What Happened to All Those Boulders Blasted into Space by DART?

It was a $325 million dollar project that was intentionally smashed to smithereens in the interest of one day, saving humanity. The DART mission (Double Asteroid Redirection Test) launched in November 2021 on route to asteroid Dimorphos. Its mission was simple, to smash into Dimorphos to see if it may be possible to redirect it from its path. On impact, it created a trail of debris from micron to meter sized objects. A new paper analyses the debris field to predict where they might end up. 

Asteroid Dimorphos orbits around its host asteroid, Didymos and together they form a binary asteroid system. Neither asteroid poses a threat to Earth but their gave a fabulous opportunity to test technology for defending Earth from potential impactors. On 11 October NASA announced that DART successfully altered the orbit of Dimorphos showing that the kinetic energy of a spacecraft could indeed alter the trajectory of a potential threat. 

DART hit Dimorphos in an almost head on collision and the resulting ejecta plume travelled at approximately 2km/s. The plume had been observed by the Les Makes Observatory and with the Hubble Space Telescope. The debris contained material from dust sized particles to meteor and even boulder sized objects. Just before the impact, the CubeSat LICIACube was released from DART so that it could offer some long term monitoring of the debris field. 

Observations that followed showed delicate structures within the ejecta with a diffuse cloud that quickly transformed into a cone shaped formation with a tail. That tail, just like the tail of a comet was then pushed away from the asteroid system by the solar radiation pressure. Using ground based imagery, the mass and velocity of the ejected particles was established. 

The analysis of ejecta enabled modelling to be undertaken to estimate that approximately 3% of all ejected boulders would remain in orbit after 83 days (within the scope of the captured data). This estimation was in line with the pre-impact simulations over a 60 day period. By varying the parameters of the simulation they also revealed that 5% of 10cm sized particles escaping with a velocity of 0.12 and 0.18m/s would remain in orbit around the system after a 60 day period of time, similar again to the observations. 

Perhaps the greatest concern though is the long term fate of the larger boulder sized ejecta. Taking integrations covering 800 and 1550 days following the impact, the results showed a gradual decline in the number of boulders bound to the asteroid system. The good news is that for the most part, the reduction was due to collisions with Didymos and Dimorphos themselves. In no simulations was there any suggestion that any of the ejecta would escape the double asteroid system. 

What of the future, well to fully understand the results a follow up study is required. Hera is a European Space Agency mission slated to travel to Didymos and Dimorphos 5 years after the DART impact. On arrival it will assess the orbit of Dimorphos to understand its orbital changes. 

Source : On the fate of slow boulders ejected after DART impact on Dimorphos

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