DART — humanity moves an asteroid

Slamming into Dimorphos at 22,000 km/h, DART shortened its orbit by 32 minutes — planetary defence's first field test.

The moment of DART — humanity moves an asteroid: https://nowsky.org/e/dart-impact — opens at the right place and time, replayable, no login.

Available views: Orbit ✓ · Sky — (not visible in the sky)

The story

At 23:14 UT on 26 September 2022 the DART spacecraft, the size of a vending machine, struck the asteroid Dimorphos head-on at 22,000 km/h. The live feed, showing boulders rushing up to fill the frame until the last instant, was broadcast worldwide, and the moment the signal cut out was the moment of success.

The goal was to change the 11-hour-55-minute period of the 160 m moonlet Dimorphos around its parent Didymos by at least 73 seconds. The result was 32 minutes — more than 25 times the minimum, showing that the recoil from ejected debris was far stronger than expected. It was humanity's first deliberate change to the orbit of a celestial body.

Fifteen days later the Italian cubesat LICIACube, following behind, photographed the debris cloud from just after impact. The tail of debris seen by ground telescopes, Hubble and Webb stretched more than 10,000 km and looked like an artificial comet for months; about a million kilograms were thrown off Dimorphos.

The Didymos system was chosen as a safe test range that poses no threat to Earth. The precise follow-up — the crater's size, Dimorphos's mass and interior — falls to ESA's Hera, due to arrive in December 2026. The orbit view shows Didymos's orbit.

Why it matters

DART proved that planetary defence is a technology, not a theory. In the thirty years since Shoemaker–Levy 9 in 1994 the reality of impacts had been accepted, but there was no means of response. With a kinetic impactor shown to change an asteroid's orbit by a measurable amount, a threatening asteroid found decades in advance could be nudged aside.

The 32-minute result also says that asteroids are rubble piles. A solid rock would have recoiled little; Dimorphos, a loosely bound heap of gravel, threw off mass in quantity and gained extra push from it. Models of asteroid interiors were updated accordingly.

The DART–Hera pairing is a model of international cooperation: the United States took the impact and Europe the verification, splitting the cost, and when Hera arrives it will provide the first complete before-and-after data set.

How to watch

Didymos, 780 m across, is around 14th magnitude and shows as a point only in large amateur telescopes. Switch on asteroids in the orbit view and select Didymos for its 2.1-year orbit reaching from outside Earth's orbit to beyond Mars.

Choose the DART impact in the orbit view's event list to reproduce the geometry of 26 September 2022, 11 million km from Earth; viewed together with Hera's scheduled arrival in December 2026, the relationship between the two missions is clear.

The change in Dimorphos's period was measured by ground telescopes timing the slight dips in Didymos's brightness each time the moonlet passed in front. The sky view shows which constellation Didymos is in now.

See this for yourself — replay the moment in the orbit view

Background

DART launched on a Falcon 9 on 24 November 2021 and flew ten months; from four hours before impact its autonomous SMART Nav system identified and targeted Dimorphos on its own. Until the final approach Dimorphos had never been resolved from the ground.

Dimorphos's period before impact was 11 h 55 m 18 s and afterwards 11 h 22 m 37 s — 32 minutes 41 seconds shorter. Simulations in 2024 suggested the whole moonlet may have been reshaped rather than cratered, which Hera is to confirm.

Hera launched on 7 October 2024, passed Mars in March 2025 and is scheduled to reach Didymos in December 2026 for a six-month survey with two cubesats.

There was a computational reason for choosing Dimorphos as the target. Changing a lone asteroid’s period around the Sun would take years to measure, but a change in a moonlet’s twelve-hour period around its parent can be measured by ground telescopes within days. The Didymos system, found to be a binary in 2003, already had precise records of the dips in brightness as Dimorphos passed in front of and behind Didymos. The change in the period of that light curve after impact is the basis of the 32-minute result.

FAQ

How much did the orbit change?

Dimorphos's period shortened by 32 minutes 41 seconds — more than 25 times the 73-second minimum goal.

Was there any danger to Earth?

No — the Didymos system was chosen because it does not cross Earth's orbit.

Why was the effect larger than expected?

Dimorphos is a loose rubble pile; the mass thrown off gave extra recoil.

When does Hera arrive?

Scheduled for December 2026, to survey the crater and Dimorphos's mass and interior.

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.