 | |
| Discovery | |
|---|---|
| Discovered by | Lucy (spacecraft) |
| Discovery date | 1 November 2023 |
| Designations | |
| Pronunciation | /səˈlɑːm/ |
| Dinkinesh I | |
| Orbital characteristics[1][2] | |
| 3.11±0.05 km | |
| Eccentricity | ≈0 |
| 52.67±0.04 h | |
| Satellite of | Dinkinesh |
| Physical characteristics[2] | |
| Dimensions |
|
| |
| Mass | 2.8×1010 kg |
| 52.44±0.14 h(likelysynchronous) | |
Selam (formal designationDinkinesh I) is acontact binarymoon of themain belt asteroid152830 Dinkinesh. It was discovered byNASA'sLucyspace 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 abinary asteroidsystem. Dinkinesh is the second binary main-belt asteroid explored by spacecraft, after243 Ida byGalileo in 1993. The Dinkinesh binary system resembles the65803 Didymosnear-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]
In the weeks prior to the flyby, theLucy 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 afterLucy's approach revealed that it is acontact 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]
This moon was named afterSelam, the fossilized remains of a three-year-oldAustralopithecus afarensis femalehominin found inDikika, Ethiopia.[8]: 5 In theAmharic 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 forLucy.[9] The name was approved by theInternational Astronomical Union on 27 November 2023.[9][8]: 5
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 theYoruba 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 thePolish word for "beautiful".[10]: 10, 15 Piękna Lobus has a narrowridge 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 includeaccretion 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]
| 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 |
Selam is expected to have a similar origin as the satellites ofrubble 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 thecentrifugal 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 theYarkovsky–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'sangular 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 chaoticgravitational 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
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| Ranked by size | ||