Selam (moon)

Selam
Discovery
Discovered byLucy (spacecraft)
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 the main belt asteroid 152830 Dinkinesh. It was discovered by NASA's Lucy space probe when it flew past Dinkinesh on 1 November 2023. The moon consists of two conjoined lobes of similar size, each around 220 metres (720 ft) in diameter.

Together, Dinkinesh and Selam form a binary asteroid system. Dinkinesh is the second binary main-belt asteroid explored by spacecraft, after 243 Ida by Galileo in 1993. The Dinkinesh binary system resembles the 65803 Didymos near-Earth asteroid binary system in size and composition, but differs in location from the Sun, which allows scientists to compare the nature of binary asteroids in different environments.[3][4]

Discovery

In the weeks prior to the flyby, the Lucy spacecraft found that Dinkinesh's brightness did not vary as predicted, which provided the first hints of Dinkinesh's binary nature.[5][6] Images of Selam taken after Lucy's approach revealed that it is a contact binary with two lobes attached to each other. While contact binary asteroids are common in the Solar System, Selam is the first known example of a contact binary satellite of an asteroid.[7]

Name

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

Geology

Geological features on Selam are formally named after words for 'wonderful,' 'marvelous,' or 'beautiful' in the languages of the world.[10]: 10  This naming theme was introduced by the International Astronomical Union, which announced the first approved 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 bearing flat facets.[2]: 1017  The larger lobe facing 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 facing towards 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 feature—named Uwoduhi Dorsum—that that is tilted 50 degrees with respect to Selam's orbital plane and Dinkinesh's equatorial ridge.[12][10]: 10  Uwoduhi Dorsum appears most prominent at its thinnest width of 17 m (56 ft), and broadens to a width of 38 m (125 ft) on the eastern side of Piękna Lobus.[10]: 16  Possible explanations for Uwoduhi Dorsum's formation include accretion of material before the two lobes of Selam merged, or the creation of Piękna Lobus via a low-velocity merger between two similarly-sized moonlets.[12]

Named features on Selam[10]: 10 
Feature Named after Name approved
(Date · Ref)
Piękna Lobus Polish word for "beautiful" 2025-12-20 · WGPSN
Lẹwa Lobus Yoruba word for "beautiful" 2025-12-20 · WGPSN
Uwoduhi Dorsum Cherokee word for "beautiful" 2025-12-20 · WGPSN

Origin

Selam is expected to have a similar origin as the satellites of rubble pile asteroids, which are thought to have originated from mass shedding events from the primary body in the past.[13][4] These mass shedding events occur when the asteroid rotates fast enough that material accumulates along the equator and becomes ejected into orbit by the centrifugal force.[13][14] The ejected material forms a disk around the asteroid, which eventually coalesces into a satellite.[13] The uneven reflection of sunlight off an asteroid's surface, which is called the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect, is responsible for rotationally accelerating asteroids to the point of mass shedding.[13] During a mass shedding event, the asteroid's angular momentum is transferred to its ejected material, which slows down the asteroid's rotation rate as a result.[13]

One possible explanation for the origin of Selam's contact binary nature is rotational fissioning by the YORP effect.[15][16] In this scenario, the fissioned satellite is split into two separate satellites in orbit around Dinkinesh, making it a triple asteroid system. This triple asteroid system is unstable due to chaotic gravitational perturbations between the satellites, and eventually leads to one of the satellites colliding with either the primary asteroid or the other satellite.[15]: 170  If the collision between two satellites occurs at slow enough speeds (less than 50 mm/s or 2.0 in/s), the impact does not disrupt the shapes of the two bodies and instead forms a contact binary.[15]: 167 

References

  1. ^ "(152830) Dinkinesh and Selam". Johnston's Archive. Wm. Robert Johnston. Retrieved 27 March 2024.
  2. ^ a b c 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. ^ a b 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. ^ a b "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. ^ a b c 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. ^ a b c d e f g 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. ^ a b c 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. ^ a b c d e 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. ^ a b c 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.
Wikimedia Commons has media related to Selam (moon).
This article is issued from Wikipedia. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.