(458271) 2010 UM26 and (796785) 2010 RN221

Pair of main-belt asteroids November 2023 yes (458271) 2010 UM 26 and (796785) 2010 RN 221 #D6D6D6 2010 UM26-2010 RN221 HST Jewitt et al. 2023.jpg 2010 UM 26 and 2010 RN 221 imaged by the Hubble Space Telescope on 4 January 2023 Spacewatch (UM 26)Mt. Lemmon Survey (RN 221) Kitt Peak Obs. (UM 26)Mt. Lemmon Obs. (RN 221) 10 November 2006 (UM 26)11 September 2010 (RN 221) (458271) 2010 UM 26(796785) 2010 RN 221 main-belt (middle) 13 September 2023 (JD 2460200.5) 0 17.15 yr (6,265 days) 14 May 2005 3.415 AU 1.738 AU 2.577 AU 0.3256 4.14 yr (1,511 days) 65.868° (UM 26)65.863° (RN 221) 0.2382991 ms / day (UM 26)0.2382985 ms / day (RN 221) 3.883° 234.869° 120.270° 3 760 m (UM 26)350 m (RN 221) 5.9 0.6 h (UM 26)20 h ≥ (RN 221) 0.20 ≈ (assumed S-type)3 17.80 (UM 26)19.24 0.36 (RN 221) (458271) 2010 UM 26 and (796785) 2010 RN 221 are a pair of main-belt asteroids that have identical orbits around the Sun. These two asteroids are thought to have dissociated from a binary system sometime in the 2000s, which would make them one of the youngest asteroid pairs known. The larger member of this pair, (458271) 2010 UM 26 (or simply 2010 UM 26), is about 760 m ft in diameter and was discovered on 10 November 2006 by the Spacewatch survey at Kitt Peak Observatory. The smaller member, (796785) 2010 RN 221 (or simply 2010 RN 221), is about 350 m ft in diameter and was discovered on 11 September 2010 by the Mount Lemmon Survey at Mount Lemmon Observatory. The similarity of the orbits of these asteroids was first noticed by astronomers in February 2022. History Observations 2010 UM 26 was discovered on 10 November 2006 by the Spacewatch survey at Kitt Peak Observatory. The asteroid was then reobserved once by Spacewatch and Mount Lemmon Survey on 13 December 2006. However, the number of observations was too sparse to accurately determine the asteroid's orbit, so it did not receive a provisional minor planet designation from the Minor Planet Center (MPC) and became a lost minor planet. On 11 September and 28–29 October 2010, Mt. Lemmon Survey and Spacewatch reobserved the asteroid and other observatories began providing follow-up observations in November 2010 to secure its orbit. The MPC gave the asteroid its provisional designation 2010 UM 26 on 7 November 2010 and later linked its 2010 observations to its 2006 discovery on 26 December 2010.313120 The MPC gave 2010 UM 26 the minor planet catalog number of 458271 on 22 February 2016 and established official discovery credit to Spacewatch's 2006 observations.1027 2010 RN 221 was discovered on 11 September 2010 by the Mount Lemmon Survey at Mount Lemmon Observatory. However, the MPC did not recognize the asteroid's existence until 2021, when additional observations were linked to the discovery observations.169 The MPC gave 2010 RN 221 the minor planet catalog number of 796785 on 22 May 2025.1176 thumb|center|upright=1.5|Canada–France–Hawaii Telescope precovery image of 2010 UM 26 and 2010 RN 221 from 14 May 2005 Asteroid pair discovery On 15 February 2022, Alessandro Odasso reported to the online Minor Planets Mailing List that 2010 UM 26 and 2010 RN 221 had extremely similar orbits. He noticed that the two asteroids approached very close to each other in 2003 according to his numerical integration of their orbits, which led him to suspect that they may have separated from a common progenitor.5 Odasso suspected that it could not be a single object mistaken as two asteroids; Peter Vereš of the MPC staff team confirmed that they are indeed two separate asteroids. On that same day, Sam Deen found precovery observations of 2010 UM 26 and 2010 RN 221 appearing close together in archival Canada–France–Hawaii Telescope images from 14 May 2005. A more rigorous study of the asteroid pair was published in Astronomy & Astrophysics by David Vokrouhlický and collaborators in November 2022, who acknowledged Odasso and Deen for independently discovering the asteroid pair.5 Orbit thumb|upright=1.5|Orbit diagram of 2010 UM 26 and 2010 RN 221. The orbits and positions of these two asteroids are virtually identical in this diagram. 