High-resolution view of Dimorphos, created by combining the final 10 full-frame images obtained byDART's Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO). Dimorphos is oriented so that its north pole is toward the top of the image. Taken seconds before impact on September 26, 2022.
Dimorphos is the smallest asteroid to be photographed andvisited by a spacecraft. It was the target of theDouble Asteroid Redirection Test (DART), aNASA space mission that deliberately collided aspacecraft with the moon on 26 September 2022 to alter its orbit around Didymos. Before the impact by DART, Dimorphos had a shape of anoblate spheroid with a surface covered inboulders but virtually nocraters.[16] The moon is thought to have formed when Didymos shed its mass due to its rapid rotation, which formed an orbiting ring of debris that conglomerated into a low-densityrubble pile that became Dimorphos today.[17][18][19]
The DART impact reduced Dimorphos'sorbital period around Didymos by 33 minutes and ejected over 1 million kilograms (2.2×10^6 lb) of debris into space, producing a dust plume that temporarily brightened the Didymos system and developed a 10,000-kilometer (6,200 mi)-longdust tail that persisted for several months.[20][21][22] The DART impact is predicted to have caused global resurfacing and deformation of Dimorphos's shape, leaving animpact crater several tens of meters in diameter.[23][15][24] Post-impact observations of brightness fluctuations within the Didymos system suggest that the impact may have either significantly deformed Dimorphos into an ellipsoidal shape or may have sent it into achaoticallytumbling rotation.[8][25] If Dimorphos was in a tumbling rotation state, the moon will be subjected to irregulartidal forces by Didymos before it will eventually return to atidally locked state within several decades.[14][26][27] TheESA missionHera is planned to arrive at the Didymos system in 2026 to further study the effects of DART's impact on Dimorphos.
Radar images of Didymos and Dimorphos taken by theArecibo Observatory in 2003
The primary asteroid Didymos was discovered in 1996 by Joe Montani of theSpacewatch Project at the University of Arizona.[1] The satellite Dimorphos was discovered on 20 November 2003, inphotometric observations byPetr Pravec and colleagues at theOndřejov Observatory in the Czech Republic. Dimorphos was detected through periodic dips in Didymos's brightness due to mutualeclipses andoccultations. With his collaborators, he confirmed from theArecibo radar delay-Doppler images that Didymos is a binary system.[28][13]
TheWorking Group for Small Bodies Nomenclature of theInternational Astronomical Union (IAU) gave the satellite its official name on 23 June 2020.[29] The nameDimorphos is derived from a Greek word (Δίμορφος) meaning 'having two forms'.[30][31][e] The justification for the new name reads: "As the target of the DART andHera space missions, it will become the first celestial body in cosmic history whose form was substantially changed as a result of human intervention (the DART impact)".[2] The name was suggested by Kleomenis Tsiganis, aplanetary scientist at theAristotle University of Thessaloniki and a member of both theDART andHera teams.[2] Prior to the IAU naming, the nicknameDidymoon was used in official communications.[32]
On 24 November 2021,NASA and theApplied Physics Laboratory launched an impactor spacecraft towards Dimorphos as part of theirDouble Asteroid Redirection Test (DART).[33][34] DART was the first experiment conducted in space to testasteroid deflection as a method of defending Earth frompotentially hazardous asteroids.[35] Following a ten-month journey to the Didymos system, the impactor collided with Dimorphos on 26 September 2022 at a speed of around 24,000 kilometers per hour (15,000 miles per hour).[35][36] The collision successfully decreased Dimorphos'sorbital period around Didymos by32±2 minutes.[37][29][38][39] Fifteen days prior to its collision, the impactor releasedLICIACube, anItalian Space AgencyCubeSat that photographed the impact and the resulting dust plume as it performed a close flyby of the Didymos system.[33][40][41][42] Spacecraft and observatories such asHubble,James Webb,Lucy,SAAO andATLAS also captured the dust plume trailing the Didymos system in the days following the impact.[43][44][45][22] As part of itsHera mission,ESA launched three spacecraft to the Didymos system in 2024 to reach this asteroid system in December 2026 to further study the aftermath of the impact.[38][46][47]
A trail of dust streams from Dimorphos in thisHubble Space Telescope photo taken about three months after the collision. The asteroid is surrounded by blue dots, which are boulders ranging from 1 to 6.7 metres across that were ejected by the impact.
The DART impact on the center of Dimorphos decreased the orbital period, previously 11.92 hours, by 33±1 minutes. This large change indicates the recoil from material excavated from the asteroid and ejected into space by the impact (known as ejecta) contributed significant momentum change to the asteroid, beyond that of the DART spacecraft itself. Researchers found the impact caused an instantaneous slowing in Dimorphos's speed along its orbit of about 2.7 millimeters per second — again indicating the recoil from ejecta played a major role in amplifying the momentum change directly imparted to the asteroid by the spacecraft. That momentum change was amplified by a factor of 2.2 to 4.9 (depending on the mass of Dimorphos), indicating the momentum change transferred because of ejecta production significantly exceeded the momentum change from the DART spacecraft alone.[48] While the orbital change was small, the change is in the velocity and over the course of years will accumulate to a large change in position.[49] For a hypothetical Earth-threatening body, even such a tiny change could be sufficient to mitigate or prevent an impact, if applied early enough. As thediameter of Earth is around 13,000 kilometers, a hypothetical asteroid impact could be avoided with as little of a shift as half of that (6,500 kilometers). A2 cm/s velocity change accumulates to that distance in approximately 10 years.
