The planetary defence target
Didymos
Didymos is a near-Earth asteroid about 800 metres across, orbited by a small moon, Dimorphos. On 26 September 2022 NASA's DART spacecraft struck the moon on purpose and changed its orbit: it was the first time humanity had moved a celestial body, and the first proof that an asteroid on a collision course could be deflected.
- Size
- 818 × 801 × 604 m
- Distance from Sun
- 246 M km
- Orbital period
- 2.11 years
- Rotation
- 2.26 hours
- Discovered
- 1996
- Moon
- Dimorphos
Physical characteristics
Didymos measures 818 × 801 × 604 metres and is shaped like a squashed spinning top, with a bulge at the equator. It spins in just 2 hours and 16 minutes, close to the limit beyond which centrifugal force would tear material off the surface. It is a rocky S-type asteroid, probably a rubble pile.
It is this fast spin that explains the existence of the moon: Didymos is thought to have shed debris from its equator in the past, and that debris, gathering in orbit, formed Dimorphos. About one near-Earth asteroid in six has a companion, and this is the most likely mechanism.
Orbit and rotation
Didymos takes 2.11 years to travel a very elongated orbit, lying between 1.01 and 2.27 AU from the Sun, 246 million km on average, tilted 3.4° to the ecliptic: at perihelion it comes level with Earth's orbit, at aphelion it goes beyond that of Mars. Neither Didymos nor its moon is a threat to Earth: the experiment was carried out here precisely because there was no risk, before or after.
Close approaches to Earth are rare: the one in 2003 led to the discovery of the moon, and the one in October 2022, at about 11 million km, was chosen for the DART impact because it allowed the system to be followed with ground-based telescopes. The next comparable approach will be in 2062.
Dimorphos and the DART impact
Before the impact Dimorphos measured 178 × 169 × 114 metres and circled Didymos about 1.2 km from its centre, in 11 hours and 55 minutes. It is a pile of boulders with no cohesion, more like a heap of gravel than a rock. Its existence was inferred in 2003 from variations in the system's brightness, and confirmed by radar.
The DART spacecraft, the size of a vending machine, struck it on 26 September 2022 at 23:14 UTC, at 6.1 km per second. Its orbital period shortened by 33 minutes, far more than the 7 expected from the collision alone: the difference came from the material blasted away, which pushed the moon like a rocket exhaust. The small Italian satellite LICIACube, released two weeks earlier, recorded the scene as it passed less than 60 km away.
Discovery and exploration
Didymos was discovered on 11 April 1996 by the Spacewatch programme at Kitt Peak in Arizona. Its name means "twin" in Greek and was given when it was found to be double; the moon was named Dimorphos, "having two forms", because it is the first celestial body whose orbit has been changed by humans.
To measure the effect of the collision precisely, the European Space Agency launched the Hera spacecraft on 7 October 2024; it flew past Mars in March 2025 and is due to arrive at Didymos in November 2026. Together with two small satellites, Milani and Juventas, it will measure the mass of Dimorphos, the shape of the crater and the internal structure of both bodies: the data still needed to turn a successful experiment into a reliable technique.
Frequently asked questions
Was Earth ever in danger?
No. Didymos and Dimorphos are not on a collision course with Earth, and the DART impact could not change that: it altered only the moon's orbit around the main asteroid, not the system's orbit around the Sun. It was chosen precisely because, being double, it made it possible to measure a tiny deflection from the ground.
How much did Dimorphos's orbit change?
Its orbital period went from 11 hours 55 minutes to 11 hours 22 minutes: 33 minutes less, give or take one minute. The mission's minimum goal was 73 seconds. The effect was so large because the debris thrown out by the collision added its push to that of the spacecraft.
What is the Hera mission for?
To understand exactly what happened. DART measured how much the orbit changed, but not the mass of Dimorphos or what the crater looks like: without those data it is impossible to calculate how much push would be needed to deflect another asteroid. Hera will measure them on site, after its arrival, planned for November 2026.
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