NASA’s DART mission hammered its target asteroid into a new shape
The shape of the asteroid Dimorphos was changed when NASA’s Double Asteroid Redirection Test (DART) deliberately crashed into it in 2022 as part of a test of humanity’s planetary defence capabilities. DART was designed to show whether we could divert a potentially hazardous asteroid away from Earth. It was sent to a binary asteroid in which the 170-metre (560-foot) wide Dimorphos orbits a larger 760-metre (2,493foot) wide space rock called Didymos. When DART impacted Dimorphos on 26 September 2022, astronomers were able to measure how much the impact had nudged the asteroid by measuring how the space rock’s orbit around Didymos changed.
Now, scientists have shown that it seems DART didn’t just give Dimorphos a push – it hit Dimorphos with enough kinetic energy to reshape it. “The entire shape of the asteroid has changed, from a relatively symmetrical object to a ‘triaxial ellipsoid’ – something more like an oblong watermelon,” said Shantanu Naidu of NASA’s Jet Propulsion Laboratory (JPL) in California. Originally, Dimorphos would have been an oblate spheroid, which is kind of like a squashed ball. The impact of DART at five kilometres (three miles) per second sent shock waves through the asteroid, resulting in it becoming more elongated and shifting its axis of rotation off centre. The new shape is inferred by astronomers from the light curve of the Didymos-Dimorphos system, which is aligned in such a way that we can see them transiting and eclipsing one another.
This conclusion from Naidu’s team is also shared in work published in February by a group spearheaded by Sabina Raducan of the University of Bern in Switzerland. Raducan’s team concluded that the impact had resulted in up to one per cent of Dimorphos’ mass being ejected into space, and another eight per cent being redistributed across the surface as the asteroid absorbed the impact energy and reshaped itself. The conclusion was that to allow itself to morph in such a way, Dimorphos must be a loose rubble pile – an agglomeration of dirt and rocks held together by weak gravity which can easily be reshaped, as opposed to a rigid structure that would not give as easily. “The results of the study agree with others that are being published,” said Tom Statler, who is program scientist in the Science Mission Directorate’s Planetary Science Division at NASA Headquarters. “Seeing separate groups analyse the data and independently come to the same conclusions is a hallmark of a solid scientific result.”
The new study also confirms how much Dimorphos’ orbit around Didymos was altered by DART’s impact. Prior to the impact, Dimorphos revolved around Didymos once every 11 hours and 55 minutes, with an orbital radius of 1,189 metres (3,900 feet). Studies of the light curve, coupled with radar observations from the Deep Space Network’s Goldstone Solar System Radar in California, show that Dimorphos’ orbital period has reduced to 11 hours, 22 minutes and 3 seconds, within an error margin of
1.5 seconds. Its orbital radius has also been reduced to 1,152 metres (3,780 feet).
Given that Dimorphos’ axis of rotation is now offset from its geographical centre, Dimorphos now rocks backwards and forwards as it orbits Didymos – a swaying motion that’s detectable through the shape of the light curve. “Before impact the times of the [transit] events occurred regularly, showing a circular orbit,” said JPL’s Steve Chesley. “After impact there were very slight timing differences, showing something was askew. We never expected to get this kind of accuracy.”
DART was designed to test whether it would be possible to alter the trajectory of a small but dangerous asteroid if it were on a collision course with Earth. The experiment exceeded scientists’ expectations in terms of how much DART nudged Dimorphos and what the impact is teaching us about how asteroids behave when faced with such kinetic violence. “DART is not only showing us the pathway to an asteroid-deflection technology, it’s revealing a new fundamental understanding of what asteroids are and how they behave,” said Statler.
But the study of Didymos and Dimorphos is not over yet. In October 2024, the European Space Agency (ESA) will launch the Hera spacecraft, which is a mission to encounter the two asteroids and inspect how much damage DART did to Dimorphos, in addition to studying the nature of the asteroids more closely.