 Dinkinesh and its satellite Selam imaged by theLucy spacecraft's L'LORRI camera | |
| Discovery[1] | |
|---|---|
| Discovered by | LINEAR |
| Discovery site | Lincoln Lab ETS |
| Discovery date | 4 November 1999 |
| Designations | |
| (152830) Dinkinesh | |
| Pronunciation | /ˈdɪŋkɪnɛʃ/ |
Named after | Dinkʼinesh (Lucy fossil) |
| 1999 VD57 · 2004 HJ78 · 2007 CB63[2] | |
| main-belt[1] · (inner) | |
| Orbital characteristics[1] | |
| Epoch 25 February 2023 (JD 2460000.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 23.06yr (8,422 days) |
| Earliestprecovery date | 15 October 1999 |
| Aphelion | 2.437AU |
| Perihelion | 1.946 AU |
| 2.191 AU | |
| Eccentricity | 0.1120 |
| 3.24 yr (1,185 days) | |
| 25.239° | |
| 0° 18m 13.874s / day | |
| Inclination | 2.094° |
| 21.380° | |
| 66.711° | |
| Knownsatellites | 1 |
| Physical characteristics | |
| 790 m (primary)[3] | |
| 3.7387±0.0013 h[4] | |
| 0.27+0.25 −0.06[5] | |
| Sq[6][7] V–R =0.455±0.025[8] | |
| 17.62±0.04 (V-band)[6][8]: L3 | |
152830 Dinkinesh (provisional designation1999 VD57) is abinarymain-belt asteroid about 790 meters (2,600 feet) in diameter. It was discovered by theLincoln Near-Earth Asteroid Research (LINEAR) survey atSocorro, New Mexico on 4 November 1999. Dinkinesh, the name borrowed from an Ethiopian word for theLucy fossil, was the firstflyby target ofNASA'sLucy mission, which approached 425 km (264 mi) from the asteroid on 1 November 2023.[9] During the flyby, theLucy spacecraft discovered that Dinkinesh has acontact-binarynatural satellite, namedSelam, which is 220 meters (720 ft) in diameter. Dinkinesh is thesmallest main-belt asteroid explored by spacecraft yet, though some smallernear-Earth asteroids have also been explored.
Dinkinesh was discovered on 4 November 1999 by theLincoln Near-Earth Asteroid Research (LINEAR) sky survey atSocorro, New Mexico.[2] The discovery observations were published by theMinor Planet Center (MPC) on 23 November 1999 and the asteroid was given theminor planetprovisional designation1999 VD57, which describes its discovery year, month, and discovery order within the month.[10] The LINEAR andSpacewatch (Kitt Peak,Arizona) surveys continued observing Dinkinesh until 15 November 1999, after which the asteroid becamelost and went unrecognized for years.[2]
On 19 April 2004, Spacewatch reobserved Dinkinesh as a seemingly new asteroid, but misattributed these observations to those of another unrelated asteroid,2004 GZ43, which was discovered by the same survey on 12 April 2004.[2][11] Dinkinesh was again reobserved as a seemingly new asteroid on 15 and 17 February 2007 byPalomar Observatory'sNear-Earth Asteroid Tracking (NEAT) survey atSan Diego County, California, which led the MPC to give Dinkinesh the provisional designation2007 CB63 on 25 February 2007.[2][12]Gareth V. Williams, the associate director of the MPC at the time, recognized that1999 VD57 and2007 CB63 were the same asteroid and published the linkage on 2 March 2007.[13][14] The linkage between Dinkinesh's 1999 and 2007 observations enabled the MPC to find additional observations from 2001 to 2007, where the asteroid was previously detected unknowingly.[15][16] The linkage and additional observations extended Dinkinesh'sobservation arc to over 7 years and greatlyreduced uncertainties in its orbit. This allowed the MPC to give Dinkinesh its permanentminor planet catalog number 152830 on 2 April 2007.[17][14]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.[18]
On 3 March 2007, the MPC established that Spacewatch's 2004 observations of Dinkinesh were not of2004 GZ43, and thus redesignated these observations as2004 HJ78.[19] However, the MPC did not recognize that2004 HJ78 was Dinkinesh until Gareth Williams made the linkage and published it on 9 February 2009.[20]
