(65803) Didymos
(65803) Didymos is a near-Earth asteroid about 780 m wide and 580 m tall, with a small moon, Dimorphos, circling it 1.2 km out. On 26 September 2022 NASA's DART spacecraft deliberately crashed into Dimorphos — the first test of whether an impact can change an asteroid's path — and shortened the moon's orbit around Didymos by 33 minutes. ESA's Hera spacecraft, launched in 2024, was sent to survey the pair after the impact.
Key facts
Science
Physical
- Dimensions
- 0.783 × 0.783 × 0.584 km · approx. triaxial diameter
- Mass
- 5.26 × 10¹¹ kg · 0.00000000000717 Moon masses
- Density
- 2,805 kg/m³ · 2.8× water
- Computed from mass and radius
- Rotation period
- 2.26 hours
Orbital
- Semi-major axis
- 1.64 AU
- From catalogued orbital elements (MPC)
- Eccentricity
- 0.3831
- From catalogued orbital elements (MPC)
- Inclination
- 3.41 °
- From catalogued orbital elements (MPC)
- Orbital period
- 2.11 years
- From catalogued orbital elements (MPC)
- Perihelion
- 1.01 AU
- Computed from the orbital elements
- Aphelion
- 2.27 AU
- Computed from the orbital elements
- Mean orbital velocity
- 23.2 km/s
- Computed from the orbital elements
Discovery
- Discovered
- April 11, 1996
- Discoverer
- Spacewatch (Kitt Peak)
Composition
- Make-up
- A stony (S-type) asteroid whose spectrum resembles the L/LL ordinary chondrites — silicate rock, olivine and pyroxene, with a little metal. Its density is lower than those meteorites', which suggests a porous rubble pile of loosely bound boulders; DART's images showed a boulder-strewn surface. It spins once every 2.26 hours, near the fastest rate at which rubble-pile asteroids are seen to spin. No global map exists: DART saw one side, in its final minutes.
Radiation & temperature
- Insolation
- 0.371 S⊕ · 37% of the sunlight Earth receives
- Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
- Geometric albedo
- 0.15
- Geometric (V-band) albedo, 0.15 ± 0.04 (Naidu et al. 2020).
- Equilibrium temperature
- 208 K (-65 °C)
- Computed from albedo + distance from the Sun (airless black body); uses the geometric albedo as an estimate
Discovery & history
- Discovered
- April 11, 1996
- By
- Spacewatch (Kitt Peak)
- Name
- Didymos is Greek for 'twin', chosen because the asteroid is a pair. The name was proposed by Joe Montani of Spacewatch and published in 2004.
Spacewatch, the University of Arizona's asteroid survey at Kitt Peak, found it on 11 April 1996, and it was catalogued as 1996 GT. In November 2003, regular dips in its brightness seen from Ondřejov Observatory in the Czech Republic, and Arecibo radar images, showed that it has a moon.
ESA's follow-up to DART, surveying the crater the impact left on Dimorphos (planned arrival).
Images
Images from open archives · credit shown per image