152830 Dinkinesh

152830 Dinkinesh (provisional designation ) is a binary main-belt asteroid about 790 m in diameter. It was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) survey at Socorro, New Mexico on 4 November 1999. Dinkinesh, the name borrowed from an Ethiopian word for the Lucy fossil, was the first flyby target of NASA's Lucy mission, which approached 425 km from the asteroid on 1 November 2023. During the flyby, the Lucy spacecraft discovered that Dinkinesh has a contact-binary natural satellite, named Selam, which is 220 m in diameter. Dinkinesh is the smallest main-belt asteroid explored by spacecraft yet, though some smaller near-Earth asteroids have also been explored.

Discovery and observational history
Dinkinesh was discovered on 4 November 1999 by the Lincoln Near-Earth Asteroid Research (LINEAR) sky survey at Socorro, New Mexico. The discovery observations were published by the Minor Planet Center (MPC) on 23 November 1999 and the asteroid was given the minor planet provisional designation, which describes its discovery year, month, and discovery order within the month. The LINEAR and Spacewatch (Kitt Peak, Arizona) surveys continued observing Dinkinesh until 15 November 1999, after which the asteroid became lost and went unrecognized for years.

On 19 April 2004, Spacewatch reobserved Dinkinesh as a seemingly new asteroid, but misattributed these observations to those of another unrelated asteroid,, which was discovered by the same survey on 12 April 2004. Dinkinesh was again reobserved as a seemingly new asteroid on 15 and 17 February 2007 by Palomar Observatory's Near-Earth Asteroid Tracking (NEAT) survey at San Diego County, California, which led the MPC to give Dinkinesh the provisional designation on 25 February 2007. Gareth V. Williams, the associate director of the MPC at the time, recognized that and  were the same asteroid and published the linkage on 2 March 2007. The linkage between Dinkinesh's 1999 and 2007 observations enabled the MPC to find additional observations from 2001–2007, where the asteroid was previously detected unknowingly. The linkage and additional observations extended Dinkinesh's observation arc to over 7 years and greatly reduced uncertainties in its orbit. This allowed the MPC to give Dinkinesh its permanent minor planet catalog number 152830 on 2 April 2007. Pre-discovery LINEAR observations of Dinkinesh from 15 October 1999 were later identified and published on 19 August 2007, extending the observation arc by another 5 years.

On 3 March 2007, the MPC established that Spacewatch's 2004 observations of Dinkinesh were not of, and thus redesignated these observations as. However, the MPC did not recognize that was Dinkinesh until Gareth Williams made the linkage and published it on 9 February 2009.

Name
Dinkinesh is the Ethiopian name for the Lucy fossil, after which NASA's Lucy mission is named. The name means "you are wonderful" in the Amharic language (ድንቅነሽ). "Din(i)k’i" means "wonderful" and "nesh" means "you are" in feminine form of this pronoun and verb. The asteroid was unnamed when it was selected for exploration by the Lucy spacecraft, so the Lucy mission team proposed the name Dinkinesh to the International Astronomical Union's Working Group for Small Bodies Nomenclature (WGSBN), which approved and announced the name on 6 February 2023.

Orbit
Dinkinesh orbits the Sun in the inner main asteroid belt on an elliptical orbit with an average distance of 2.19 astronomical units (328 e6km) and an orbital period of 3.24 years. With an orbital eccentricity of 0.112, Dinkinesh comes as close as 1.95 AU from the Sun at perihelion to as far as 2.44 AU at aphelion. The asteroid's orbit is inclined 2.1° with respect to the plane of the Solar System. Dinkinesh is possibly a member of the Flora family, a group of asteroids that share similar orbital characteristics as the family's parent asteroid 8 Flora.

Exploration
The Lucy spacecraft made a flyby of Dinkinesh from a distance of 425 km on 1 November 2023 16:54 UTC. Lucy's flyby of Dinkinesh was announced by NASA and the Lucy science team on 25 January 2023, more than one year after Lucy had launched in October 2021. The asteroid was initially overlooked as a potential flyby target because it was too small. It was identified in August 2022 by Raphael Marschall, mission collaborator of the Nice Observatory, who investigated 500,000 asteroids for potential close approaches with the spacecraft. The original trajectory of Lucy took it within 64000 km of Dinkinesh, but a series of planned trajectory correction maneuvers from May to September 2023 allowed Lucy to approach much closer.

