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Stacked image of Dimorphos.jpg
Dimorphos in September 2022, stacked from images taken by DART
Discovered byPravec et al.[a]
Discovery date20 November 2003
Didymos I
Named after
Greek word for "having two forms"[2]
S/2003 (65803) 1
Orbital characteristics[3]
Epoch 7.5 November 2011 (JD 2455873.0)
1.19±0.03 km (pre-impact)
Eccentricity≤ 0.03
11.9216262±0.0000027 hr (11h 55m 18s; pre-impact)[3]
11h 23m ± 2m (post-impact)[4][b]
0.174 m/s (pre-impact)
Inclination168.6°±1.8° with respect to the ecliptic[c]
Satellite of65803 Didymos
Physical characteristics[5]
Dimensions208 m × 160 m × 133 m[7]
Mean diameter
171±11 m[5]
164±18 m[7]
Mass~5×109 kg (assumed)[7]
Spectral type
21.3±0.2 (difference from primary)[1]

(65803) Didymos I Dimorphos (provisional designation S/2003 (65803) 1) is a minor-planet moon of the near-Earth asteroid 65803 Didymos, with which it forms a binary system. It has a diameter of 170 metres (560 ft) and has been characterised as a low-density rubble pile. Discovered in 2003 at the Ondřejov Observatory, Dimorphos was the target of the Double Asteroid Redirection Test (DART), a NASA space mission that deliberately collided a spacecraft with the moon in September 2022 to alter its orbit around Didymos. The ESA mission Hera is planned to arrive at the Didymos system in 2026 to further study the effects of the collision on the moon.


The Working Group for Small Bodies Nomenclature of the International Astronomical Union (IAU) gave the satellite its official name on 23 June 2020.[8] The name Dimorphos is derived from a Greek word (Δίμορφος) meaning 'having two forms'.[9][10][d] The justification for the new name reads: "As the target of the DART and Hera 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] Prior to the IAU naming, the nickname Didymoon was used in official communications.[11]


Surface of Dimorphos, captured by DART two seconds before impact.

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 an eccentricity of 0.38 and an inclination of 3° with respect to the ecliptic. On 4 October 2022 Didymos made an Earth approach of 10.6 million km (6.6 million mi).[12] Dimorphos moves in a nearly equatorial, nearly circular orbit around Didymos, with an orbital 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 is retrograde relative to the ecliptic plane, in conformity with Didymos's retrograde rotation.[13]

The final few minutes of pictures from the DART mission revealed an egg-shaped body covered with boulders, suggesting it has a rubble pile structure.[14][15]

Dimorphos is around 170 metres (560 ft) in diameter, compared to Didymos at 780 metres (2,560 ft).[16] Dimorphos does not have a confirmed mass, but it is estimated to be about 5×109 kg, or about the same mass and size as the Great Pyramid of Giza, when assuming a density of 2.17 g/cm3 similar to Didymos.[17] It is currently the smallest celestial object given a formal name by the IAU.[2]


Radar images of Didymos and Dimorphos taken by the Arecibo Observatory in 2003

The primary asteroid Didymos was discovered in 1996 by Joe Montani of the Spacewatch Project at the University of Arizona.[1] The satellite Dimorphos was discovered on 20 November 2003, in photometric observations by Petr Pravec and colleagues at the Ondřejov Observatory in the Czech Republic. Dimorphos was detected through periodic dips in Didymos's brightness due to mutual eclipses and occultations. With his collaborators, he confirmed from the Arecibo radar delay-Doppler images that Didymos is a binary system.[18][19]


On 24 November 2021, NASA and the Applied Physics Laboratory launched an impactor spacecraft towards Dimorphos as part of their Double Asteroid Redirection Test (DART).[20][21] DART was the first experiment conducted in space to test asteroid deflection as a method of defending Earth from potentially hazardous asteroids.[22] Following a ten-month journey to the Didymos system, the impactor collided with Dimorphos on 26 September 2022 at a speed of around 15,000 miles per hour (24,000 kilometres per hour).[22][23] The collision successfully decreased Dimorphos's orbital period around Didymos by 32±2 minutes.[4][8][24][25] Fifteen days prior to its collision, the impactor released LICIACube, a 6U CubeSat operated by the Italian Space Agency that photographed the impact and the resulting dust plume as it performed a close flyby of the Didymos system.[20][26][27][28] Spacecraft and observatories such as Hubble, James Webb, Lucy, SAAO and ATLAS also captured the dust plume trailing the Didymos system in the days following the impact.[29][30][31][32] As part of its Hera mission, ESA currently plans to launch three spacecraft to the Didymos system in 2024 to further study the aftermath of the impact.[24][33][34]


