Dark comet
Class of asteroids
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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
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.