happened when the asteroids were in the same direction as the Sun'. The astronomers plan to use Hubble again in 2011 to view the remnant asteroid and hope to see how far the dust has been swept back by the Sun's radiation and how the mysterious X-shaped structure has evolved. The results of the study appear in the journal Nature.