Dinkinesh was Lucy's first and smallest asteroid flyby during its mission, and is the smallest main-belt asteroid explored by spacecraft yet. The Dinkinesh flyby served to test Lucy's autonomous tracking capabilities before it will apply them to its main science targets, the Jupiter trojans. Lucy took its first images of Dinkinesh on 3–5 September 2023, when the asteroid was 23 e6km away from the spacecraft. The spacecraft continued imaging Dinkinesh from afar to aid its optical navigation over the days before the flyby. Because Dinkinesh is very small, Lucy did not resolve surface detail on Dinkinesh until the day of the flyby. At closest approach, Lucy was moving 4.5 km/s relative to Dinkinesh and it was expected to take 2 meters/pixel resolution images of the asteroid with the panchromatic L'LORRI imager, 15 m/pixel color images with the L'Ralph imager, and 24 m/pixel near-infrared spectra and thermal measurements with the L'TES spectrometer. After the flyby, Lucy's L'LORRI instrument continued observing Dinkinesh for four days to measure the asteroid's light curve.

Satellite
During the flyby, the Lucy spacecraft discovered that Dinkinesh has a natural satellite 220 m in diameter. The satellite is named Selam (full designation Dinkinesh I Selam), after the fossil remains of a three-year-old Australopithecus afarensis female hominin (the same species as the Lucy fossil) found in Dikika, Ethiopia in 2000. Selam means "peace" in the Amharic language (ሰላም) and it was proposed by Raphael Marschall. The name was approved by the International Astronomical Union's WGSBN on 27 November 2023.

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. 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 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.

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 (less than 50 mm/s), the impact does not disrupt the shapes of the two bodies and instead forms a contact binary.

Geology
The surfaces of Dinkinesh and Selam are covered with boulders and craters. The outline of Dinkinesh's shape is not smooth, which suggests that the asteroid is relatively old. Dinkinesh bears an equatorial ridge, which suggests that the asteroid has experienced mass shedding in the past. Dinkinesh's equatorial ridge also has a secondary ridge that branches off it. Dinkinesh's shape resembles the near-Earth asteroids 101955 Bennu and 162173 Ryugu, which are known to have rubble pile interior structures consisting of rocks and dust loosely held by gravity. Because of this similarity, Dinkinesh is thought to likely have a rubble pile structure as well.

A ridge is also present on Selam, but it is not oriented along its equator. The reason for Selam's unaligned ridge remains yet to be determined.

Surface composition
Visible light spectroscopy of Dinkinesh by two independent teams of researchers in November–December 2022 showed that it is an S-type asteroid, meaning it is mainly composed of rocky silicates and small amounts of metal. Spectral data obtained from the 10-meter Keck I telescope at Mauna Kea, Hawaii indicates that Dinkinesh belongs to the Sq subclass of S-type asteroids because it exhibits the 1 μm olivine and pyroxene spectral absorption band that is characteristically seen in Q-type asteroids. On the other hand, spectral data from the 8.1-meter Gemini South telescope at Cerro Pachón, Chile showed that Dinkinesh's spectrum more closely resembles a standard S-type asteroid with a shallower 1 μm band. This difference between the two measured spectra of Dinkinesh may be caused by either observational artifacts or compositional variations across Dinkinesh's surface as it rotates. If the latter possibility is true, then Dinkinesh's varying 1 μm band would indicate that there is space-weathered material that is unevenly distributed across its surface, likely due to impacts and surface topography.

Rotation and light curve
Dinkinesh rotates every $790 m$. As Dinkinesh rotates, its brightness from Earth fluctuates due to its non-spherical shape, which can be inferred from the amplitude of the asteroid's rotational light curve. The first photometric observations of Dinkinesh's rotational light curve were attempted with the Teide Observatory's 0.8-m IAC-80 telescope at Tenerife, Spain in November 2022, but it did not observe Dinkinesh long enough to make conclusive findings. Longer photometric observations of Dinkinesh were made with the Calar Alto Observatory's 1.23-m telescope at Almería, Spain from November 2022–February 2023, which discovered that Dinkinesh rotates slowly and had a light curve amplitude of $3.739 h$ magnitudes.

Diameter and albedo
Lucy images of Dinkinesh show that it measures approximately 790 m across its equator. This is in agreement with the previous diameter estimates from measured absolute magnitude and average S-type asteroid albedo. Reprocessing of archival infrared thermal emission observations by the Wide-field Infrared Survey Explorer (WISE) from March 2010 give a consistent result.