How DART changed an asteroid's orbit

On 26 September 2022, NASA's DART spacecraft (the Double Asteroid Redirection Test) flew into Dimorphos, a small moon of the asteroid Didymos, at about 6 kilometres per second. It was the first time people had changed the motion of a natural body in space. Neither asteroid was, or is, any danger to Earth: the point was to test whether a fast impact could nudge an asteroid, and by how much.

1. Step 1 of 7

   Choosing the target

   The target was a binary asteroid: Didymos, about 780 metres across, with a moon called Dimorphos, about 150 metres across, circling it roughly 1.2 kilometres away once every 11 hours and 55 minutes. Dimorphos is about the size of the asteroids that planetary defence is most concerned with: big enough to devastate a region if one struck Earth.

   A binary was chosen for a clever reason. Changing an asteroid's path around the Sun by a tiny amount would be very hard to measure, but changing a moon's orbit around its partner is easy to time from Earth, because the two dim each other as they pass in front of and behind one another. And because Dimorphos was bound to Didymos, the test could not send either of them anywhere new.

2. Step 2 of 7

   Launch and cruise

   DART launched on 24 November 2021 from California. It was a fairly simple spacecraft of about 610 kilograms at launch, with large roll-out solar arrays, an ion engine that was tested along the way and a single camera, DRACO, to see the target. It carried no explosive: the impact relied on speed alone.

   Its route was timed so that it would meet Didymos in late September 2022, when the pair passed within about 11 million kilometres of Earth, close enough for telescopes to watch. About two weeks before impact it released a small Italian satellite, LICIACube, to follow behind and photograph the result.

3. Step 3 of 7

   Finding a tiny moon

   Dimorphos was too small and faint to see from Earth as a separate object, so DART had to find it by itself. For the final four hours it steered autonomously, using its camera and on-board software to lock on, because signals took about 38 seconds to reach Earth and the same to come back, far too slow for anyone to steer by hand.

   Dimorphos first appeared as a separate dot only about an hour before impact. In the last minutes it grew to fill the camera's view, revealing for the first time an egg-shaped pile of boulders and rubble. The final complete image was taken from about 12 kilometres away, two seconds before impact.

4. Step 4 of 7

   Impact

   At 23:14 UTC on 26 September 2022, DART flew into Dimorphos at about 6.1 kilometres per second. By then it weighed about 580 kilograms, having used some of its fuel. It hit only about 25 metres from the centre of the moon, and was aimed to strike roughly against the direction of the moon's orbit, so as to slow it down.

   A body moving at that speed carries far more energy than the same mass of explosive. The spacecraft was destroyed instantly, and its momentum was passed on to the asteroid moon, a body millions of times heavier.

5. Step 5 of 7

   The ejecta did more of the pushing

   The impact threw out a great spray of rock and dust, probably thousands of tonnes of it, backwards along the path DART had come in on. Just as a rifle kicks back when it fires, that blast of ejecta pushed Dimorphos the other way, adding to the spacecraft's own push.

   Scientists express this as a momentum enhancement factor. Their best estimates are that the ejecta increased the push by a factor of roughly 2 to 5 compared with DART alone, depending on the moon's mass, which is not known exactly. Dimorphos, a loose rubble pile, gave up a lot of material easily, which helped. Telescopes, including Hubble, watched a tail of debris more than 10,000 kilometres long stream away over the following weeks.

6. Step 6 of 7

   Measuring the change

   Telescopes around the world, and radar, then watched the pair. As Dimorphos passes in front of or behind Didymos, the total brightness dips slightly. Timing those dips over many nights showed that the orbit had changed from 11 hours 55 minutes to about 11 hours 22 minutes: shorter by about 33 minutes.

   The mission's minimum goal had been a change of 73 seconds. The real change was more than 25 times larger, which showed that the method works, and that the ejecta matters a great deal. Later analysis found that Dimorphos's orbit had also become slightly smaller and more stretched, and that the moon itself had probably been reshaped by the impact.

7. Step 7 of 7

   What it means, and what comes next

   DART showed that a kinetic impactor, a spacecraft simply flown into an asteroid, can change its motion in a predictable way. The push is tiny, but applied years or decades before a predicted impact, a change of a fraction of a millimetre per second grows into a miss of thousands of kilometres. The key requirement is warning time, which is why finding near-Earth asteroids early matters so much.

   The follow-up is under way. The European Space Agency's Hera spacecraft launched in October 2024 and is due to reach Didymos in late 2026. It will measure Dimorphos's mass, map the crater or reshaping DART caused and so pin down exactly how much push the impact delivered, turning one experiment into a reliable tool.

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