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Selam (moon)

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Selam
Selam photographed by the Lucy spacecraft on 29 May 2024
Discovery
Discovered byLucy
Discovery date1 November 2023
Designations
Pronunciation/səˈlɑːm/
Dinkinesh I
Orbital characteristics[1][2]
3.11±0.05 km
Eccentricity≈0
52.67±0.04 h
Satellite ofDinkinesh
Physical characteristics[2]
Dimensions
  • 240 × 200 × 200 m (inner lobe)
  • 280 × 220 × 210 m (outer lobe)
  • 212±21 m (inner lobe)
  • 234±23 m (outer lobe)
Mass2.8×1010 kg
52.44±0.14 h (likely synchronous)

Selam (formal designation Dinkinesh I) is a contact binary moon of 152830 Dinkinesh, a main-belt asteroid. It was discovered by NASA's Lucy spacecraft during its flyby of Dinkinesh on 1 November 2023. The moon consists of two similarly sized lobes, each about 220 m (720 ft) in diameter.

Together, Dinkinesh and Selam form a binary asteroid system. Dinkinesh is the second binary main-belt asteroid to be explored by a spacecraft, after 243 Ida, which was visited by the Galileo spacecraft in 1993. The Dinkinesh system is similar in size and composition to the near-Earth binary asteroid system 65803 Didymos, but its greater distance from the Sun provides an opportunity to compare binary asteroids in different environments.[3][4]

Discovery

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In the weeks before the flyby, Lucy observations showed that Dinkinesh's brightness varied differently than expected, providing the first indication that it had a companion.[5][6] Images taken after the encounter revealed that Selam is a contact binary composed of two lobes joined together. Although contact binary asteroids are common in the Solar System, Selam is the first known contact binary satellite of an asteroid.[7]

Name

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The moon is named after Selam, the fossilized remains of a three-year-old female Australopithecus afarensis discovered in Dikika, Ethiopia.[8]: 5  In Amharic, Selam means "peace" (ሰላም).[9] The name was proposed by Raphael Marschall, the scientist who first identified Dinkinesh as a potential flyby target for Lucy.[9] The name was approved by the International Astronomical Union on 27 November 2023.[9][8]: 5 

Orbit

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Selam orbits Dinkinesh at a semi-major axis of 3.11±0.05 km.[2] Its orbit is nearly circular, with an eccentricity of approximately 0,[2] and it takes 52.67±0.04 hours to complete one revolution.[1] Selam has a synodic rotation period of 52.44±0.14 hours, suggesting that it is likely tidally locked to Dinkinesh.[2]

Physical characteristics

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Geology

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Gallery of Lucy images of Selam at different locations, phase angles (α), and pixel resolutions, with geological features labeled

Geological features on Selam are formally named after words meaning "wonderful", "marvelous", or "beautiful" in languages from around the world.[10]: 10  This naming theme was introduced by the International Astronomical Union, which approved the first names for Selam's features on 20 December 2024.[11]

Selam consists of two conjoined lobes of similar size. Both lobes have blocky, angular shapes with flat facets.[2]: 1017  The larger lobe, which faces away from Dinkinesh, is about 234 ± 23 m (768 ± 75 ft) in diameter and is officially named Lẹwa Lobus, after the Yoruba word for "beautiful".[10]: 10, 15  The smaller lobe, which faces Dinkinesh, is about 212 ± 21 m (696 ± 69 ft) in diameter and is officially named Piękna Lobus, after the Polish word for "beautiful".[10]: 10, 15  Piękna Lobus has a narrow ridge named Uwoduhi Dorsum, which is tilted 50 degrees relative to Selam's orbital plane and Dinkinesh's equatorial ridge.[12][10]: 10  Uwoduhi Dorsum is most prominent at its narrowest point, where it is 17 m (56 ft) wide, and broadens to 38 m (125 ft) on the eastern side of Piękna Lobus.[10]: 16  Possible explanations for the formation of Uwoduhi Dorsum include the accretion of material before Selam's two lobes merged or the formation of Piękna Lobus through a low-velocity merger between two similarly sized moonlets.[12]

Named features on Selam[10]: 10 
FeatureNamed afterName approved
(Date · Ref)
Piękna LobusPolish word for "beautiful"2024-12-20 · WGPSN
Lẹwa LobusYoruba word for "beautiful"2024-12-20 · WGPSN
Uwoduhi DorsumCherokee word for "beautiful"2024-12-20 · WGPSN

Origin

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Simulation of two colliding moonlets illustrating the possible formation of the Uwodohi Dorsum ridge on Selam's smaller inner lobe, Piękna Lobus

Selam is expected to have a similar origin to the satellites of rubble-pile asteroids, which are thought to form from material shed by the primary body.[13][4] Mass-shedding events can occur when an asteroid rotates rapidly enough for material to accumulate along its equator and be ejected into orbit by centrifugal forces.[13][14] The ejected material forms a disk around the asteroid that eventually coalesces into a satellite.[13] The Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect, in which the uneven reflection of sunlight from an asteroid's surface alters its rotation, can accelerate an asteroid's rotation to the point of mass shedding.[13] During such an event, the asteroid transfers angular momentum to the ejected material, slowing its rotation.[13]