Dinkinesh is theAmharic name for theLucy fossil, after whichNASA'sLucy mission is named.[21] The name means "you are wonderful" in the Amharic language (ድንቅነሽ).[22][23] "Dinkʼi" means "wonderful"[24] and "nesh" means "you are" in feminine form.[25] The asteroid was unnamed when it was selected for exploration by theLucy spacecraft, so theLucy mission team proposed the name Dinkinesh to theInternational Astronomical Union'sWorking Group for Small Bodies Nomenclature (WGSBN), which approved and announced the name on 6 February 2023.[21][22]
Dinkinesh orbits the Sun in theinnermain asteroid belt on anelliptical orbit with an average distance of 2.19 astronomical units (328 million km, 204 million mi) and anorbital period of 3.24 years.[1] With anorbital eccentricity of 0.112, Dinkinesh comes as close as 1.95 AU (292 million km; 181 million mi) from the Sun atperihelion to as far as 2.44 AU (365 million km; 227 million mi) ataphelion.[1] The asteroid's orbit isinclined 2.1° with respect to theplane of the Solar System.[1] Dinkinesh is possibly a member of theFlora family, a group of asteroids that share similar orbital characteristics as the family's parent asteroid8 Flora.[5]: 5
TheLucy spacecraft made aflyby of Dinkinesh from a distance of 425 km (264 mi) on 1 November 2023 16:54UTC.[26]Lucy's flyby of Dinkinesh was announced by NASA and theLucy science team on 25 January 2023, more than one year afterLucy had launched in October 2021.[27] The asteroid was initially overlooked as a potential flyby target because it was too small.[27] It was identified in August 2022 by Raphael Marschall, mission collaborator of theNice Observatory, who investigated 500,000 asteroids for potential close approaches with the spacecraft.[27][28] The original trajectory ofLucy took it within 64,000 km (40,000 mi) of Dinkinesh, but a series of plannedtrajectory correction maneuvers from May to September 2023 allowedLucy to approach much closer.[27][29][30]
Dinkinesh wasLucy's first and smallest asteroid flyby during its mission, and is thesmallest main-belt asteroid explored by spacecraft yet.[27] The Dinkinesh flyby served to testLucy's autonomous tracking capabilities before it will apply them to its main science targets, theJupiter trojans.[27][31]Lucy took its first images of Dinkinesh on 3–5 September 2023, when the asteroid was 23 million km (14 million mi) away from the spacecraft.[9] The spacecraft continued imaging Dinkinesh from afar to aid its optical navigation over the days before the flyby.[9] Because Dinkinesh is very small,Lucy did not resolve surface detail on Dinkinesh until the day of the flyby.[9] At closest approach,Lucy was moving 4.5 km/s (2.8 mi/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/pixelnear-infrared spectra and thermal measurements with the L'TESspectrometer.[9][31] After the flyby,Lucy's L'LORRI instrument continued observing Dinkinesh for four days to measure the asteroid'slight curve.[26]
_1999_VD57_Lucy_flyby_diagram.webp)
 | |
| Discovery | |
|---|---|
| Discovered by | Lucy |
| Discovery date | 1 November 2023 |
| Designations | |
| Pronunciation | /səˈlɑːm/ |
| Dinkinesh I | |
| Orbital characteristics[32][4] | |
| 3.11±0.05 km | |
| Eccentricity | ≈0 |
| 52.67±0.04 h | |
| Satellite of | Dinkinesh |
| Physical characteristics[32][4] | |
| Dimensions | 240 m × 200 m × 200 m(inner lobe) 280 m × 220 m × 210 m(outer lobe) |
| 220 m | |
| 52.44±0.14 h(likelysynchronous) | |
During the flyby, theLucy spacecraft discovered that Dinkinesh has anatural satellite 220 m (720 ft) in diameter.[3] The satellite is namedSelam (/səˈlɑːm/; formal designation152830 Dinkinesh I), afterthe fossil remains by the same name of a three-year-oldAustralopithecus afarensis female hominin (the same species as the Lucy fossil) found inDikika, Ethiopia in 2000.[33]: 5 Selam means "peace" in the Amharic language (ሰላም) and it was proposed by Raphael Marschall.[34] The name was approved by the International Astronomical Union's WGSBN on 27 November 2023.[34][33]: 5
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.[35][36] 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.[3][37] 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.[38]
