Didymos's moon,Dimorphos, was the target of theDART mission to test the viability ofasteroid impact avoidance by collision with a spacecraft, while the impact was witnessed byLICIACube, a flybyCubeSat component of the mission.
Didymos orbits the Sun at a distance of 1.0–2.3AU once every 770 days (2 years and 1 month). Its orbit has aneccentricity of 0.38 and aninclination of 3° with respect to theecliptic. The minimum distance between the orbit of Earth and the orbit of Didymos is currently 0.04 AU (6.0 million km),[1] but will change as the asteroid isperturbed. In November 2003 it passed 7.18 million km from Earth; it will not come that near again until November 2123, with a distance of 5.86 million km. Didymos also occasionally passes very close to Mars: it will fly by Mars at a distance of 4.68 million km in July 2144.[1] The Earth approach of October 2184, at a distance of 8.6 million km, is listed with an uncertainty region of roughly ±105 km.[14]
Didymos spends one third of its time orbiting far enough from the Sun to cross the inner parts of themain asteroid belt, where collisions with other asteroids are more probable. This means that about every 73–84 thousand years, an object impacts Didymos with the energy of theDART mission satellite. Over its median NEA lifetime of 8 to 10 million years, Didymos probably has been hit several tens of times.[15]
In theSMASS classification, Didymos was classified as an Xk-type asteroid, which transitions from theX-type to the rareK-type asteroids.[1] Subsequent visible and near-infrared spectroscopy showed it to be silicate in nature, which also qualifies it as a stonyS-type asteroid.[16] Itrotates rapidly, with a period of 2.26 hours and a low brightness variation of 0.08magnitude (U=3/3), which indicates that the body has a nearly spheroidal shape.[11][8][17] Radar observations confirmed this spheroidal shape, showing it to beoblate due to its rapid rotation.[6]
The proper name for the satelliteDidymos B comes from the word "Dimorphos", Greek for "having two forms".[23] The meaning of the name represents how the form of Dimorphos's orbit will change after the collision with NASA's Double Asteroid Redirection Test (DART) spacecraft,[18] though in fact the change will be only a very slight change in its orbital parameters. Appropriately, Dimorphos serves dual roles as both a test target and as a part of a blueprint for a modality for future planetary protection.[18] The name of the moon was suggested by planetary scientistKleomenis Tsiganis at theAristotle University of Thessaloniki, Greece.[24]
Two boulders (saxa) have been given names of traditional drums.[25]
Telescope image of Didymos before DART's impactArtist's impression of theDART spacecraftTimelapse of the Didymos system's expanding dust plume from the DART impact, as seen by theSouth African Astronomical Observatory's 1-meter Lesedi telescopeAnimation of DART's trajectory DART· 65803 Didymos·Earth·Sun· 2001 CB21·3361 OrpheusAnimation of DART around Didymos - Impact on Dimorphos DART· Didymos· Dimorphos
NASA redefined mission requirements and decided to proceed with a 2020s mission to visit Didymos with an impactor, which had been considered as a part of the earlier AIDA mission, named theDouble Asteroid Redirection Test or DART. The NASA mission was intended to test whether a spacecraft impact could successfully deflect an asteroid on a collision course with Earth. The DART spacecraft was launched on 24 November 2021, and impacted Dimorphos on September 26, 2022.[28][29][30] It was accompanied by theItalian Space Agency's (ASI) six-unitLICIACube flybyCubesat that was released 15 days before impact to observe the asteroid and DART's impact.[31]
DART was the first spacecraft to intentionally target and successfully visit an asteroid known to have aminor-planet moon (The binary asteroid2000 DP107 was targeted by thePROCYON mission before it failed,243 Ida was visited by theGalileo spacecraft but its moon was unknown until then,Pluto was considered a planet until a few months after the launch ofNew Horizons, and3548 Eurybates's and15094 Polymele's moons were not discovered until months before and afterLucy's launch, respectively). Didymos is the most easily reachable asteroid of its size from Earth, requiring adelta-v of only5.1 km/s for a spacecraft to rendezvous, compared to6.0 km/s to reach theMoon.[32]
After two weeks of analysis, NASA announced that the collision shortened Dimorphos's orbital period around Didymos by 32 minutes,[33] far more than the minimum requirement of 73 seconds and the success benchmark of 10 minutes. The measurement has an uncertainty of ±2 minutes.[34]
Dart Impact seen by LICIACube
Another mission to Didymos was approved in November 2019 and launched in October 2024, with the arrival at Didymos being expected in 2026.[35] ESA'sHera mission is planning to survey the dynamical effects of the DART impact and measure the characteristics of the crater made by DART.[36]
^Volume-equivalent spherical diameter is calculated from an ellipsoid's volume given Didymos's dimensions of851 × 849 × 620 m,[5]: 28–29 and then solving for radius with.
^Naidu et al. (2020) give the Didymos's north pole direction in terms ofecliptic coordinates, whereλ is ecliptic longitude andβ is ecliptic latitude.[6]: 12 β is the angular offset from the ecliptic plane whereas inclinationi with respect to the ecliptic is the angular offset from the ecliptic north pole atβ = +90°;i with respect to the ecliptic would be thecomplement ofβ.[9] Therefore, givenβ = –84°,i = 90° – (–84°) = 174° from the ecliptic.
^"Coordinate transformations".Astronomy and Astrophysics. European Southern Observatory. January 1998.Archived from the original on 17 June 2021. Retrieved17 June 2022.
^Pravec, P.; Benner, L. A. M.; Nolan, M. C.; Kusnirak, P.; Pray, D.; Giorgini, J. D.; et al. (November 2003)."(65803) 1996 GT".IAU Circular (8244): 2.Bibcode:2003IAUC.8244....2P.Archived from the original on 3 October 2022. Retrieved12 March 2017.
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