BULLETIN
DART successfully impacts asteroid
NASA’s planetary defence strategy takes a step forward
Clouds of ejecta spray from Dimorphos after the strike, as seen by the Italian Space Agency’s LICIACube 
The DART team celebrate the impact 
Chile’s SOAR telescope captured this 10,000km trail of debris streaming from the distant moonlet
Could humanity save itself from a killer asteroid? On 26 September, the Double Asteroid Redirection Test (DART) mission took us one step closer to taking on the challenge when it slammed into space rock Dimorphos at a speed of 22,530km/h (over 6km/s). Confirmation of the impact reached Earth at 7:14pm EDT (11:14pm UT), when the signal cut out halfway through sending back its final image.
Launched on 24 November 2021, DART’s destination was Dimorphos, a 160-metre-wide ‘moon’ orbiting the larger Didymos. Though DART’s camera was only able to target Dimorphos in the last hour of its mission, it transmitted images of its journey right up until the last moment.
“As we were getting close there was both joy and terror,” said Elena Adams, DART’s systems engineer. “We had no idea what to expect – we didn’t even know what shape the asteroid was.”
The impact was also witnessed by Italy’s asteroid- imaging spacecraft LICIACube, which flew past Dimorphos about three minutes later and captured the cloud of ejecta thrown up by the impact.
The goal is to shorten Dimorphos’s orbit around Didymos, currently 11 hours 55 minutes, by around 10 minutes. Given enough time, even a change that small could alter the path of an asteroid on a collision course with Earth, should one ever be discovered. Over the next two months, ground and space observatories, including the James Webb Space Telescope, will scrutinise the system to confirm how much its orbit has changed.
“As far as we can tell, our first planetary defence test was a success,” said Adams. “Earthlings should sleep a little better. I definitely will.” dart.jhuapl.edu
Comment by Chris Lintott
Back in 2005, I watched from Palomar Observatory as the Deep Impact spacecraft collided with comet Tempel 1. Despite widespread excitement, the mighty 200-inch telescope saw nothing at all. This time, it was the other way round. No one expected to see much, but even small back-garden telescopes could see the asteroid brightening, while larger telescopes saw a cloud of debris thrown up by the impact. If DART had a bigger effect than expected, it confirms the impression that Dimorphos is more like a pile of rubble than a solid rock. Whether DART succeeded in changing the asteroid’s orbit or not, we’ve learnt much already from its spectacular end
Chris Lintott co-presents The Sky at Night