NASA's Double Asteroid Redirection Test, or DART, was the first real attempt to move an asteroid on purpose. The spacecraft launched on a SpaceX Falcon 9 from Vandenberg Space Force Base in California on 24 November 2021 (late on 23 November, local time). Ten months later, on 26 September 2022, it slammed into Dimorphos, a small asteroid moon about 160 metres across, roughly 11 million kilometres from Earth.
The result was clear. Before the impact, Dimorphos took 11 hours and 55 minutes to circle its larger partner, Didymos. Afterwards the orbit was about 32 minutes shorter. NASA had set 73 seconds as the minimum change that would count as success. For the first time, humans had measurably changed the motion of a natural object in space.
Why NASA crashed a spacecraft into an asteroid
DART was a test of the "kinetic impactor" idea: if a dangerous asteroid were found years before it reached Earth, a spacecraft could hit it hard enough to nudge it off course. A tiny change in speed, applied early, adds up to a large miss distance over many orbits.
Neither Didymos nor Dimorphos was ever a threat to Earth, and the impact could not make them one. The pair was chosen because Dimorphos orbits Didymos. Changing the moon's orbit produces a change in its period that telescopes on Earth can measure within weeks, rather than waiting years to see a shift in an orbit around the Sun.
Before launch, NASA's Planetary Defense Officer Lindley Johnson explained the reasoning: there was no known asteroid on course to hit Earth, but "we don't want to be in a situation where an asteroid is headed towards Earth and then have to test this capability."
Liftoff! pic.twitter.com/Kx5n6TwriC
— SpaceX (@SpaceX) November 24, 2021
The spacecraft and the journey
DART itself was about the size of a small car, with long roll-out solar arrays. It weighed a little over half a tonne at the moment of impact. Its only instrument was a camera called DRACO, which fed images to an on-board navigation system known as SMART Nav, so the spacecraft could find and steer towards Dimorphos by itself in the final hours. Dimorphos was too small and too faint to pick out until about an hour before impact.
The mission, built and run by the Johns Hopkins Applied Physics Laboratory (APL) for NASA, cost around $330 million. It carried a tiny companion, LICIACube, a cubesat from the Italian Space Agency that separated about two weeks before impact and flew past afterwards to photograph the plume of debris.
Impact day and the measured result
DART struck Dimorphos at roughly 22,500 kilometres per hour (about 14,000 mph), a little over 6 kilometres per second. Its last images showed a boulder-strewn surface coming up fast before the signal ended. Telescopes on Earth, the Hubble and James Webb space telescopes, and LICIACube all watched a huge cloud of rock and dust blast away from the asteroid, later stretching into a tail thousands of kilometres long.
In October 2022 NASA announced the key measurement: the orbit had shrunk from 11 hours 55 minutes to about 11 hours 23 minutes. Later analysis refined the change to around 33 minutes, give or take a minute. Before launch, APL planetary scientist Nancy Chabot had described the expected effect as only a small nudge, roughly a 1% change in the orbital period, perhaps from 11 hours 55 minutes to 11 hours 45 minutes. The real effect was about three times bigger.
Why the nudge was bigger than expected
The extra push came from the debris. When DART hit, it threw out a large mass of material. That spray of rock acted a bit like a rocket exhaust, pushing Dimorphos further in the opposite direction. Studies published in Nature in 2023 put the momentum boost at about 3.6 times what the spacecraft alone would have delivered, meaning the ejecta pushed harder than DART itself.
Dimorphos appears to be a loosely bound "rubble pile" of boulders and gravel rather than a solid rock, which helps explain the large plume. Earlier predictions had treated its make-up as one of the big unknowns. Scientists had suggested it was similar to ordinary chondrite meteorites, the most common type of stony meteorite, a mix of rock and metal grains formed around 4.5 billion years ago.
The lesson is not that every asteroid would react the same way. A solid, metallic or fast-spinning body might respond differently, which is why the follow-up mission matters.
What comes next: Hera and finding asteroids early
The European Space Agency's Hera mission launched in October 2024 to visit Didymos and Dimorphos and study the aftermath up close. It is due to arrive in November 2026, a month earlier than first planned, and will measure Dimorphos's mass, shape and the crater (or reshaping) left by DART. That will turn DART from a single experiment into a calibrated tool that could be used in a real emergency.
Deflection only works if a threat is spotted early, ideally years or decades in advance. NASA is building NEO Surveyor, an infrared space telescope designed to find hard-to-see near-Earth asteroids. Recent scares show why: in February 2025 the asteroid 2024 YR4 briefly had an estimated impact chance of 3.1% for December 2032, before further observations within days ruled out an Earth impact.
Common misconceptions about DART
- "NASA tried to destroy the asteroid." No. The aim was a measurable push, not to blow Dimorphos apart. Breaking up a threatening asteroid could create several dangerous pieces instead of one.
- "The asteroid was heading for Earth." Neither body was or is on a collision course. It was a safe target chosen for testing.
- "The launch was the deflection." The launch was in November 2021; the impact and orbit change came in September 2022.
- "This means we are safe from all asteroids." DART proved one method works on one kind of asteroid. Planetary defence still depends on finding objects early and understanding what they are made of.
Sources: NASA: DART mission impact changed asteroid's motion in space (October 2022); Johns Hopkins APL press release; IBTimes: pre-launch interviews with Lindley Johnson and Nancy Chabot (November 2021); Wikipedia: Double Asteroid Redirection Test; Wikipedia: Hera (space mission); Wikipedia: 2024 YR4; launch video: SpaceX on X/Twitter.
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