2010 UM 26 and 2010 RN 221 both orbit the Sun in the middle zone of the main asteroid belt. They share extremely similar elliptical orbits with semi-major axes of 2.58 astronomical units (386 e6km e6mi unit comma) and orbital periods of 4.14 years.3 With orbital eccentricities of 0.326, the two asteroids come as close as 1.74 AU e6km e6mi unit from the Sun at perihelion to as far as 3.42 AU e6km e6mi unit at aphelion. The asteroids' orbits are inclined 3.9° with respect to the plane of the Solar System.3 The asteroids' mean anomalies, or angular positions along their orbits, differ by less than 0.01°.3 Formation Asteroid pairs and binaries are thought to have formed from the rotational fissioning of a single precursor asteroid, as a result of rotational acceleration by the uneven reflection of sunlight off the asteroid's surface—a phenomenon known as the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect. For asteroids with diameters under 1 km mi, the YORP effect can double their rotation rates within timescales of 1 million years, eventually leading to the ejection of surface material and disruption of their internal structures due to excessive centrifugal forces.79–80 Rotational fission preferentially occurs if the precursor asteroid's internal structure is weak, like a rubble pile consisting of rocks and dust loosely held together by gravity.80 Orbit simulations of 2010 UM 26 and 2010 RN 221 indicate that there is a 55% probability that the two asteroids separated after the year 2000, with the most likely date being March 2003.1 According to these simulations, the two asteroids came within 1000 km mi of each other in March 2003, with a range of possible close approach distances that could reach inside the ≈230 km mi-radius Hill sphere of 2010 UM 26.2 The relative velocities between the two asteroids in March 2003 were extremely small; there is a 99% probability that their relative velocities were less than 3 cm/s in/s at that time, which is much less than the ≈50 cm/s in/s escape velocity of 2010 UM 26.2–3 It is possible but very unlikely that 2010 UM 26 and 2010 RN 221 could have separated before the 2000s.5 On 4 January 2023, David C. Jewitt and collaborators observed 2010 UM 26 and 2010 RN 221 with the Hubble Space Telescope to search for dusty debris that might have been ejected from the separation of 2010 UM 26 and 2010 RN 221. They found no evidence of a dust trail nor macroscopic fragments larger than 36 m ft in diameter, which implies that the asteroid pair did not separate directly through rotational fission like the fragmenting active asteroid 331P/Gibbs.3, 6 Jewitt and collaborators proposed that the pair separated by dissociating from a preexisting binary system, as a result of orbital expansion by gravitational perturbations and solar radiation pressure.6 A binary system like this can be formed by rotational fissioning of a single asteroid, but the split components can remain in orbit for over 100 years, enough time for solar radiation pressure to completely clear any dust and macroscopic fragments from the binary system by the time they dissociate.6 Physical characteristics Little is known about the sizes, albedos, and spectral types of 2010 UM 26 and 2010 RN 221, since none these properties have been directly measured.3 Jewitt and collaborators examined archival Wide-field Infrared Survey Explorer (WISE) images of the asteroid pair from 2010, but were unable to detect any conclusive signs of infrared thermal emissions from either asteroid, thus precluding the measurement of their albedos.3 Nevertheless, Jewitt and collaborators assume a silicate (S-type) composition with a geometric albedo of 0.20 for 2010 UM 26 and 2010 RN 221, since other asteroid pairs are known to have these characteristics.3 Using this assumed albedo for the asteroids' absolute magnitudes, they estimate diameters of 760 m ft and 350 m mi for 2010 UM 26 and 2010 RN 221, respectively.3 Hubble observations of 2010 UM 26 and 2010 RN 221 in 2023 showed that 2010 UM 26 is slightly elongated (aspect ratio 1.15:1) and rotates in 5.9 hours, whereas 2010 RN 221 is