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Radar images of Didymos and its satellite Dimorphos by the Arecibo Observatory in 2003
Discovered byPetr Pravec et al.
Discovery date20 November 2003
Didymos I
Named after
Greek word for "having two forms"
S/2003 (65803) 1
Orbital characteristics
1.19±0.03 km
Eccentricity< 0.05
0.4971±0.0004 d
or 11.93±0.01 hr
Satellite of65803 Didymos
Physical characteristics
Mean diameter
0.170±0.030 km
or 170±30 m
21.3±0.2 (difference from primary)

Dimorphos is a small asteroid satellite that was discovered in 2003. It is the minor-planet moon of a synchronous binary system with 65803 Didymos as the primary asteroid. After being provisionally designated as S/2003 (65803) 1 with informal nicknames such as "Didymos B" and "Didymoon", the Working Group Small Body Nomenclature (WGSBN) of the International Astronomical Union gave the satellite its official name on 23 June 2020. At a diameter of 170 metres (560 ft), it is one of the smallest astronomical objects that has been given a permanent name.

Dimorphos is the target of NASA's Double Asteroid Redirection Test (DART), a mission spacecraft which was launched on 24 November 2021 with a planned impact between 26 September and 1 October 2022. The DART mission is a test of planetary defence against asteroids. A flyby mini-satellite named LICIACube, provided by the Italian Space Agency, will photograph the collision. The European Space Agency's Hera mission will observe the asteroid after the impact.


The primary asteroid was discovered in 1996 by Joe Montani of the Spacewatch Project at the University of Arizona. The name Didymos was officially approved in 2004. Petr Pravec of the Ondřejov Observatory in the Czech Republic found in 2003 that the asteroid had a satellite orbiting it. With his collaborators, he confirmed from the Arecibo radar delay-Doppler images that Didymos is a binary system.


The name is derived from a Greek word Dimorphos, meaning "having two forms". The name was suggested by planetary scientist Kleomenis Tsiganis at the Aristotle University of Thessaloniki. The justification for the new name reads: "As the target of the DART and Hera space missions, it will become the first celestial body in cosmic history whose form was substantially changed as a result of human intervention (the DART impact)". Tsiganis further explained that the name "has been chosen in anticipation of its changes. It will be known to us in two very different forms, the one seen by DART before the impact, and the other seen by Hera a few years later".

Prior to the IAU naming, the nickname "Didymoon" was used in official communications.


Shape model of Didymos and its satellite Dimorphos, based on photometric lightcurve and radar data

The primary body of the binary system, Didymos, orbits the Sun at a distance of 1.0–2.3 AU once every 2 years and 1 month (770 days). The pathway of the orbit has an eccentricity of 0.38 and an inclination of 3° with respect to the ecliptic. It was known to be closest to Earth in November 2003 with a distance of 7.18 million km (4.46 million mi). It is not expected to pass that near again for 120 years.

Dimorphos moves in a nearly equatorial, nearly circular orbit around Didymos, with an orbital period of 11.9 hours. Its orbital motion is synchronous with the rotation of Didymos, so that Dimorphos always faces the same side of Didymos. Dimorphos's orbit is retrograde relative to the ecliptic plane, in conformity with Didymos's retrograde rotation.

Dimorphos measures approximately 160 metres (520 ft) in diameter, compared to Didymos which is 780 metres (2,560 ft) across.


On 24 November 2021, NASA launched the Double Asteroid Redirection Test (DART) spacecraft on a SpaceX Falcon 9 rocket from Space Launch Complex 4 East at Vandenberg Space Force Base in California, US. DART is the first experiment for defending Earth from hazardous asteroids, and the spacecraft will try to deflect Dimorphos slightly from its position. The spacecraft is expected to crash into Dimorphos at a speed of around 15,000 mph (6.6 km/s) between 26 September and 1 October 2022. The collision is expected to bring Dimorphos and Didymos closer to each other. Dimorphos will then circle Didymos faster than before, so that its orbit is shortened by at least 10 minutes.

The DART spacecraft is accompanied by LICIACube, a flyby Cubesat of the Italian Space Agency (ASI) 6-Unit, that will be released 10 days before impact to record the collision. Then the European Space Agency will send its Hera spacecraft to Dimorphos in 2024 to study the impact crater and the new orbit of the binary system.