Didymos.space

Binary near-Earth asteroid · Apollo group

Didymos

(65803) Didymos · 1996 GT

The “twin” asteroid whose small moon was struck by NASA's DART in the first test of planetary defense — and where ESA's Hera is arriving now.

Key factsBackyard stargazing kit builder
Didymos at a glance
Object typeBinary near-Earth asteroid, Apollo group; S-type; potentially hazardous asteroid[5]
DiscoveredApril 11, 1996 — Spacewatch, Kitt Peak, Arizona (Joseph Montani)[1]
NameGreek for “twin,” for its binary nature[1]
Diameter≈ 780 m (NASA: about 805 m)[3]
Bulk density≈ 2.17 g/cm³[3]
Rotation period≈ 2.26 hours[3]
MoonDimorphos, ≈ 170 m across, discovered 2003[1]
Distance from Sun1.01–2.27 AU (average ≈ 1.64 AU)[3]
Orbital period≈ 770 days (2.1 years)[3]
Orbital inclination≈ 3.4°[3]
Close approach (2003)≈ 7.2 million km from Earth[1]
ReachabilityDelta-v of 5.1 km/s to rendezvous — less than the 6.0 km/s to reach the Moon[5]
DART impact on moonSeptember 26, 2022 — shortened Dimorphos's orbit by 33 minutes[1]
HeraESA spacecraft launched Oct. 7, 2024; rendezvous November 2026[4]

Discovery and name

Didymos was discovered on April 11, 1996, by Joseph Montani of the Spacewatch survey at Kitt Peak National Observatory in Arizona, who also suggested its name.[1] Scientists first suspected it had a moon from multiple echoes in NASA Goldstone radar data; the moon was confirmed using optical light curves and Arecibo radar images in November 2003.[1] The name Didymos means “twin” in Greek.[1]

Size and spin

Didymos is about 780 meters across in JPL's database, and NASA describes it as about half a mile (805 meters) wide.[3][1] It spins very quickly, once every 2.26 hours, and its small brightness variations suggest a nearly spheroidal shape.[5] Its moon, Dimorphos, is about 170 meters across.[1]

Orbit

Didymos is an Apollo-group near-Earth asteroid. Its distance from the Sun ranges from about 1.01 to 2.27 AU — from just outside Earth's orbit to beyond Mars — and it takes about 770 days to go around once.[3][1] In 2003 the pair passed about 7.2 million km from Earth.[1] Neither Didymos nor Dimorphos poses an impact threat to Earth, but their characteristics made them an ideal target for a deflection test.[6]

Sun Earth (1 AU)Mars (1.52 AU) aphelion ≈ 2.27 AU perihelion ≈ 1.01 AU
Top-down orbit of Didymos to scale, computed from JPL orbital elements (a ≈ 1.64 AU, e ≈ 0.383).[3] Earth and Mars orbits shown for comparison.[9]

DART: the first planetary-defense test

NASA's DART (Double Asteroid Redirection Test) launched on November 24, 2021, and deliberately struck Dimorphos on September 26, 2022, at about 6.1 km/s, roughly 11 million km from Earth.[1] Before the impact, Dimorphos took about 12 hours to orbit Didymos. The collision shortened that by 33 minutes — more than 25 times NASA's 73-second success threshold.[1]

A study published in March 2026 found that the impact also changed the pair's 770-day orbit around the Sun by a fraction of a second.[1]

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Hera arrives

ESA's Hera mission launched on October 7, 2024, and is due to rendezvous with the Didymos system in November 2026, carrying 12 payloads including two CubeSats.[4] Hera imaged Didymos for the first time on October 7, 2026, as a single pixel, and performed braking manoeuvres on October 15 and October 22, 2026.[7] It will survey DART's aftermath for about six months, measuring Dimorphos's mass and the effects of the impact in detail.[7][4]

Can you see Didymos?

Not with backyard gear — Didymos is less than a kilometer across.[3] But the same night sky is full of things worth a look: meteor showers, the Moon, star clusters and the Milky Way. Our stargazing kit builder helps you put together a comfortable backyard setup.

Read more

DART: the day humanity moved an asteroid

How a 610 kg spacecraft hit Dimorphos at 6 km/s, shortened its orbit by 33 minutes and proved kinetic-impact deflection works.

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Hera's arrival: Europe's planetary-defense detective reaches Didymos

From a Falcon 9 launch and a Mars flyby to October 2026 braking burns — what Hera and its two CubeSats will do now.

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How Didymos got a moon

Fast spin, a ring of debris and a rubble-pile moonlet: the leading theory of how binary asteroids like Didymos form.

Read →

Sources

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

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