65803 Didymos

65803 Didymos
The radar images of Didymos and its satellite taken by the Arecibo Observatory in 2003.
Discovery [1]
Discovered by Spacewatch
Discovery site Kitt Peak National Observatory
Discovery date 11 April 1996
(65803) Didymos
Pronunciation /ˈdɪdɪmɒs/[4]
Named after
Greek word for "Twin"[2]
1996 GT
Apollo[1] · Amor[3]
Orbital characteristics[1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 0
Observation arc 21.04 years (7686 days)
Aphelion 2.2760 AU
Perihelion 1.0133 AU
1.6446 AU
Eccentricity 0.3839
2.11 yr (770 days)
0° 28m 2.28s / day
Inclination 3.4083°
Known satellites Dimorphos[5][6]
Earth MOID 0.0404 AU (15.7 LD)
Physical characteristics
Mean diameter
0.78±0.08 km[1]
0.800 km (taken)[7]
2.2593±0.0002 h[8]
2.26±0.01 h[7][6]
0.15 (derived)[7]
SMASS = Xk[1] · X[7]
18.0[1] · 18.16[7][8][9]

65803 Didymos (provisional designation 1996 GT) is a sub-kilometer asteroid and synchronous binary system that is classified as a 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, named Dimorphos, was discovered in 2003. Due to its binary nature, the asteroid was then named Didymos, the Greek word for 'twin'.

Didymos's moon, Dimorphos, is the target of the DART mission to test the viability of asteroid impact avoidance by collision with a spacecraft, while the whole system is to be visited by LICIACube, a flyby CubeSat to witness the impact.


Didymos was discovered on 11 April 1996, by the University of Arizona Steward Observatory's, and Lunar and Planetary Laboratory's, Spacewatch survey using its 0.9-meter 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.[3]

Orbital characteristics

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.[1]

Physical characteristics

Shape model of Didymos and its satellite Dimorphos, based on photometric light curve and radar data

In the SMASS classification, Didymos was classified as an Xk-type asteroid, which transitions from the X-type to the rare K-type asteroids.[1] Subsequent visible and near-infrared spectroscopy showed it to be silicate in nature.[11] 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.[7][8][6]


Didymos is a binary asteroid with a satellite in its orbit. The minor-planet moon, named Dimorphos,[12] moves in a mostly circular retrograde orbit[13] with an orbital period of 11.9 hours.[7][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).[5] It was previously known by its provisional designation S/2003 (65803) 1 and had been informally called "Didymoon" or "Didymos B".[14][12]


This minor planet was named "Didymos", Greek for "twin", due to its binary nature.[2] The name was suggested by the discoverer, University of Arizona Lunar and Planetary Laboratory astronomer Joseph L. "Joe" 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).[15]

The proper name for the satellite Didymos B comes from the word "Dimorphos", Greek for "having two forms".[16] The meaning of the name represents how the form of Dimorphos's orbit will change after the DART spacecraft may successfully impact the moon.[12] Appropriately, Dimorphos serves dual roles as both a test target and as a part of a blueprint for a modality for future planetary protection.[12] The name of the moon was suggested by planetary scientist Kleomenis Tsiganis at the Aristotle University of Thessaloniki, Greece.[17]


Artist's impression of the DART spacecraft
Animation of DART's trajectory
  DART ·   65803 Didymos ·   Earth ·   Sun ·   2001 CB21 ·   3361 Orpheus

Didymos's moon, Dimorphos was the target of the proposed robotic Asteroid Impact & Deflection Assessment (AIDA) mission, a collaboration between ESA and NASA, which was cancelled in 2016.[18][19] NASA 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, and 3548 Eurybates' moon was not discovered until the year when Lucy was due to launch). 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.[20]

The DART spacecraft was launched on 24 November 2021 for an impact with Dimorphos in late September or early October 2022.[21] It is accompanied by ASI's 6-Unit LICIACube flyby Cubesat that will be released 10 days before impact to observe the asteroid and DART's impact.[22] The collision is expected to shorten[23] Dimorphos' orbital period around Didymos by at least 73 seconds.[24]

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.[25]

See also