Dimorphos

**Dimorphos** (formal designation **(65803) Didymos I**; provisional designation **S/2003 (65803) 1**) is a natural satellite or moon of the near-Earth

## Dimorphos

Dimorphos as seen by DART, a few seconds before impact.

## Designations

### Alternative Names

S/2003 (65803) 1Didymos B

"Didymoon"

## Orbit Data

### Eccentricity

~0 (pre-impact)0.021±0.014 or 0.0247±0.0002  (post-impact)

### Orbital Period

11.921473±0.000044 hr (Sidereal)(11h 55m 17.3s ± 0.2s; pre-impact)

(11h 22m 03.4s ± 5.0s; post-impact)

## Physical Characteristics

### Mass

(1.33±0.30)×10⁹ kg (if density is 0.6–0.7 g/cm3)≈ 4.3×10⁹ kg (if same density as Didymos)

### Synodic Rotation Period

≈ 11.9 hr (synchronous; pre-impact)chaotic (post-impact)

### Mean Density

0.6–0.7 g/cm³2.4±0.9 g/cm³ (if same as Didymos)

### Absolute Magnitude (H)

21.4±0.2

asteroid 65803 Didymos, with which it forms a binary system. The moon was discovered on 20 November 2003 by Petr Pravec in collaboration with other astronomers worldwide. Dimorphos has a diameter of 177 meters (581 ft) across its longest extent and it was the target of the Double Asteroid Redirection Test (DART), a NASA space mission that deliberately collided a spacecraft with the moon on 26 September 2022 to alter its orbit around Didymos. Before the impact by DART, Dimorphos had a shape of an oblate spheroid with a surface covered in boulders but virtually no craters. The moon is thought to have formed when Didymos shed its mass due to its rapid rotation, which formed an orbiting ring of debris that conglomerated into a low-density rubble pile that became Dimorphos today.

## Discovery

Didymos was discovered in 1996 by Joe Montani of the Spacewatch Project at the University of Arizona.

Dimorphos was discovered on the twentieth of November 2003, in photometric observations by Petr Pravec and colleagues at the Ondřejov Observatory in the Czech Republic. Dimorphos was detected through periodic dips in Didymos's brightness due to mutual eclipses and occultations. With his collaborators, he confirmed from the Arecibo radar delay-Doppler images that Didymos is a binary system.

## Name

The Working Group for Small Bodies Nomenclature of the International Astronomical Union (IAU) gave the satellite its official name on 23 June 2020. The name *Dimorphos* is derived from a Greek word (Δίμορφος) meaning 'having two forms'. 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)". Prior to the IAU naming, the nickname *Didymoon* was used in official communications.

## Size and Shape

Dimorphos is approximately 170 meters (560 ft) in diameter, compared to Didymos at 780 meters (2,560 ft). Dimorphos does not have a confirmed mass, but it is estimated to be about 5×109 kg (5.5 million tons), or about the same mass and size as the Great Pyramid of Giza, when assuming a density of 2.17 g/cm3 similar to Didymos. It is one of the smallest celestial objects given a formal name by the IAU, after 367943 Duende and 469219 Kamoʻoalewa.

The final few minutes of pictures from the DART mission revealed an egg-shaped body covered with boulders, suggesting it has a rubble pile structure.

## Surface

Five boulders (*saxa*) have been given names of traditional drums from several cultures. They are approximately 10 meters across or smaller:

## Orbit

The parent body, Didymos, orbits the sun at a distance of 1.0 to 2.3 AU once every 2 years. The pathway of the orbit has an eccentricity of 0.38 and an inclination of 3° with respect to the ecliptic. On 4 October 2022 Didymos made an Earth approach of 10.6 million km (6.6 million mi). Dimorphos moves in a nearly equatorial, nearly circular orbit around Didymos, with an orbital period of 11.9 hours. Its orbit period is synchronous with its rotation, so that the same side of Dimorphos always faces Didymos. Dimorphos's orbit is retrograde relative to the ecliptic plane, in conformity with Didymos's retrograde rotation.

Dimorphos's rotation is being slowed down by the YORP effect, with an estimated rotation period doubling time of 86,000 years. However, because it is in orbit around Didymos, tidal forces keep the moon locked in synchronous rotation.