DART — humanity moves an asteroid
On 26 September 2022 the DART spacecraft deliberately struck the asteroid Dimorphos and shortened its orbit by about 33 minutes — the first human change to a celestial body’s orbit and planetary defence’s first real test.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
At 23:14 UT on 26 September 2022, the DART spacecraft, about the size of a large vending machine, slammed head-on into the asteroid Dimorphos at about 22,000 km/h. That is fast enough to cross from London to Paris (about 340 km) in under a minute. The pictures from its camera were broadcast live around the world. Boulders on the asteroid’s surface grew larger and larger until they filled the screen, and then the signal stopped. That moment was the moment of success.
Dimorphos is a small asteroid about 160 m across that circles a larger asteroid, Didymos, about once every 11 hours 55 minutes. The goal was to change that period by at least 73 seconds. In fact it became about 33 minutes shorter, more than 25 times the goal. The rock and dust thrown out by the impact pushed Dimorphos a second time in the opposite direction, and that push was much stronger than expected. It was the first time people had deliberately changed the orbit of a natural object in space.
Fifteen days before impact, a small Italian satellite called LICIACube separated from DART. It flew past about three minutes after the impact and photographed the rising cloud of debris. Telescopes on the ground, along with Hubble and Webb, also watched the aftermath. More than a million kilograms of material were thrown off, and the dust formed a tail more than 10,000 km long that made the asteroid look like a comet for a while.
Didymos and Dimorphos were chosen as a safe test target: they pose no danger to Earth, and the impact could not make them dangerous. A closer look at the results falls to the European Space Agency’s Hera spacecraft, due to reach Didymos in late 2026. The orbit view shows Didymos circling the Sun on a path that runs from just outside Earth’s orbit to beyond the orbit of Mars.
Mission goals
DART showed that planetary defence, protecting Earth from asteroid impacts, can be a real technology rather than an idea. After comet Shoemaker–Levy 9 crashed into Jupiter in 1994, everyone knew that large objects really can hit planets. But no way of stopping one had ever been tested. Now it is possible to calculate that if a dangerous asteroid is found early enough, for example decades in advance, a spacecraft could strike it and nudge its path aside.
The large change of about 33 minutes also says something about what Dimorphos is like inside. If it were one solid rock, less material would have been thrown out and the change would have been smaller. Scientists think Dimorphos is a loosely held “rubble pile” of rocks and gravel, so a lot of material flew off and gave it a much bigger push.
DART and Hera are a good example of international cooperation. The United States carried out the impact and Europe will study the results closely. When Hera arrives, there will be data to compare the asteroid before and after the impact, making it possible to work out more precisely how big a spacecraft to send, and when, if a real threat ever appears.
Mission chronology and background
DART was launched on a Falcon 9 rocket on 24 November 2021 and flew for about ten months. About four hours before impact, from roughly 90,000 km away, an autonomous navigation system called SMART Nav took over, finding and aiming at Dimorphos on its own. Steering by signals from Earth would have been too slow. Dimorphos was so small and distant that its shape had never been seen before this final approach.
There was a good reason for choosing Dimorphos as the target. Changing the period of a lone asteroid circling the Sun would take years to measure. But the roughly twelve-hour orbit of a small asteroid around a larger one can be measured by ground telescopes within days. Didymos was found to be a pair in 2003, and its brightness changes had been recorded ever since, which made it easy to compare before and after the impact.
Hera launched on 7 October 2024 and swung past Mars in March 2025 to change course. At Didymos it is to survey the two asteroids for about six months together with two small satellites, Milani and Juventas. Computer simulations published in 2024 suggested that DART may have reshaped all of Dimorphos rather than leaving a single crater, and Hera is expected to check this.
Other milestones of this mission
Where to see it — sky and simulator
Didymos is about 780 m across, but it is very faint, so even when it comes nearer to Earth it shows only as a small point in large telescopes. Switch on asteroids in the orbit view and select Didymos to see its orbit, which takes about 2.1 years to go around the Sun. Choose the DART impact in the orbit view’s event list to see the arrangement on 26 September 2022, when the impact happened about 11 million km from Earth.
The change in Dimorphos’s period was found using telescopes on the ground. Each time Dimorphos passes in front of or behind Didymos, their combined brightness dips slightly, and timing those dips reveals the period of its orbit. Select Didymos in the sky view to see which constellation it is in now.
See this for yourself — replay the moment in the orbit view
FAQ
How much did the orbit change?
Dimorphos’s period became about 33 minutes shorter. The first announcement said 32 minutes, later refined to 33 minutes, give or take one. That is more than 25 times the 73-second minimum goal.
Was there any danger to Earth?
No. Didymos and Dimorphos are not on a path to hit Earth, and they were chosen because the test could not make them dangerous.
Why was the effect bigger than expected?
Dimorphos seems to be a loose rubble pile, so a lot of material flew off, and the recoil pushed it harder.
When does Hera arrive?
It is due in late 2026 and will study the impact’s effects and Dimorphos’s mass and inner structure.
Related
- Hera arrives at Dimorphos
- Hera launch
- Shoemaker–Levy 9 hits Jupiter
- Hera — object index
- Earth — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.