Dart Impact seen by LICIACube
By smashing into the asteroid DART made Dimorphos anactive asteroid. Scientists had proposed that some active asteroids are the result of impact events, but no one had ever observed the activation of an asteroid. The DART mission activated Dimorphos under precisely known and carefully observed impact conditions, enabling the detailed study of the formation of an active asteroid for the first time.[48][50] Observations show that Dimorphos lost approximately 1 million kilograms after the collision.[51] Impact produced a dust plume that temporarily brightened the Didymos system and developed a 10,000-kilometer (6,200 mi)-longdust tail that persisted for several months.[20][21][22] The DART impact is predicted to have caused global resurfacing and deformation of Dimorphos's shape, leaving animpact crater several tens of meters in diameter.[23][15][24] The impact has likely sent Dimorphos into achaoticallytumbling rotation that will subject the moon to irregulartidal forces by Didymos before it will eventually return to atidally locked state within several decades.[14][26][27] Additionally, the impact changed Dimorphos shape from a roughly symmetrical "oblate spheroid" to "a flat-topped oval", or "triaxial ellipsoid".[52][53][54]
DART spacecraft's final images of Dimorphos, (from 11.5 seconds before impact).
Dimorphos is approximately 170 meters (560 ft) in diameter, compared to Didymos at 780 meters (2,560 ft). Dimorphos does not have a confirmed mass, but it is estimated to be about5×109 kg (5.5 million tons), or about the same mass and size as theGreat Pyramid of Giza, when assuming a density of2.17 g/cm3 similar to Didymos.[55] It is one of the smallest celestial objects given a formal name by the IAU, after367943 Duende and469219 Kamoʻoalewa.[2]
The final few minutes of pictures from the DART mission revealed an egg-shaped body covered with boulders, suggesting it has arubble pile structure.[56][57]
Five boulders (saxa) and six craters have been given names of traditional drums from several cultures. They are approximately 10 meters across or smaller:[58]
Animation of DART around Didymos - Impact on Dimorphos DART· Didymos· Dimorphos
The primary body of the binary system, Didymos, orbits the Sun at a distance of 1.0 to 2.3 AU once every 770 days (2 years and 1 month). The pathway of the orbit has aneccentricity of 0.38 and aninclination of 3° with respect to theecliptic. On 4 October 2022 Didymos made an Earth approach of 10.6 million km (6.6 million mi).[59] Dimorphos moves in a nearly equatorial, nearly circular orbit around Didymos, with anorbital period of 11.9 hours. Its orbit period is synchronous with its rotation, so that the same side of Dimorphos always faces Didymos. Dimorphos's orbit isretrograde relative to theecliptic plane, in conformity with Didymos's retrograde rotation.[60]
^For acircular orbit with negligible eccentricity, as in the case of Dimorphos, the meanorbital speed can be approximated by the timeT it takes to complete one revolution around its orbitalcircumference, with the radius being itssemi-major axisa: .
^Absolute magnitude of Dimorphos calculated from the addition of its magnitude difference to Didymos's absolute magnitude: 18.07 + 3.29 ≈ 21.4.[1]
^The nameDimorphos was suggested byplanetary scientist Kleomenis Tsiganis at theAristotle University of Thessaloniki. Tsiganis explained that the name "has been chosen in anticipation of its changes: It will be known to us in two very different forms, the one seen by DART before the impact, and the other seen byHera a few years later."[2]
^abcdeJohnston, Wm. Robert (9 October 2021)."(65803) Didymos and Dimorphos".Asteroids with Satellites Database. Johnston's Archive. Retrieved11 November 2021.
^Fodde, Iosto; Feng, Jinglang; Vasile, Massimiliano; Gil-Fernández, Jesús; et al. (October 2023). "Design and Analysis of Robust Ballistic Landings on the Secondary of a Binary Asteroid".arXiv:2310.19844 [astro-ph.IM].
^Naidu, Shantanu P.; Chesley, Steven R.; Moskovitz, Nick; Benner, Lance A. M.; Brozovic, Marina; Pravec, Petr; et al. (2 April 2023).Change in the mutual orbit of Dimorphos due to the DART impact. 8th IAA Planetary Defense Conference. Vienna, Austria. p. 17.Archived from the original on 4 April 2023.
^"Coordinate transformations".Astronomy and Astrophysics. European Southern Observatory. January 1998.Archived from the original on 17 June 2021. Retrieved17 June 2022.
^Barnouin, O. S.; Ballouz, R. L.; Marchi, S.; Vincent, J.-B.; Pajola, M.; Lucchetti, A.; et al. (March 2023).The Geology of the Didymos System(PDF). 54th Lunar and Planetary Science Conference 2023. Lunar and Planetary Institute. Retrieved4 February 2023.
^abRaducan, S. D.; Jutzi, M.; Zhang, Y.; Cheng, A. F.; Collins, G. S.; Davison, T. M.; et al. (March 2023).Low Strength of Asteroid Dimorphos As Demonstrated by the Dart Impact(PDF). 54th Lunar and Planetary Science Conference 2023. Lunar and Planetary Institute. Retrieved4 February 2023.
^abMeyer, A. J.; Noiset, G.; Karatekin, Ö.; McMahon, J.; Agrusa, H. F.; Nakano, R.; et al. (March 2023).Tidal Dissipation in Didymos Following the DART Impact(PDF). 54th Lunar and Planetary Science Conference 2023. Lunar and Planetary Institute. Retrieved4 February 2023.
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