highly elongated (aspect ratio >2.5:1) and rotates slowly with a period of at least 20 hours.3 The particularly elongated shape of 2010 RN 221 challenges previous theories that elongated secondary components of binary asteroid systems should have been destroyed by rotational fission; this implies that 2010 RN 221 had either somehow avoided rotational fission or had undergone reshaping after it had fissioned from the primary component.6 See also P/2016 J1 (PanSTARRS) – Pair of main-belt comets thought to have recently split apart (300163) 2006 VW 139 – A widely-separated binary main-belt comet References 2022-07-09 last obs. JPL Small-Body Database Lookup: 458271 (2010 UM26) https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=20458271 Jet Propulsion Laboratory 4 November 2023 2022-07-09 last obs. JPL Small-Body Database Lookup: 796785 (2010 RN221) https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=20796785 Jet Propulsion Laboratory 4 November 2023 (458271) = 2010 UM26 https://www.minorplanetcenter.net/db_search/show_object?object_id=458271 Minor Planet Center 12 November 2023 (796785) = 2010 RN221 https://www.minorplanetcenter.net/db_search/show_object?object_id=796785 Minor Planet Center 12 November 2023 M. P. S. 353965 https://minorplanetcenter.net/iau/ECS/MPCArchive/2010/MPS_20101107.pdf Minor Planet Circulars Supplement Minor Planet Center 7 November 2010 353965 313 12 November 2023 live https://web.archive.org/web/20160914052357/https://minorplanetcenter.net/iau/ECS/MPCArchive/2010/MPS_20101107.pdf 14 September 2016 M. P. S. 362802 https://minorplanetcenter.net/iau/ECS/MPCArchive/2010/MPS_20101226.pdf Minor Planet Circulars Supplement Minor Planet Center 26 December 2010 362802 120 12 November 2023 live https://web.archive.org/web/20170828091222/https://minorplanetcenter.net/iau/ECS/MPCArchive/2010/MPS_20101226.pdf 28 August 2017 M. P. C. 98597 https://minorplanetcenter.net/iau/ECS/MPCArchive/2016/MPC_20160222.pdf Minor Planet Circular Minor Planet Center 22 February 2016 98597 1027 12 November 2023 live https://web.archive.org/web/20211013230010/https://minorplanetcenter.net/iau/ECS/MPCArchive/2016/MPC_20160222.pdf 13 October 2021 M. P. S. 1524895 https://www.minorplanetcenter.net/iau/ECS/MPCArchive/2021/MPS_20211214.pdf Minor Planet Circulars Supplement Minor Planet Center 14 December 2021 1524895 169 12 November 2023 M. P. C. 184458 https://www.minorplanetcenter.net/iau/ECS/MPCArchive/2025/MPC_20250522.pdf Minor Planet Circular Minor Planet Center 22 May 2025 184458 1176 7 July 2025 Alessandro Odasso 2010 RN221 and (458271) 2010 UM26 https://groups.io/g/mpml/message/37322 Minor Planets Mailing List Groups.io 15 February 2022 12 November 2023 Alessandro Odasso 2010 RN221 and (458271) 2010 UM26 https://odassoastro.blogspot.com/2022/02/2010-rn221-and-458271-2010-um26.html astro Blogspot 15 February 2022 12 November 2023 Sam Deen Re: 2010 RN221 and (458271) 2010 UM26 https://groups.io/g/mpml/message/37323 Minor Planets Mailing List Groups.io 15 February 2022 12 November 2023 D. Polishook Spin axes and shape models of asteroid pairs: Fingerprints of YORP and a path to the density of rubble piles Icarus October 2014 241 79–96 10.1016/j.icarus.2014.06.018 1406.3359 2014Icar..241...79P 118618128 D. Vokrouhlický P. Fatka M. Micheli P. Pravec E. J. Christensen Extremely young asteroid pair (458271) 2010 UM26 and 2010 RN221 Astronomy & Astrophysics August 2022 664 L17 6 free 10.1051/0004-6361/202244589 2208.06207 2022A&A...664L..17V 251554531 David Jewitt Yoonyoung Kim Jing Li Max Mutchler Physical Properties of the Young Asteroid Pair 2010 UM26 and 2010 RN221 The Astronomical Journal November 2023 166 5 191 7 free 10.3847/1538-3881/acfc1f 2309.07024 2023AJ....166..191J 261705684 External links (458271) 2010 UM26 (796785) 2010 RN221 (458271) 2010 UM 26 20458271 458271 (796785) 2010 RN 221 20796785 796785 458271 (458271) 2010 UM 26 796785 (796785) 2010 RN 221 Astronomy Stars Spaceflight Solar System Category:Main-belt asteroids Category:Asteroid groups and families Category:Discoveries by the Spacewatch project 20061110 Category:Discoveries by MLS 20100911