High-resolution view of Dimorphos created by combining the final 10 full-frame images obtained by DART’s Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO). Credit: NASA/Johns Hopkins APL.
Dimorphos is a 160 metre-long pile of rubble in orbit around the larger Didymos asteroid, which gets to within a few million kilometres of Earth as it circles the Sun. Dimorphos was struck by the 570-kilogram box-shaped Double Asteroid Redirection Test (DART) on September 26, 2022, shedding more than 10,000 tonnes of material in the process. The mission aimed to test whether an asteroid heading towards the Earth could be deflected by launching a spacecraft at it.
Telescopes on the ground and in space tracked the sunlight reflected off debris in the weeks following the impact. This way, scientists were able to estimate that the collision reduced Dimorphos’s orbital period of 11 hours and 55 minutes by more than half an hour.
Observations of the ejected material in the aftermath of the impact were instead made by the Italian Space Agency's Light Italian CubeSat for Imaging of Asteroids (LICIACube). The size of a small rucksack, LICIACube was released from inside DART about two weeks before the collision and then went on to record the event from as close as 55 kilometres away.
According to Maurizio Pajola of the Astronomical Observatory of Padova in Italy, one of the authors of the study, LICIACube would have done its job having captured just one photo before impact and a few afterwards. "But in fact we got 600 images," he says, "so it's an amazing success".
The researchers simulated Dimorphos with different material properties and structures, comparing the amount and trajectory of debris generated in each simulated collision with that observed by LICIACube. They found that only when they modelled the asteroid moon as a very weakly-bound pile of rubble did simulation match observation. This told the researchers that Dimorphos probably formed when Didymos, either spinning or struck by another object, shed material that then coalesced.
The researchers also found that DART's impact probably deformed Dimorphos globally, rather than leaving a crater – having modelled its shape from various angles and found no sign of the latter. The final word will go to the European Space Agency mission Hera, due to launch this year and make detailed maps of Dimorphos from up close.
As for planetary defence, Pajola points out that small, stony asteroids like Dimorphos are dangerous because they can get close to Earth. But he says that future missions might be able to knock such asteroids even further out of kilter than DART did, if the objects in question are loosely bound and smaller than Dimorphos.
