Didymos and Dimorphos
Key facts
- What it is
- Double near-Earth asteroid (Apollo group)
- Size
- About 780 m (Didymos) and 150 m (Dimorphos)
- Discovered
- 1996
- Spacecraft
- DART impact, 2022; Hera due late 2026
- Orbit group
- Apollo
- Average distance from the Sun
- 1.64 au (about 250 million km)
- Time to go round the Sun
- 2.1 years
- Closest and farthest from the Sun
- 1.01 to 2.27 au
- Tilt of its orbit
- 3.4°
- Closest its orbit comes to Earth's
- 5,900,000 km (15 times the Moon's distance)
- Spins once every
- 2.3 hours
An au (astronomical unit) is Earth's average distance from the Sun, about 150 million km. Orbit figures are from open tracking data, figures last updated 1 Nov 2026.
Didymos is a near-Earth asteroid about 780 metres across with a small moon, Dimorphos, about 150 metres wide, circling it at a distance of a little over a kilometre. The pair were chosen for the first real test of whether humans could change the path of an asteroid. Neither was ever any danger to Earth, which is exactly why they made a safe test.
On 26 September 2022 NASA's DART spacecraft, about the mass of a small car, deliberately crashed into Dimorphos at about 6 kilometres per second. A small Italian spacecraft, LICIACube, released beforehand, photographed the plume of debris thrown out by the impact.
Before the impact, Dimorphos took 11 hours and 55 minutes to circle Didymos. Afterwards, telescopes on Earth measured that its orbit had shortened by about 33 minutes, far more than the minimum the mission needed to count as a success. Much of the extra push came from the large amount of rock blasted away, which acted like the exhaust of a rocket. The impact also changed Dimorphos's shape, since it is a loose rubble pile rather than solid rock.
The European Space Agency's Hera spacecraft, launched in October 2024, is due to reach the pair in late 2026 to measure the crater, the mass of Dimorphos and the exact effect of the impact. That will turn a successful experiment into a reliable technique.
The test showed that, given enough warning, a small asteroid could be nudged off course. Finding asteroids early, years or decades ahead, is what makes that possible.
Coming close to Earth
Its next passes within about 30 million km of Earth, worked out from its orbit. Each one is a safe miss.
| Date | Distance | In Moon distances |
|---|---|---|
| 20 Oct 2062 | 7,400,000 km | 19 |
| 29 Nov 2083 | 22,000,000 km | 57 |
| 19 Sept 2102 | 23,000,000 km | 59 |
| 4 Nov 2123 | 5,900,000 km | 15 |
| 2 Dec 2144 | 27,000,000 km | 69 |
| 17 Sept 2163 | 27,000,000 km | 71 |