(458271) 2010 UM26 and (796785) 2010 RN221
Young asteroid pair from a dissociated binary system.
(458271) 2010 UM26 and (796785) 2010 RN221 are a pair of main-belt asteroids traveling on nearly identical paths around the Sun. Astronomers believe they split apart from a binary system sometime in the 2000s, which would make them one of the youngest known asteroid pairs. The larger of the two, 2010 UM26, measures roughly 760 meters across and was first spotted on November 10, 2006, by the Spacewatch survey at Kitt Peak Observatory. Its smaller companion, 2010 RN221, is about 350 meters in diameter and was discovered on September 11, 2010, by the Mount Lemmon Survey at Mount Lemmon Observatory. The striking similarity of their orbits was first noted by astronomers in February 2022.
**History**
2010 UM26 was initially observed by Spacewatch on November 10, 2006, and again on December 13, 2006, by both Spacewatch and the Mount Lemmon Survey. However, these observations were too sparse to pin down its orbit, so the Minor Planet Center (MPC) did not assign it a provisional designation, and it became a lost minor planet. On September 11 and October 28–29, 2010, the Mount Lemmon Survey and Spacewatch reobserved the asteroid, and other observatories began tracking it in November 2010 to secure its orbit. The MPC gave it the provisional designation 2010 UM26 on November 7, 2010, and linked its 2010 observations to its 2006 discovery on December 26, 2010. The asteroid received the permanent catalog number 458271 on February 22, 2016, with official discovery credit going to Spacewatch's 2006 observations.
2010 RN221 was discovered on September 11, 2010, by the Mount Lemmon Survey. The MPC did not recognize its existence until 2021, when additional observations were connected to the original discovery. It was assigned the catalog number 796785 on May 22, 2025.
**Asteroid pair discovery**
On February 15, 2022, Alessandro Odasso posted to the online Minor Planets Mailing List that the orbits of 2010 UM26 and 2010 RN221 were extremely similar. His numerical integration of their paths showed they had passed very close to each other in 2003, leading him to suspect they came from a common parent body. Odasso thought it might be a single object mistaken for two, but Peter Vereš of the MPC confirmed they were separate asteroids.
- Larger diameter
- 760 m (2,490 ft)
- Smaller diameter
- 350 m (1,150 ft)
- Discovery date 2010 um26
- 10 November 2006
- Discovery site 2010 um26
- Kitt Peak Observatory by the Spacewatch survey
- Discovery date 2010 rn221
- 11 September 2010
- Discovery site 2010 rn221
- Mount Lemmon Observatory by the Mount Lemmon Survey
- Orbital period
- 4.14 years
- Semi major axis
- 2.58 AU (386 million km)
- Orbital eccentricity
- 0.326
- Orbital inclination
- 3.9°
Lore & Background
2010 UM26 was discovered on 10 November 2006 by the Spacewatch survey at Kitt Peak Observatory. It was reobserved once in December 2006 but became a lost minor planet until reobservations in September and October 2010 allowed its orbit to be secured. The Minor Planet Center gave it the provisional designation 2010 UM26 on 7 November 2010 and later linked its 2010 observations to its 2006 discovery. It received the minor planet number 458271 on 22 February 2016. 2010 RN221 was discovered on 11 September 2010 by the Mount Lemmon Survey, but its existence was not recognized until 2021 when additional observations were linked; it received the number 796785 on 22 May 2025.
The similarity of their orbits was first noticed by Alessandro Odasso on 15 February 2022, who reported it to the Minor Planets Mailing List. Numerical integration suggested they approached very close in 2003, indicating a common progenitor. Sam Deen found precovery images from 14 May 2005 showing them close together. A rigorous study by David Vokrouhlický and collaborators was published in November 2022. Hubble Space Telescope observations on 4 January 2023 found no dust trail or fragments larger than 36 m, implying the pair separated by dissociating from a preexisting binary system rather than direct rotational fission.
Reader's Guide
The pair's significance lies in being one of the youngest known asteroid pairs, with orbit simulations indicating a 55% probability that they separated after the year 2000, most likely in March 2003. Their relative velocities at that time were extremely small—less than 3 cm/s with 99% probability—well below the escape velocity of 2010 UM26. The lack of dust or fragments in Hubble observations suggests the pair did not form through direct rotational fission like active asteroid 331P/Gibbs, but rather by dissociation of a binary system, where components can remain in orbit for over 100 years, allowing solar radiation pressure to clear debris. The highly elongated shape of 2010 RN221 (aspect ratio >2.5:1) challenges previous theories that such elongated secondary components should have been destroyed by rotational fission, implying it either avoided fission or was reshaped afterward. Their identical orbits—with semi-major axes of 2.58 AU, eccentricity 0.326, inclination 3.9°, and mean anomalies differing by less than 0.01°—make them a key object for studying binary asteroid evolution and the YORP effect.
Did You Know?
- The two asteroids have identical orbits with semi-major axes of 2.58 AU and orbital periods of 4.14 years.
- Their orbits differ in mean anomaly by less than 0.01°.
- Hubble observations in 2023 showed 2010 RN221 is highly elongated with an aspect ratio greater than 2.5:1.
- The pair is thought to have dissociated from a binary system, not through direct rotational fission.
Frequently Asked Questions
What are (458271) 2010 UM26 and (796785) 2010 RN221?
They are a pair of main-belt asteroids that share nearly identical orbital paths around the Sun. Astronomers consider them to be a recently separated binary pair, making them one of the youngest known asteroid pairs in the catalog.
How big are 2010 UM26 and 2010 RN221?
The larger member, 2010 UM26, spans roughly 760 meters (about 2,490 feet) across, while its companion 2010 RN221 measures approximately 350 meters (about 1,150 feet) in diameter.
When and where were the two asteroids first detected?
2010 UM26 was spotted on 10 November 2006 by the Spacewatch survey operating at Kitt Peak Observatory. Its smaller partner, 2010 RN221, was not identified until 11 September 2010 by the Mount Lemmon Survey at Mount Lemmon Observatory.
Why is this asteroid pair considered significant?
Because the pair is believed to have broken apart from a single binary system sometime in the 2000s, it represents one of the youngest known examples of a dissociated asteroid pair. That makes it a valuable natural laboratory for studying how binary asteroid systems evolve and eventually split.
How did 2010 UM26 and 2010 RN221 end up on separate orbits?
Current understanding holds that they were once gravitationally bound as a binary system and later separated, likely through tidal or rotational effects, during the 2000s. Their nearly identical orbital elements today are the key evidence that they share a common origin.
More in Notable Asteroids 1-24
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