  1. ^ Astronomers involved in the discovery of Dimorphos include P. Pravec, L.A.M. Benner, M.C. Nolan, P. Kusnirak, D. Pray, J.D. Giorgini, R.F. Jurgens, S.J. Ostro, J.-L. Margot, C. Magri, A. Grauer, and S. Larson. The discovery used lightcurve and radar observations from the Jet Propulsion Laboratory, Pasadena, CA; National Astronomy and Ionosphere Center / Arecibo Observatory, Arecibo, PR; and Ondrejov Observatory, Ondřejov, CZ. [1]
  2. ^ Orbital parameters are those after the impact of DART, which changed the orbital period by 32 minutes. Other possibly changed parameters remain to be reevaluated.[4]
  3. ^ Scheirich and Pravec (2022) give the Dimorphos's orbit pole orientation in terms of ecliptic coordinates, where λ is ecliptic longitude and β is ecliptic latitude.[5]: 4  β is the angular offset from the ecliptic plane, whereas inclination i with respect to the ecliptic is the angular offset of the orbital pole from the ecliptic north pole, at β = +90° ; i with respect to the ecliptic would be the complement of β.[6] Therefore, given β = –78.6° , i = 90° – (–78.6°) = 168.6° from the ecliptic.
  4. ^ The name Dimorphos was suggested by planetary scientist Kleomenis Tsiganis at the Aristotle 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 by Hera a few years later."[2]


