65803 Didymos

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Discovery and designation
Discovered by Spacewatch
Discovery site Kitt Peak
Discovery date April 11, 1996
MPC designation 65803
1996 GT
Orbital characteristics[1]
Epoch JD 2456200.5 (30 September 2012)
Aphelion 2.275 AU
Perihelion 1.013 AU
1.644 AU
Eccentricity 0.384
2.108 yr (770.1 d)
Inclination 3.408°
Known satellites 1
Physical characteristics
Dimensions ~800 m (primary)
~150 m (secondary)
Mean density
1.7±0.4 g/cm3
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 satellite orbiting it with a period of 11.9 hours, hence the appellation "Didymos", meaning "twin". The primary asteroid is about 800 m in diameter. Its satellite is about 150 m in diameter in an orbit about 1.1 km from the primary. Didymos rotates rapidly, with a period of 2.26 hours. Its density is 1.7±0.4 g/cm3. 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.

Its 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.

Didymos is the target of the proposed AIDA mission by ESA and collaboration by NASA.[3][4] It will be the first asteroid with a known moon visited by a spacecraft, and the second with a moon, known or unknown, visited by a spacecraft, after 243 Ida.

The discoverer of Didymos at the Spacewatch 0.9-m telescope in 1996, Joe (Joseph L.) Montani, provides this copy of the citation he submitted for the formal naming of the object by the IAU. The word applied as the name, "didymos", is the Greek word for "twin", and Montani initiated the naming proposal process only after the binary nature of the object was discovered by others via light-curve analysis, and planetary radar: Suspicions of binarity arose in the Goldstone delay-Doppler echoes, and these were confirmed with the optical light-curve analysis, along with Arecibo radar imaging on 2003 Nov. 23. The satellite has an an orbital period of 11.9 hr. It has been informally named "Didymoon".[5]

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