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.[39][36] 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.[39][40] The ejected material forms a disk around the asteroid, which eventually coalesces into a satellite.[39] 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.[39] 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.[39]
One possible explanation for the origin of Selam's contact binary nature is rotational fissioning by the YORP effect.[41][42] 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.[41]: 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.[41]: 167
The surfaces of Dinkinesh and Selam are covered with boulders andcraters.[39] The outline of Dinkinesh's shape is not smooth, which suggests that the asteroid is relatively old.[35] Dinkinesh bears anequatorial ridge, which suggests that the asteroid has experienced mass shedding in the past.[39] Dinkinesh's equatorial ridge also has a secondaryridge that branches off it.[35] Dinkinesh's shape resembles the near-Earth asteroids101955 Bennu and162173 Ryugu, which are known to haverubble 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.[39]
A ridge is also present on Selam, but it is not oriented along its equator.[35] The reason for Selam's unaligned ridge remains yet to be determined.[35]
Visible lightspectroscopy of Dinkinesh by two independent teams of researchers in November–December 2022 showed that it is anS-type asteroid, meaning it is mainly composed of rockysilicates and small amounts of metal.[7]: 1 [6] Spectral data obtained from the 10-meterKeck I telescope atMauna Kea,Hawaii indicates that Dinkinesh belongs to the Sq subclass of S-type asteroids because it exhibits the 1 μmolivine andpyroxenespectral absorption band that is characteristically seen inQ-type asteroids.[6][43]: 4, 6 On the other hand, spectral data from the 8.1-meterGemini South telescope atCerro Pachón, Chile showed that Dinkinesh's spectrum more closely resembles a standard S-type asteroid with a shallower 1 μm band.[6] This difference between the two measured spectra of Dinkinesh may be caused by either observationalartifacts or compositional variations across Dinkinesh's surface as it rotates.[6] If the latter possibility is true, then Dinkinesh's varying 1 μm band would indicate that there isspace-weathered material that is unevenly distributed across its surface, likely due toimpacts and surfacetopography.[6]
Dinkineshrotates every3.7387±0.0013 h. As Dinkinesh rotates, its brightness from Earth fluctuates due to its non-spherical shape, which can be inferred from theamplitude of the asteroid's rotationallight curve.[44][8] The firstphotometric observations of Dinkinesh's rotational light curve were attempted with theTeide Observatory's 0.8-m IAC-80 telescope atTenerife, Spain in November 2022, but it did not observe Dinkinesh long enough to make conclusive findings.[7]: 4 Longer photometric observations of Dinkinesh were made with theCalar Alto Observatory's 1.23-m telescope atAlmería, Spain from November 2022 to February 2023, which observed that Dinkinesh's brightness fluctuates by0.39±0.02magnitudes every 52.67 hours.[44][8]: 1 This light curve period is not caused by Dinkinesh's rotation; rather it caused by the tidally locked rotation of Selam, whose elongated shape produces the observed brightness fluctuations.[4]
Lucy images of Dinkinesh show that it measures approximately 790 m (2,590 ft) across its equator.[3][39] This is in agreement with the previous diameter estimates from measuredabsolute magnitude and average S-type asteroid albedo.[6][7][8] Reprocessing of archivalinfraredthermal emission observations by theWide-field Infrared Survey Explorer (WISE) from March 2010 give a consistent result.[5][39]
| Planetary satellites of |    | |
|---|---|---|
| Dwarf planet satellites of | ||
| Minor-planet moons | ||
| Ranked by size | ||
.png)
