Dark comets are small bodies with no detected coma that have significant accelerations explainable by outgassing of volatiles, analogous to the first interstellar object 1I/2017 U1 ‘Oumuamua. These objects represent a potentially widespread class of small bodies that further populate the continuum between asteroids and comets and for which the active nature is inferred from their orbital motion. Michigan State University astronomer Darryl Seligman and colleagues have now detected seven new dark comets and demonstrated that there are two distinct populations of these objects.
In 2003, astronomers noticed the trajectory of the asteroid 2003 RM had moved slightly from its expected orbit. That movement could not be explained by accelerations typical for asteroids.
Then, interstellar object ‘Oumuamua passed through our Solar System and perplexed astronomers in 2017.
Like typical asteroids and comets, it was moving partly due to the gravitational pull of other objects in the Solar System, like the Sun.
However, astronomers also found ‘Oumuamua was accelerating beyond that, likely due to the release of previously trapped gas.
The strength of the added propulsion from such outgassing defied that of typical asteroids and matched a comet’s pace.
The problem? ‘Oumuamua was missing the signature bright dust tail of a comet. Thus, it could not be classified easily as an asteroid or a comet.
In 2023, astronomers identified seven objects within our Solar System that mirrored the same unusual characteristics of ‘Oumuamua.
The objects were categorized as members of an entirely new class of objects — the first seven dark comets.
“One of the most important reasons why we study small bodies, like asteroids and comets, is because they tell us about how material is transported around the Solar System,” Dr. Seligman said.
“Dark comets are a new class of near-Earth objects that may contain water, so they’re a new potential source for delivering materials to…
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