NASA's real-life Armageddon mission to move an asteroid off course was even more of a smashing success than early results indicated.
The world watched on as the Double Asteroid Redirection Test (DART) spacecraft ploughed into Dimorphos, the tiny moon of near-Earth asteroid (65803) Didymos, last September.
Full results of the DART mission, published in a series of five papers today in the journal Nature, reveal more detail about the moment of impact and exactly how far the tiny moon shifted.
Measurements of Dimorphos taken for two weeks following impact suggest the moon's orbital period, or the time it takes to complete one orbit of Didymos, was reduced by around 33 minutes — nearly five times greater than predicted.
"The change in its orbit was greater than what many of us, myself included, had expected," said DART mission lead Andy Cheng of Johns Hopkins University Applied Physics Laboratory in Maryland.
"The DART kinetic impact was highly effective for deflecting the asteroid Dimorphos."
Anatomy of asteroid smashing
As DART zoomed towards Dimorphos, cameras on board the spacecraft sent data back to Earth right up until two seconds before the crash.
The images revealed the surface of the egg-shaped moonlet was strewn with boulders resembling a rubble pile.
"Dimorphos is typical of near-Earth objects in terms of composition," Dr Cheng said.
Analysis of the images shows the table-sized spacecraft, travelling at 22,530 kilometres per hour, crashed between two 2-metre-high boulders, hitting one of the boulders with its solar arrays as it came in.
Meanwhile the Hubble Space Telescope and a global network of citizen science telescopes captured stunning images of plumes of debris spewing up to 1,500km into space within the first three hours.
The scientists estimated between 0.3 to 0.5 per cent of the moonlet's mass was ejected into space from the impact.
Over the next two to three weeks, the main dust tail fanned out and faded, giving scientists a glimpse of what happens when space rocks collide.
Why did the moon move so far?
The mission was the first to test the "kinetic impact technique" — aka slamming a spacecraft into an asteroid — as a planetary defence strategy.
The movement of the moonlet was due not so much from the impact of the spacecraft, but the transfer of energy caused by the release of material blasted off its surface.
"[This] put a recoil force into Dimorphos in the same way that when you fire a gun, the gun puts a recoil force into your hand or shoulder," Dr Cheng explained.
"The kinetic impact causes an orbit deflection that is more than twice what would have been expected if there had been no release of ejecta."
What does this mean for planetary defence?
Dr Cheng said the larger momentum produced by debris blasting off the moon in the DART experiment was important for planetary defence.
"This means the same-sized collision that was created by DART would prevent an asteroid strike on Earth for a similar-sized incoming asteroid with less warning time, or prevent an asteroid strike by a larger asteroid with the same warning time," he said.
Estimating warning time matters because an incoming asteroid would need to be smashed into while it was still far enough away from hitting the Earth.
"Otherwise, if the kinetic impact deflection occurs too late and the asteroid is already too close, the asteroid would still hit the Earth."
