Selam (moon)
Selam 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 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.
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. Images of Selam taken after Lucys 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.Name
This moon was named after Selam, the fossilized remains of a three-year-old Australopithecus afarensis female hominin found in Dikika, Ethiopia. In the Amharic language the name Selam means "peace". The name was proposed by Raphael Marschall, the scientist who first identified Dinkinesh as a flyby target for Lucy. The name was approved by the International Astronomical Union on 27 November 2023.Geology
Geological features on Selam are formally named after words for 'wonderful,' 'marvelous,' or 'beautiful' in the languages of the world. This naming theme was introduced by the International Astronomical Union, which announced the first approved names for Selam's features on 20 December 2024.Selam consists of two conjoined lobes of similar size. Both lobes have blocky, angular shapes bearing flat facets. The larger lobe facing away from Dinkinesh is about in diameter and is officially named Lẹwa Lobus, after the Yoruba word for "beautiful". The smaller lobe facing towards Dinkinesh is about in diameter and is officially named Piękna Lobus, after the Polish word for "beautiful". 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. Uwoduhi Dorsum appears most prominent at its thinnest width of, and broadens to a width of on the eastern side of Piękna Lobus. 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.
| Feature | Named after | Name approved (Date |
| Piękna Lobus | Polish word for "beautiful" | 2025-12-20 |
| Lẹwa Lobus | Yoruba word for "beautiful" | 2025-12-20 |
| Uwoduhi Dorsum | Cherokee word for "beautiful" | 2025-12-20 |
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. 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. The ejected material forms a disk around the asteroid, which eventually coalesces into a satellite. 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. 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.One possible explanation for the origin of Selam's contact binary nature is rotational fissioning by the YORP effect. 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. If the collision between two satellites occurs at slow enough speeds, the impact does not disrupt the shapes of the two bodies and instead forms a contact binary.