Last September, NASA’s Double Asteroid Redirection Test (DART) spacecraft smashed into an asteroid, deliberately altering the rock’s trajectory through space in a first test of planetary defence. Now scientists have deconstructed the collision and its aftermath — and learnt just how successful humanity’s punch at the cosmos really was.

DART, which was the size of a golf cart, collided with a Great Pyramid-sized asteroid called Dimorphos. The impact caused the asteroid’s orbit around another space rock to shrink — Dimorphos now completes an orbit 33 minutes faster than before the impact, researchers report¹ today in Nature. This means that if a dangerous asteroid were ever detected heading for Earth, a mission to smash into it would probably be able to divert it away from the planet.

DART’s success has been reported before; now, five studies in Nature describe the final moments of the crash and how it affected the asteroid. One group combined data on the spacecraft’s trajectory with photographs of the asteroid’s surface just before impact². As DART hurtled towards Dimorphos at more than 6 kilometres per second, the first part that hit was one of its solar panels, which smashed into a 6.5-metre-wide boulder. Microseconds later, the main body of the spacecraft collided with the rocky surface next to the boulder — and the US$330-million DART shattered to bits.

The impact ejected at least one million kilograms of rock from Dimorphos’s 4.3-billion-kilogram mass. The debris formed a tail that stretched for tens of thousands of kilometres behind the asteroid. Various telescopes watched over weeks as the tail shifted and evolved under the pressure of the Sun’s rays; the Hubble Space Telescope even detected a second tail, which had disappeared by 18 days after the impact³.

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