Dark comet
Dark comets are a class of asteroids that exhibit some behavior consistent with comets, but do not grow a comet's tail or produce a coma, making them visually appear as asteroids. They are distinguished from other asteroids by their non-gravitational acceleration, which causes them to deviate from their expected orbital path. Dark comets were first identified as a distinct class of objects in 2023.[1] They mostly fall into two families, the outer dark comets, and the inner dark comets, which are distinguished by their size and orbital characteristics.[2]
Discovery
[edit]The first object that exhibited “dark comet” behavior was ‘Oumuamua, an interstellar object discovered in 2017, which was found to exhibit “non-gravitational acceleration,” that is too strong to be explained by the Yarkovsky effect or solar radiation pressure.[3] What made this notable was that, while this is typical of comets, observations by the Spitzer Space Telescope did not find any cometary activity.[3][1] As astrophysicist Darryl Seligman put it, “It moved like a comet, but didn't have the classic coma, or tail, of a comet.”
Since the unexplained acceleration is away from the direction of the sun, the scientists hypothesized that the acceleration is produced by outgassing from the sunlit side of the surface.[3]
A team of astronomers began searching the asteroid belt for more objects that behaved the same way. By 2024, astronomers had identified 14 asteroids that moved like comets.[4] These were divided into two families.[4] The outer family was made up of larger asteroids, measuring hundreds of meters or more, and seemed to reflect more light. They had larger orbits, resembling those of Jupiter-family comets, which pass near Jupiter's orbit at aphelion. The inner family asteroids were smaller, all 50 meters or less in diameter, and had more circular orbits.
Exploration
[edit]1998 KY26, the target of Japan's Hayabusa2 mission in July 2031, was originally speculated by astronomers to be a dark comet.[4] However, this hypothesis was disproven in 2025 when the asteroid showed no evidence of ejected dust from outgassing,[5] and a reanalysis showed that the asteroid's non-gravitational acceleration can be fully explained by the Yarkovsky effect using its updated properties.[6]
References
[edit]- 1 2 Seligman, Darryl Z. (2023-02-15). "Dark Comets? Unexpectedly Large Nongravitational Accelerations on a Sample of Small Asteroids". The Planetary Science Journal. 4, Number 2. doi:10.3847/PSJ/acb697. hdl:10150/674306. Retrieved 2025-07-13.
- ↑ Seligman, Darryl Z.; Farnocchia, Davide; et al. (2024-12-09). "Two distinct populations of dark comets delineated by orbits and sizes". Proceedings of the National Academy of Sciences of the United States of America. 1. doi:10.1073/pnas.2406424121. PMC 11665882. Retrieved 2025-07-13.
- 1 2 3 Hoang, Thiem; Loeb, Abraham (2020-08-17). "Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 ('Oumuamua)". The Astrophysical Journal Letters. 899, Number 2. arXiv:2006.08088. doi:10.3847/2041-8213/abab0c. Retrieved 2025-07-13.
- 1 2 3 "Dark Comets". Scientific American (May 2025): 58–63. 2025-05-01. Retrieved 2025-07-13.
- ↑ Santana-Ros, T.; Bartczak, P.; Muinonen, K.; Rożek, A.; Müller, T.; Hirabayashi, M.; et al. (September 2025). "Hayabusa2 extended mission target asteroid 1998 KY26 is smaller and rotating faster than previously known". Nature Communications. 16 (1): 8275. Bibcode:2025NatCo..16.8275S. doi:10.1038/s41467-025-63697-4. PMC 12446490. PMID 40968122.
- ↑ Farnocchia, Davide; Vokrouhlický, David; Santana-Ros, Toni; Bartczak, Przemysław; Micheli, Marco; Chesley, Steven R. (November 2025). "Radiation Forces and Trajectory of Hayabusa2♯ Target 1998 KY26". The Astrophysical Journal Letters. 993 (1): L9. Bibcode:2025ApJ...993L...9F. doi:10.3847/2041-8213/ae10b8.
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