Free Astronomy Magazine September-October 2023

servations by Hubble also give an estimate for the size of the DART impact crater, Hera will eventually determine the actual crater size. Long ago, Dimorphos may have formed from material shed into space by the larger as- teroid Didymos. The parent body may have spun up too quickly or could have lost material after a glancing colli- sion with another ob- ject, among other scenarios. The ejected material formed a ring that gravitationally co- alesced to form Dimor- phos. This would make it a flying rubble pile of rocky debris loosely held together by the relatively weak pull of its gravity. Therefore, the interior is probably not solid, but has a structure more like a bunch of grapes. It’s not clear how the boulders were lifted off the asteroid’s surface. They could be part of an ejecta plume that was photographed by Hubble and other ob- servatories. Or a seismic wave from the impact may have rattled through the asteroid — like hitting a bell with a hammer — shaking loose the surface rubble. The DART and LICIACube (Light Ital- ian CubeSat for Imaging of Aster- oids) teams have also been studying boulders detected in images taken by LICIACube’s LUKE (LICIACube Unit Key Explorer) camera in the minutes immediately following DART’s kinetic impact. 21 SEPTEMBER-OCTOBER 2023 tailed post-impact survey of the tar- get asteroid Dimorphos. Hera will turn the grand-scale experiment into a well-understood and repeat- able planetary defence technique that might one day be used for real. The boulders are most likely not shattered pieces of the diminutive asteroid caused by the impact. They were already scattered across the as- teroid’s surface, as evident in the last close-up picture taken by the DART spacecraft just two seconds before collision, when it was only 11 kilo- metres above the surface. The science team that observed these boulders with Hubble esti- mates that the impact shook off two percent of the boulders on the aster- oid’s surface. While the boulder ob- T his NASA/ESA Hubble Space Telescope image of the as- teroid Dimorphos was taken on 19 December 2022, nearly four months after the asteroid was impacted by NASA’s DART (Double Asteroid Redirection Test) mission. Hubble’s sensi- tivity reveals a few dozen boulders knocked off the aster- oid by the force of the colli- sion. These are among the faintest objects Hubble has ever photographed inside the Solar System. The ejected boul- ders range in size from 1 metre to 6.7 metres across, based on Hubble photometry. They are drifting away from the asteroid at around a kilometre per hour. The discovery yields invaluable insights into the behaviour of a small asteroid when it is hit by a projectile for the purpose of altering its trajectory. [NASA, ESA, D. Jewitt (UCLA)] !