  1. ^ a b c d "(65803) Didymos". www.johnstonsarchive.net. Retrieved 11 November 2021.
  2. ^ a b c d "IAU approves name of target of first NASA and ESA planetary defence missions". iau.org (Press release). International Astronomical Union. 23 June 2020. Retrieved 1 July 2020.
  3. ^ a b Naidu, Shantanu P.; Chesley, Steven R.; Farnocchia, Davide; Moskovitz, Nick; Pravec, Petr; Scheirich, Petr; Thomas, Cristina; Rivkin, Andrew S. (October 2022). "Anticipating the DART impact: Orbit estimation of Dimorphos using a simplified model". The Planetary Science Journal. 3 (10): 234. arXiv:2210.05101. Bibcode:2022PSJ.....3..234N. doi:10.3847/PSJ/ac91c0. S2CID 252815897.
  4. ^ a b c Dunbar, Brian (11 October 2022). "NASA Confirms DART Mission Impact Changed Asteroid's Motion in Space". NASA. Retrieved 11 October 2022.
  5. ^ a b c Scheirich, P.; Pravec, P. (July 2022). "Preimpact Mutual Orbit of the DART Target Binary Asteroid (65803) Didymos Derived from Observations of Mutual Events in 2003–2021". The Planetary Science Journal. 3 (7): 12. Bibcode:2022PSJ.....3..163S. doi:10.3847/PSJ/ac7233. S2CID 250650906. 163.
  6. ^ "Coordinate transformations". Astronomy and Astrophysics. European Southern Observatory. January 1998. Archived from the original on 17 June 2021. Retrieved 17 June 2022.
  7. ^ a b c Nakano, Ryota; Hirabayashi, Masatoshi; Brozovic, M.; Nolan, M. C.; Ostro, S. J.; Margot, J. L.; et al. (July 2022). "NASA's Double Asteroid Redirection Test (DART): Mutual Orbital Period Change Due to Reshaping in the Near-Earth Binary Asteroid System (65803) Didymos". The Planetary Science Journal. 3 (7): 16. Bibcode:2022PSJ.....3..148N. doi:10.3847/PSJ/ac7566. S2CID 250327233. 148.
  8. ^ a b Temming, Maria (29 June 2020). "An asteroid's moon got a name so NASA can bump it off its course". Science News. Retrieved 1 July 2020.
  9. ^ "MPEC 2020-M83". minorplanetcenter.net. Cambridge, MA: Minor Planet Center. Retrieved 1 July 2020.
  10. ^ δίμορφος. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
  11. ^ "Target: Didymoon". esa.int. European Space Agency. 31 March 2015. Retrieved 24 November 2021.
  12. ^ 65803 Didymos (Report). JPL Small-Body Database Browser. NASA / Jet Propulsion Laboratory. Retrieved 30 December 2021 – via ssd.jpl.nasa.gov.
  13. ^ Scheirich, P.; Pravec, P.; Jacobson, S.A.; Ďurech, J.; Kušnirák, P.; Hornoch, K.; et al. (2015). "The binary near-Earth asteroid (175706) 1996 FG3 – an observational constraint on its orbital evolution". Icarus. 245: 56–63. arXiv:1406.4677. Bibcode:2015Icar..245...56S. doi:10.1016/j.icarus.2014.09.023. S2CID 119248574.
  14. ^ Tariq Malik (26 September 2022). "NASA crashes DART spacecraft into asteroid in world's 1st planetary defense test". Space.com.
  15. ^ Savitsky, Zack (27 September 2022). "'Holy $@*%!' Science captures behind-the-scenes reactions to asteroid-smashing mission". Science. Retrieved 28 September 2022.
  16. ^ "(65803) Didymos". johnstonsarchive.net. Retrieved 1 July 2020.
  17. ^ Nakano, Ryota; Hirabayashi, Masatoshi; Agrusa, Harrison F.; Ferrari, Fabio; Meyer, Alex J.; Michel, Patrick; Raducan, Sabina D.; Sánchez, Paul; Zhang, Yun (1 July 2022). "NASA's Double Asteroid Redirection Test (DART): Mutual Orbital Period Change Due to Reshaping in the Near-Earth Binary Asteroid System (65803) Didymos". The Planetary Science Journal. 3 (7): 148. Bibcode:2022PSJ.....3..148N. doi:10.3847/PSJ/ac7566. ISSN 2632-3338. S2CID 250327233.
  18. ^ Pravec, P.; Benner, L.A.M.; Nolan, M.C.; Kusnirak, P.; Pray, D.; Giorgini, J. D.; Jurgens, R.F.; Ostro, S.J.; Margot, J.-L.; Magri, C.; Grauer, A. (2003). (65803) 1996 GT (Report). IAU Circular. Vol. 8244. Cambridge, MA: International Astronomical Union / Central Bureau for Astronomical Telegrams. p. 2. Bibcode:2003IAUC.8244....2P – via Harvard U.
  19. ^ Naidu, S. P.; Benner, L. A. M.; Brozovic, M.; Nolan, M. C.; Ostro, S. J.; Margot, J. L.; et al. (September 2020). "Radar observations and a physical model of binary near-Earth asteroid 65803 Didymos, target of the DART mission" (PDF). Icarus. 348: 113777. Bibcode:2020Icar..34813777N. doi:10.1016/j.icarus.2020.113777. S2CID 219091858. 134777.
  20. ^ a b Greshko, Michael (23 November 2021). "This NASA spacecraft will smash into an asteroid – to practice saving Earth". National Geographic. Archived from the original on 23 November 2021. Retrieved 25 November 2021.
  21. ^ Potter, Sean (23 November 2021). "NASA, SpaceX Launch DART: First test mission to defend planet Earth". NASA (Press release). Retrieved 25 November 2021.
  22. ^ a b Rincon, Paul (24 November 2021). "NASA DART asteroid spacecraft: Mission to smash into Dimorphos space rock launches". BBC News. Retrieved 25 November 2021.
  23. ^ Potter, Sean (23 November 2021). "NASA, SpaceX Launch DART: First test mission to defend planet Earth". NASA (Press release). National Aeronautics and Space Administration. Retrieved 25 November 2021.
  24. ^ a b Crane, Leah (23 November 2021). "NASA's DART mission will try to deflect an asteroid by flying into it". New Scientist. Archived from the original on 23 November 2021. Retrieved 25 November 2021.
  25. ^ Nelson, Bill; Saccoccia, Giorgio. "Update on DART Mission to Asteroid Dimorphos (NASA News Conference)". YouTube. Retrieved 11 October 2022.
  26. ^ Witze, Alexandra (27 September 2022). "Fresh images reveal fireworks when NASA spacecraft plowed into asteroid". Nature. doi:10.1038/d41586-022-03067-y. PMID 36167993. S2CID 252566403. Retrieved 28 September 2022.
  27. ^ Paoletta, Rae (26 September 2022). "See DART's final images before it smashed into an asteroid". The Planetary Society. Retrieved 28 September 2022.
  28. ^ "DART's small satellite companion tests camera prior to Dimorphos impact". NASA.gov (Press release). National Aeronautics and Space Administration. Retrieved 25 September 2022.
  29. ^ Lakdawalla, Emily (27 September 2022). "Photos Show Drama of DART Asteroid Impact". Sky & Telescope. Retrieved 28 September 2022.
  30. ^ George Dvorsky (27 September 2022). "Ground Telescopes Capture Jaw-Dropping Views of DART Asteroid Impact". Gizmodo. Telescopes around the world honed in on the historic collision, revealing a surprisingly large and bright impact plume.
  31. ^ George Dvorsky (27 September 2022). "First Asteroid Impact Images from DART's Companion Show Tentacle-Like Debris Plume". Gizmodo.
  32. ^ "LICIACube Impact Images". NASA. 27 September 2022. Retrieved 2 October 2022.
  33. ^ Witze, Alexandra (19 November 2021). "NASA spacecraft will slam into asteroid in first planetary-defence test". Nature. 600 (7887): 17–18. Bibcode:2021Natur.600...17W. doi:10.1038/d41586-021-03471-w. PMID 34799719. S2CID 244428237.
  34. ^ Michel, Patrick; et al. (1 July 2022). "The ESA Hera Mission: Detailed Characterization of the DART Impact Outcome and of the Binary Asteroid (65803) Didymos". The Planetary Science Journal. 3 (7): 160. Bibcode:2022PSJ.....3..160M. doi:10.3847/psj/ac6f52. ISSN 2632-3338. S2CID 250599919.

External links[edit]

  • Media related to Dimorphos at Wikimedia Commons