Selam (moon)

Selam, formal designation Dinkinesh I, is a 220 m (720 ft) contact binary and natural satellite of the main belt asteroid 152830 Dinkinesh. It was discovered on 1 November 2023 during Lucy's flyby of Dinkinesh. Lucy took pictures of the asteroid, and sent data back to NASA leading to the discovery of the moon.^[1][2][3]

Selam

Low resolution image of Selam
Discovery
Discovered by	Lucy
Discovery date	1 November 2023
Designations
Named after	Selam (Australopithecus)
Alternative designations	Dinkinesh I
Orbital characteristics
Semi-major axis	3.11±0.05 km
Eccentricity	≈0
Orbital period (sidereal)	52.67±0.04 h
Physical characteristics
Dimensions	240 m × 200 m × 200 m (inner lobe) 280 m × 220 m × 210 m (outer lobe)
Synodic rotation period	Likely synchronous

Naming

Selam was named after the fossilized remains of an Australopithecus, named Selam (ሰላም) also known as "Lucy's daughter" although being some 100,000 years older than the fossilized remains of another Australopithecus known as Lucy, but Dink'inesh in Amharic. Selam (which also means "peace" in the Amharic language) was chosen for the moon, as the name of the asteroid it orbits around is "Dink'inesh".^{[clarification needed]} The name Selam was proposed by astronomer Raphael Marschall.^[4] The spaceship Lucy which discovered Selam was also named after the Australopithecus Lucy.

Theorized origin

Selam is theorized to have been formed out of the same material and same surface texture as other rubble pile asteroids found in the asteroid belt. These rubble pile asteroids were thought to be created out of a mass shedding event^{[citation needed]} that happened to the asteroids themselves. Selam is believed to be very young at about 2 to 3 million years old when it first separated from Dinkinesh.^[5][6]

Lucy mission

On 1 November 2023, Lucy successfully flew by its first target, the main-belt asteroid 152830 Dinkinesh, at a relative speed of 4.5 km/s (2.8 mi/s). On the following day, NASA released images from the flyby and announced the discovery of a small satellite orbiting Dinkinesh. The first images from the flyby showed that Dinkinesh is approximately 790 m (2,590 ft) in diameter, while the satellite is approximately 220 m (720 ft) in diameter. Later images showed that the satellite was actually two objects in direct contact, known as a contact binary. The discovery of Dinkinesh's satellite brought the total number of Lucy's planned asteroid visits up to eleven.^[7]

Discovery

In January 2017, Lucy and the Psyche missions were chosen as Discovery Mission 13 and 14 respectively. Lucy's first target for its mission was the asteroid, Dinkinesh, which was discovered in 1999.^[8]

On 1 November 2023 Lucy flew by the Dinkinesh system, taking pictures and sending data back to NASA. A day after the flyby, NASA released pictures of the Dinkinesh system, with the discovery of Selam.^[9]