On September 26, 2022, a NASA spacecraft about the size of a vending machine flew into a small asteroid at several kilometers per second, on purpose. It was the first time humans measurably changed the motion of a natural object in space. The mission was called DART, short for Double Asteroid Redirection Test, and its result is the foundation of the idea that we could one day push a dangerous asteroid out of the way.
Surveys have found most of the very large near-Earth objects, and none of them is on course to hit Earth in the coming century. But smaller asteroids, large enough to destroy a city, are far more numerous and many remain undiscovered. If one of them were found on a collision course years ahead, the best response might be to nudge it. Until 2022 nobody had tried that for real, so the idea rested on computer models.
DART was designed to turn the model into a measurement. The question was simple: if we hit an asteroid with a spacecraft, how much does its path change?
The target was Dimorphos, a small moonlet about 160 meters across (roughly the size of a large stadium) that circles a larger asteroid called Didymos, which is about 780 meters wide. Before the impact Dimorphos orbited Didymos once every 11 hours and 55 minutes.
Choosing a double asteroid was clever. Astronomers on Earth can time the small moon's orbit very precisely by watching the combined brightness of the pair dim each time one body passes in front of the other. A change of a few minutes is easy to detect that way. Neither body posed any threat to Earth, before or after the test.
DART launched on a SpaceX Falcon 9 from California on November 24, 2021. The spacecraft weighed roughly 600 kilograms, and it carried one main instrument, a camera called DRACO, plus an autonomous navigation system. Dimorphos was too small to see from far away, so in the final hours the spacecraft had to find it and steer itself, without any help from controllers.
It struck on September 26, 2022, at about 6 kilometers per second, or roughly 22,500 kilometers per hour. A small Italian cubesat, LICIACube, had been released days earlier and flew past a few minutes later, photographing the plume of debris. Telescopes on the ground and in space, including Hubble and the James Webb Space Telescope, also watched.
NASA had set a modest success threshold: shorten the orbit by at least 73 seconds. In October 2022 the team announced that the orbit had shrunk by about 32 minutes, to 11 hours and 23 minutes, with an uncertainty of a couple of minutes. A later peer-reviewed analysis, published in Nature in 2023, refined the change to roughly 33 minutes. That is many times the minimum goal.
The bigger surprise was where the extra push came from. The spacecraft's own momentum was only part of the story. The impact threw a large cloud of rock and dust off Dimorphos, and that ejecta flying away acted like a rocket exhaust, pushing the moon the other way. Scientists describe the total effect with a number called beta, the ratio of the momentum actually transferred to the momentum the spacecraft brought. The Nature paper reported a value of about 3.6 under its main assumptions, meaning the impact delivered several times more push than the spacecraft alone. Beta depends on the asteroid's density and structure, so the exact figure carries uncertainty.
Images showed that Dimorphos is not a solid rock but a loose pile of rubble held together by its own weak gravity. That is probably why it shed so much material. Telescopes saw a long tail of debris streaming away for weeks after the impact. Researchers also found that the impact changed the moon's shape, and it may have shifted the way it wobbles. Follow-up studies are still refining these details, and a few of the conclusions are debated.
This matters because real hazardous asteroids differ from one another. A dense metallic body would respond differently than a rubble pile. DART tested one case, and engineers now know that this approach works on at least one common type.
DART left several questions open, such as the exact mass of Dimorphos and the shape of the crater. ESA's Hera spacecraft, launched in October 2024, is designed to answer them by visiting the Didymos pair and surveying the aftermath up close. Its arrival is planned for the end of 2026, so check ESA's pages for the latest schedule. The data will help scientists scale the results to other asteroids. You can read the wider story in our article on planetary defense.
It is worth keeping the test in proportion. Moving a small moon by a few minutes of orbit time is not the same as moving a large asteroid in time to save a region. Real deflection would need early warning, ideally years or decades, which is why tracking is just as important as spacecraft. See how NASA tracks asteroids.
No deflection is planned for Apophis, because it poses no impact threat, as explained in Is Apophis Dangerous?. But the same planetary-defense community is preparing to study Apophis up close during its 2029 flyby. Learn more in our Apophis 2029 guide.
Facts in this article are drawn from public scientific sources. For primary information, see: