Dark comet

Lore & Background

The first object to exhibit dark comet behavior was 'Oumuamua, an interstellar object discovered in 2017. It showed non-gravitational acceleration too strong to be explained by the Yarkovsky effect or solar radiation pressure, yet observations by the Spitzer Space Telescope found no cometary activity. Astrophysicist Darryl Seligman noted, 'It moved like a comet, but didn't have the classic coma, or tail, of a comet.' Scientists hypothesized the acceleration was produced by outgassing from the sunlit side of the surface. A team of astronomers began searching the asteroid belt for similar objects. By 2024, they had identified 14 asteroids that moved like comets, divided into two families. The outer family consists of larger asteroids, hundreds of meters or more, that reflect more light and have orbits resembling those of Jupiter-family comets. The inner family comprises smaller asteroids, all 50 meters or less in diameter, with more circular orbits.

1998 KY26, the target of Japan's Hayabusa2 mission in July 2031, was originally speculated by astronomers to be a dark comet. However, this hypothesis was disproven in 2025 when the asteroid showed no evidence of ejected dust from outgassing, and a reanalysis showed that the asteroid's non-gravitational acceleration can be fully explained by the Yarkovsky effect using its updated properties.

Reader's Guide

The identification of dark comets as a distinct class in 2023 marked a shift in understanding small solar system objects, bridging the traditional categories of asteroids and comets. Their non-gravitational acceleration, unexplained by standard effects like the Yarkovsky effect or solar radiation pressure, suggests a mechanism—likely outgassing—that does not produce visible cometary features. This challenges observational classification and raises questions about the prevalence of such objects. The division into outer and inner families, based on size and orbital characteristics, indicates two distinct populations with potentially different origins or evolutionary paths. The case of 1998 KY26, initially classified as a dark comet but later found to have its acceleration explained by the Yarkovsky effect after updated measurements, highlights the difficulty in confirming dark comet status. This uncertainty underscores the need for further study, as the model derived from 1998 KY26 may or may not apply to other small inner-family dark comets. The legacy of dark comets lies in their role as a new category that forces a reexamination of how objects are classified and how non-gravitational forces operate in the solar system.

Did You Know?

• The first object to exhibit dark comet behavior was the interstellar object 'Oumuamua, discovered in 2017.
• 1998 KY26, a target of Japan's Hayabusa2 mission, was originally speculated to be a dark comet, but this hypothesis was disproven in 2025.