When NASA’s DART spacecraft crashed into the asteroid Dimorphos on September 22, 2022, it was humanity’s first test of a planetary defence technique. It also marked the first time that our species successfully changed the motion of a celestial object. Now, the Hubble Telescope has detected another consequence of that impact. Astronomers used the powerful telescope to discover a through of rocks that might have been shaken off when the half-ton DART spacecraft crashed into Dimorphos at more than 22,000 kilometres per hour. The 37 rocks detected by Hubble range in size from around one metre to nearly 7 metres across. They are moving away from the asteroid at a speed of a little less than a kilometre per hour. "This is a spectacular observation – much better than I expected. We see a cloud of boulders carrying mass and energy away from the impact target. The numbers, sizes, and shapes of the boulders are consistent with them having been knocked off the surface of Dimorphos by the impact. This tells us for the first time what happens when you hit an asteroid and see material coming out up to the largest sizes. The boulders are some of the faintest things ever imaged inside our solar system,” said David Jewitt, a planetary scientist tracking Dimorphos, in a press statement. According to NASA, the rocks are likely not pieces that broke off from the small asteroid but they were probably already scattered across the asteroid’s surface. Dimorphos orbits the larger asteroid Didymos, which was why it was selected for the DART mission in the first place, since it would be easier to calculate the change in its motion with respect to the larger asteroid. Scientists now speculate that Dimorphos may have formed out of material that was shaken off Didymos, just like what happened with the 37 asteroids detected by Dimorphos. This must have happened when Didymos spun too quickly or had an impact on another object. The ejected material may have formed a ring that eventually coalesced to form Dimorphos. This would mean that Dimorphos is basically a flying pile of rubbly held together by a weak gravitational force.
