Dimorphos.space

Asteroid moon · Didymos binary system (near-Earth)

Dimorphos

(65803) Didymos I · S/2003 (65803) 1

The 170-meter moonlet that NASA's DART deliberately struck in 2022 — the first time humans changed the motion of a celestial body.

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Dimorphos at a glance
Object typeNatural satellite (moon) of near-Earth asteroid 65803 Didymos[5]
DiscoveredNovember 20, 2003 — Petr Pravec and colleagues, Ondřejov Observatory[5]
NamedJune 23, 2020 — Greek for “having two forms”[5]
Former nickname“Didymoon”[5]
Diameter≈ 170 m (560 ft); 177 m along its longest extent[1]
Estimated mass≈ 5 × 10⁹ kg — assuming a density like Didymos's (not measured)[5]
Orbit around Didymos (before DART)≈ 11 h 55 min (11.92 h)[5]
Change from DARTOrbit shortened by 33 minutes[1]
StructureBoulder-covered rubble pile[5]
Surface ageLess than 0.3 million years[5]
Named features5 boulders and 6 craters, named after traditional drums[5]
DART impactSeptember 26, 2022, at ≈ 6.1 km/s[1]
Observed byDART, LICIACube, Hubble, Webb and ground telescopes; ESA's Hera from November 2026[6]

Discovery and name

Dimorphos was discovered on November 20, 2003, from brightness measurements by Petr Pravec and colleagues at Ondřejov Observatory in the Czech Republic.[5] For years it was informally called “Didymoon.”[5]

On June 23, 2020, the IAU named it Dimorphos, from a Greek word meaning “having two forms.” The name was suggested by planetary scientist Kleomenis Tsiganis, a member of both the DART and Hera teams — fitting, since the moon would be seen in two forms: before and after DART.[5]

Size and makeup

Dimorphos is about 170 meters (560 feet) across, compared with roughly 805 meters for Didymos.[1] Its mass has not been measured directly; assuming a density similar to Didymos's, it would be about 5 billion kg — similar to the Great Pyramid of Giza.[5]

DART's final images revealed an egg-shaped body covered with boulders, suggesting a rubble pile.[5] Five boulders and six craters have been named after traditional drums from cultures around the world.[5]

Orbit around Didymos

Dimorphos circles Didymos, and the pair together orbits the Sun between about 1.0 and 2.3 AU every 770 days.[5] Before DART, Dimorphos took about 11.92 hours to complete one orbit around Didymos.[5] Because the two bodies pass in front of each other as seen from Earth, astronomers can time that orbit precisely from brightness dips.[6]

Sun Earth (1 AU)Mars (1.52 AU) aphelion ≈ 2.27 AU perihelion ≈ 1.01 AU
Top-down orbit of the Didymos–Dimorphos pair to scale, computed from JPL orbital elements (a ≈ 1.64 AU, e ≈ 0.383).[10] Earth and Mars orbits shown for comparison.[9]

The DART impact

NASA's DART spacecraft struck Dimorphos on September 26, 2022, at about 6.1 km/s, when it was roughly 11 million km from Earth.[1] The collision shortened Dimorphos's orbit by 33 minutes, far beyond NASA's 73-second success threshold.[1] Debris thrown off by the impact multiplied the push: the momentum transferred was about 3.6 times that of the spacecraft alone.[6]

The impact also reshaped the moon, changing it from a roughly symmetrical oblate spheroid to a flatter, triaxial ellipsoid, and may have set it tumbling.[5]

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A witness: LICIACube

The Italian Space Agency's LICIACube, a six-unit CubeSat, separated from DART 15 days before impact and flew past the system at 56.7 km about 165 seconds after the collision, photographing the plume.[7][4]

Next: Hera

ESA's Hera spacecraft, launched on October 7, 2024, is due to rendezvous with Dimorphos and Didymos in November 2026. ESA lists Dimorphos as a 151-meter target asteroid and Hera as the first binary-asteroid survey.[3] Hera will measure Dimorphos's mass and map DART's crater.[8]

Read more

Before and after: how DART reshaped Dimorphos

From a boulder-strewn oblate spheroid to a flattened, possibly tumbling moonlet — what the impact did to Dimorphos itself.

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LICIACube: the tiny Italian photographer at DART's impact

How a six-unit CubeSat rode along with DART, peeled away 15 days early and captured the plume from 56.7 km.

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Dimorphos by the numbers: name, size and orbit

Why it's called “having two forms,” how big it really is, and the numbers behind its 33-minute orbit change.

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Sources

  1. Asteroid Didymos (Didymos and Dimorphos) — NASA Science
  2. DART (Double Asteroid Redirection Test) — NASA Science
  3. Hera — European Space Agency
  4. LICIACube — Italian Space Agency (ASI)
  5. Dimorphos — Wikipedia
  6. Double Asteroid Redirection Test — Wikipedia
  7. LICIACube — Wikipedia
  8. Hera (space mission) — Wikipedia
  9. Planetary Fact Sheet (orbit comparison) — NASA NSSDCA
  10. Small-Body Database Lookup: 65803 Didymos (1996 GT) — NASA JPL Solar System Dynamics

Figures are rounded; where sources differ we note it. Spotted an error? Tell us.