65803 Didymos

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Discovered by Spacewatch
Discovery site Kitt Peak
Discovery date 11 April 1996
MPC designation 65803
1996 GT
Orbital characteristics[1]
Epoch 13 January 2016 (JD 2457400.5)
Uncertainty parameter 0
Observation arc 6979 days (19.11 yr)
Aphelion 2.2755 AU (340.41 Gm)
Perihelion 1.0131 AU (151.56 Gm)
1.6443 AU (245.98 Gm)
Eccentricity 0.38388
2.11 yr (770.13 d)
Inclination 3.4078°
Known satellites 1
Earth MOID 0.0394978 AU (5.90879 Gm)
Jupiter MOID 3.16497 AU (473.473 Gm)
Physical characteristics
Dimensions ~800 m (primary)
~150 m (secondary)
Mean radius
0.39 ± 0.04 km
Mean density
1.7±0.4 g/cm3
2.2593 h (0.09414 d)
2.259 h[1]

65803 Didymos (1996 GT) is an Apollo asteroid discovered on April 11, 1996 by Joe Montani at Spacewatch at Kitt Peak. It has a moon, whence the appellation "Didymos", meaning "twin". The primary is about 800 m in diameter and the moon 150 m in diameter. The moon is in an orbit about 1.1 km from the primary and with an orbital period of 11.9 hours. Didymos is the most easily reachable asteroid of its size from Earth, requiring a delta-v of only 5.1 km/s[2] for a spacecraft to rendezvous, compared to 6.0 km/s to reach the Moon. It is the target of the proposed AIDA spacecraft, an unmanned mission that would test the possibility of changing an asteroid's orbit via impacting its surface.

Discovery and naming[edit]

Didymos was discovered by Joseph L. Montani using the Spacewatch 0.9-m telescope in 1996. The binary nature of the asteroid was discovered by others; suspicions of binarity first arose in Goldstone delay-Doppler echoes, and these were confirmed with an optical light-curve analysis, along with Arecibo radar imaging on November 23, 2003. It has been informally named "Didymoon".[3]

Montani proposed a name to the International Astronomical Union only after the binary nature of the object was discovered: the name "Didymos" is Greek for "twin". The moon has been nicknamed "Didymoon".

Orbital characteristics[edit]

Didymos's approach to Earth in November 2003 was especially close with a distance of 7.18 million km; it will not come that near until November 2123, with a distance of 5.9 million km. Didymos also passes very close to Mars: 4.69 million km in 2144.

The satellite has an orbital period of 11.9 hr.

Physical characteristics[edit]

Didymos rotates rapidly, with a period of 2.26 hours. Its density is 1.7±0.4 g/cm3.

Proposed exploration[edit]

Didymos is the target of the proposed Asteroid Impact & Deflection Assessment (AIDA) mission, a collaboration between ESA and NASA.[4][5] This will be the first spacecraft to target an asteroid known to have a moon (243 Ida was visited by the Galileo spacecraft but its moon was a surprise). The mission is intended to test whether a spacecraft could successfully deflect an asteroid on a collision course with Earth; it would study Didymos from orbit, while also crashing a smaller spacecraft into Didymoon, in order to study the effect on its orbit.

See also[edit]


  1. ^ a b c d "JPL Small-Body Database Browser: 65803 Didymos". Jet Propulsion Laboratory. Retrieved 25 March 2016. 
  2. ^ "Delta-v for spacecraft rendezvous with all known near-Earth asteroids". 2010. Retrieved 2010-10-07. 
  3. ^ "Telescopes focus on target of ESA's asteroid mission" at phys.org (30 June 2015)
  4. ^ "AIDA: Asteroid Impact and Deflection Assessment mission under study at ESA and NASA" (PDF). Observatoire de la Côte d'Azur. February 2015. Retrieved 2015-03-29. 
  5. ^ Asteroid Impact & Deflection Assessment (AIDA) study.

External links[edit]