Dimorphos

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Dimorphos: The Moon of Didymos

What it is
Dimorphos is a small moon orbiting the near-Earth asteroid Didymos. The two bodies form a binary system. Dimorphos is about 150 meters across, a rubble-pile world covered with boulders. Its name comes from a Greek word meaning “having two forms,” reflecting the system’s two bodies.

Discovery and name
Dimorphos was discovered on November 20, 2003, by Petr Pravec and colleagues at Ondřejov Observatory. It was officially named Dimorphos by the IAU in 2020 (it was previously nicknamed Didymoon).

Orbit around Didymos
Dimorphos orbits Didymos in a nearly circular, equatorial path. Before the DART impact, it completed a loop roughly every 11.9 hours. The moon’s orbit is retrograde relative to the main asteroid’s motion.

DART impact and what happened
In September 2022, NASA’s DART mission intentionally collided with Dimorphos at about 24,000 km/h to test asteroid deflection. The hit shortened Dimorphos’s orbital period around Didymos by about 32–33 minutes. A pile of debris, ejected during the impact, amplified the momentum change—estimates suggest ejecta added roughly 2 to 5 times the effect of the spacecraft alone. The collision created a bright dust plume and a 10,000-km-long dust tail that lasted for months. The impact likely reshaped Dimorphos from a simple oval into a more flattened, ellipsoid shape and may have put it into a chaotic spin, though tides will gradually re-lock it with Didymos over decades.

Observations and future study
The DART mission’s camera and the LICIACube CubeSat carefully photographed the event and its aftermath. ESA’s Hera mission is planned to visit the Didymos system in 2026 to study the long-term effects of DART’s impact.

Didymos system context
Didymos, the larger primary asteroid, is about 780 meters across. Dimorphos is the smaller moon in this binary pair, which made it the first celestial body altered by human-made impact.