One possible explanation for Selam's contact-binary nature is rotational fission caused by the YORP effect.[15][16] In this scenario, the fissioned material would form two separate satellites orbiting Dinkinesh, creating a triple asteroid system. Gravitational perturbations between the satellites would make the system unstable, eventually causing one satellite to collide with either Dinkinesh or the other satellite.[15]: 170  If the collision occurred at a sufficiently low speed, less than 50 mm/s (2.0 in/s), it would not disrupt the shapes of the two bodies and could instead produce a contact binary.[15]: 167 

References

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  1. 1 2 "(152830) Dinkinesh and Selam". Johnston's Archive. Wm. Robert Johnston. Retrieved 27 March 2024.
  2. 1 2 3 4 5 6 Levison, Harold F.; Marchi, Simone; Noll, Keith S.; et al. (29 May 2024). "A contact binary satellite of the asteroid (152830) Dinkinesh" (PDF). Nature. 629 (8014): 1015–1020. arXiv:2406.19337. Bibcode:2024Natur.629.1015L. doi:10.1038/s41586-024-07378-0. PMC 11136651. PMID 38811709.
  3. ↑ Bartels, Meghan (3 November 2023). "NASA Asteroid Mission Discovers Tiny Surprise Moon with 'Really Bizarre' Shape". Scientific American. Archived from the original on 3 November 2023. Retrieved 3 November 2023.
  4. 1 2 Merrill, Colby; Kubas, Alexia; Meyer, Alex; Raducan, Sabina (April 2024). "Age of (152830) Dinkinesh I Selam constrained by secular tidal-BYORP theory". Astronomy and Astrophysics. 684: L20. Bibcode:2024A&A...684L..20M. doi:10.1051/0004-6361/202449716.
  5. ↑ Kretke, Katherine (2 November 2023). "NASA's Lucy Spacecraft Discovers 2nd Asteroid During Dinkinesh Flyby". NASA. Archived from the original on 3 November 2023. Retrieved 2 November 2023.
  6. ↑ "NASA's Lucy Mission Gets an Unexpected Twofer in First Asteroid Encounter". Johns Hopkins University Applied Physics Laboratory. 2 November 2023. Archived from the original on 3 November 2023. Retrieved 2 November 2023.
  7. ↑ Kretke, Katherine (7 November 2023). "NASA's Lucy Surprises Again, Observes 1st-ever Contact Binary Orbiting Asteroid". NASA. Archived from the original on 7 November 2023. Retrieved 7 November 2023.
  8. 1 2 "WGSBN Bulletin 3, #16" (PDF). WGSBN Bulletin. 3 (16). International Astronomical Union: 12. 27 November 2023. Archived (PDF) from the original on 28 November 2023. Retrieved 27 November 2023.
  9. 1 2 3 Morton, Erin (29 November 2023). "Satellite Discovered by NASA's Lucy Mission Gets Name". NASA Blogs. NASA. Archived from the original on 30 November 2023. Retrieved 29 November 2023.
  10. 1 2 3 4 5 6 Bierhaus, E. B.; Marchi, S.; Robbins, S. J.; Mottola, S.; Bottke, W. F.; Noll, K.; et al. (December 2025). "The Geology of a Small Main-belt S-class Binary Asteroid System: Dinkinesh and Its Contact Binary Satellite Selam as Observed by the Lucy Mission". The Planetary Science Journal. 6 (12): 299. Bibcode:2025PSJ.....6..299B. doi:10.3847/PSJ/ae1968.
  11. ↑ "Selam Nomenclature". Gazetteer of Planetary Nomenclature. United States Geological Survey. Archived from the original on 6 January 2026. Retrieved 6 January 2026.
  12. 1 2 Raducan, S. D.; Madeira, G.; Agrusa, H. F.; Merrill, C. C.; Marschall, R.; Ferrari, F.; et al. (December 2025). "Multiple moonlet mergers as the origin of the Dinkinesh-Selam system". Nature Communications. 16 (1): 11033. Bibcode:2025NatCo..1611033R. doi:10.1038/s41467-025-66484-3. PMC 12698862. PMID 41381555.
  13. 1 2 3 4 5 Lakdawalla, Emily (3 November 2023). "Lucy Mission Flies By Asteroid Dinkinesh, Finds a Little Surprise". Sky & Telescope. Archived from the original on 3 November 2023. Retrieved 3 November 2023.
  14. ↑ Hyodo, Ryuki; Sugiura, Keisuke (October 2022). "Formation of Moons and Equatorial Ridge around Top-shaped Asteroids after Surface Landslide". The Astrophysical Journal Letters. 937 (2): 6. arXiv:2209.07045. Bibcode:2022ApJ...937L..36H. doi:10.3847/2041-8213/ac922d. S2CID 252280407. L36.
  15. 1 2 3 Jacobson, Seth A.; Scheeres, Daniel J. (July 2011). "Dynamics of rotationally fissioned asteroids: Source of observed small asteroid systems". Icarus. 214 (1): 161–178. arXiv:1404.0801. Bibcode:2011Icar..214..161J. doi:10.1016/j.icarus.2011.04.009. S2CID 119245876.
  16. ↑ Petersen, Collins (9 November 2023). "What? Wow! That New Asteroid Image from Lucy Just Got Even More Interesting". Universe Today. Archived from the original on 10 November 2023. Retrieved 9 November 2023.
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