|Discovery site||Kitt Peak National Observatory|
|Discovery date||11 April 1996|
|Greek word for "Twin"|
|NEO · PHA|
Apollo  · Amor 
|Orbital characteristics |
|Epoch 4 September 2017 (JD 2458000.5)|
|Uncertainty parameter 0|
|Observation arc||21.04 years (7686 days)|
|2.11 yr (770 days)|
|0° 28m 2.28s / day|
|Earth MOID||0.0404 AU · 15.7 LD|
0.800 km (taken)
|SMASS = Xk · X|
|18.0 · 18.16 · 18.16±0.03|
65803 Didymos, provisional designation 1996 GT, is a sub-kilometer asteroid and synchronous binary system, classified as potentially hazardous asteroid and near-Earth object of both the Apollo and Amor group. The asteroid was discovered in 1996, by the Spacewatch survey at Kitt Peak, and its small 160-metre minor-planet moon was discovered in 2003, named Dimorphos. Due to its binary nature, it was then named "Didymos", the Greek word for twin.
Didymos was discovered on 11 April 1996, by the University of Arizona Steward Observatory's Spacewatch survey using its 0.9-metre telescope at Kitt Peak National Observatory in Arizona, United States. 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 lightcurve analysis, along with Arecibo radar imaging on 23 November 2003.
Didymos orbits the Sun at a distance of 1.0–2.3 AU once every 770 days (2 years and 1 month). Its orbit has an eccentricity of 0.38 and an inclination of 3° with respect to the ecliptic. Its approach to Earth in November 2003 was especially close (in relative terms), 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 occasionally passes very close to Mars: 4.69 million km in 2144.
In the SMASS classification, Didymos was classified as an Xk-type asteroid, which transitions from the X-type to the rare K-type asteroids. Subsequent visible and near-infrared spectroscopy showed it to be silicate in nature. It rotates rapidly, with a period of 2.26 hours and a brightness variation of 0.08 magnitude (U=3/3), which indicates that the body has a nearly spheroidal shape.
Didymos is a binary asteroid with a satellite in its orbit. The minor-planet moon, named Dimorphos, moves in a mostly circular retrograde orbit  with an orbital period of 11.9 hours.[a] It measures approximately 160 metres (520 ft) in diameter compared to 780 metres (2,560 ft) for its primary (a mean-diameter-ratio of 0.22). Prior to the official naming of Dimorphos, it was known by its provisional designation S/2003 (65803) 1 and has been informally known as "Didymoon" or "Didymos B".
This minor planet was named "Didymos", Greek for "twin", due to its binary nature. The name was suggested by the discoverer, University of Arizona Lunar and Planetary Lab astronomer Joseph Montani, who made the naming proposal to the International Astronomical Union after the binary nature of the object was detected. The approved naming citation was published on 13 July 2004 (M.P.C. 52326).
The asteroid moon's name comes from the word "Dimorphos", Greek for "having two forms". The meaning of the name represents how the form of Dimorphos's orbit will change after the DART spacecraft impacts the moon. Appropriately, Dimorphos serves dual roles as a both a test target and a part of a blueprint for future planetary protection. The name of the moon was suggested by planetary scientist Kleomenis Tsiganis at the Aristotle University of Thessaloniki.
Didymos was the target of the proposed robotic Asteroid Impact & Deflection Assessment (AIDA) mission, a collaboration between ESA and NASA, which was cancelled in 2016. NASA will proceed with the impactor portion of the mission, called Double Asteroid Redirection Test or DART. The NASA mission is intended to test whether a spacecraft impact could successfully deflect an asteroid on a collision course with Earth. DART will be the first spacecraft to intentionally target an asteroid known to have a minor-planet moon (243 Ida was visited by the Galileo spacecraft but its moon was unknown until then). Didymos is the most easily reachable asteroid of its size from Earth, requiring a delta-v of only 5.1 km/s for a spacecraft to rendezvous, compared to 6.0 km/s to reach the Moon. DART will be launched in July 2021 for an impact in October 2022. ESA's Hera mission is approved in November 2019 for a launch in 2024, to arrive at Didymos in January 2027. It will survey the dynamical effects of the DART impact and measure the characteristics of the crater made by DART.
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