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(153591) 2001 SN263
(153591) 2001 SN263 is a carbonaceous trinary[3] asteroid, classified as near-Earth object and former potentially hazardous asteroid of the Amor group, approximately 2.6 kilometers (1.6 miles) in diameter. It was discovered by the Lincoln Near-Earth Asteroid Research project at Lincoln Lab's Experimental Test Site in Socorro, New Mexico, on 20 September 2001.[1] The two synchronous minor-planet moons measure approximately 770 and 430 meters and have an orbital period of 16.46 and 150 hours, respectively.[4][10] Numbering and namingThis minor planet was numbered by the Minor Planet Center on 2 April 2007.[11] As of 2018, the primary and its moons have not been named.[1] In the scientific literature, the components of the trinary system are generically referred to as Alpha, Beta and Gamma, but these labels are not recognized by the IAU.[5][4] Primary2001 SN263, the primary object of this trinary system, is an unusual carbonaceous near-Earth asteroid of a C- or somewhat brighter B-type.[8][b] It orbits the Sun at a distance of 1.0–2.9 AU once every 2 years and 10 months (1,023 days). Its orbit has an eccentricity of 0.48 and an inclination of 7° with respect to the ecliptic.[2] A first precovery was taken at Palomar Observatory during the Digitized Sky Survey in 1990, extending the body's observation arc by 11 years prior to its official discovery observation at Socorro.[1] It has an Earth minimum orbital intersection distance (MOID) of 0.0520 AU (7,780,000 km), which translates into 20.3 lunar distances.[2] With an Earth MOID above 0.05 AU, 2001 SN263 is no longer a potentially hazardous asteroid, but it was classified as such by the MPC until early 2017.[1][12] Radar observations show that it measures 2.5 kilometers in diameter.[4] Its surface has a low albedo of 0.048.[6] Rotational lightcurves obtained from photometric observations gave a rotation period of 3.423 hours (best result) with a brightness variation between 0.13 and 0.27 magnitude (U=2/3/3).[8][9][a] Radar observations gave a concurring period of 3.4256 hours, and subsequent modeling of both radiometric and photometric observations gave a spin axis of (309.0°, −80.0°) in ecliptic coordinates (λ, β).[4] Trinary systemIn 2008, scientists using the planetary radar at Arecibo Observatory discovered that the object is orbited by two satellites, when the triple asteroid made a close approach to Earth of 0.066 AU (nearly 10 million kilometers). The largest body (preliminarily called Alpha) is spheroid in shape, with principal axes of 2.8±0.1 km, 2.7±0.1 km, and 2.9±0.3 km, with an effective diameter of 2.5±0.3 km and a density of 1.1±0.2 g/cm3. The satellites, named Beta and Gamma, are several times smaller in size. Beta is 0.77±0.12 km in diameter and Gamma 0.43±0.14 km.[4] The only other unambiguously identified triple asteroids in the near-Earth population are (136617) 1994 CC, which was discovered to be a triple system in 2009, and 3122 Florence, which was found to be a triple system in September 2017.[13] Orbital characteristics of satellitesThe orbital properties of the satellites are listed in this table.[7] The orbital planes of both satellites are inclined relative to each other; the relative inclination is about 14 degrees. Such a large inclination is suggestive of past evolutionary events (e.g. close encounter with a terrestrial planet, mean-motion-resonance crossing) that may have excited their orbits from a coplanar configuration to an inclined state.
ExplorationThis triple asteroid system is the target for the planned ASTER mission scheduled for launch in 2025 by the Brazilian Space Agency.[14] Notes
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