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The objects that should not move this way

Robin George Andrews frames “Dark Comets” around a simple but unnerving astronomical problem. Some small bodies in the solar system look like ordinary asteroids: they appear as bare points of light, with no fuzzy coma and no tail. Yet when researchers calculate their orbits, those bodies do not behave like ordinary asteroids at all. Gravity alone cannot explain their paths, and even the subtle shove from sunlight and the Yarkovsky effect is not enough. Something is nudging them.

The story begins with Davide Farnocchia of NASA’s Center for Near-Earth Object Studies, who in 2016 noticed that the asteroid 2003 RM was drifting in a way that standard models could not explain. The most natural comparison was a comet, because comets get pushed around when solar heat vaporizes ice on or near their surfaces. That outgassing acts like a tiny thruster. But if 2003 RM was behaving like a comet, it was doing so invisibly. No one could see the dust or gas that should have announced such activity.

The mystery deepened when astronomers discovered the interstellar visitor ‘Oumuamua in 2017. It also accelerated in a way that seemed hard to explain by gravity alone, and it too showed no obvious cometary tail. Andrews is careful not to indulge the alien-spacecraft speculation that briefly swamped public discussion of ‘Oumuamua. The scientific point is not that the object was artificial. It is that the solar system already seemed to contain at least one body, 2003 RM, whose behavior rhymed with this stranger’s. What looked at first like an isolated oddity was starting to resemble a category.

Why “dark comet” is more than a catchy name

The label Darryl Seligman and his collaborators chose is apt. These bodies move like comets but remain dark in the observational sense: whatever is propelling them is hidden from view. That makes them scientifically important because comets are usually easy to diagnose. Even a small amount of ejected dust tends to spread out and scatter light dramatically. Astronomers are used to seeing the evidence.

Andrews uses this discrepancy to explain why the phenomenon is so provocative. If the standard explanation for cometary acceleration is outgassing, and if outgassing should usually make itself visible, then dark comets imply that researchers are missing some part of the picture. Perhaps the outgassing is extremely weak or intermittent. Perhaps the expelled material is mostly gas rather than dust. Perhaps some of these objects sit on the blurry continuum between asteroid and comet that astronomers have increasingly come to recognize, alongside hybrid bodies such as main-belt comets.

That ambiguity is part of the article’s appeal. Dark comets are not presented as a fully formed new class with tidy boundaries. They are more like a challenge to existing categories. The older language of asteroid versus comet may be too rigid for what the solar system actually contains.

Two families, two puzzles

The article’s most useful conceptual move is to split the known dark comets into two populations. By 2024 researchers had identified 14 examples, enough to notice a pattern. One group, the “outies,” tends to be larger and to follow more elongated orbits that keep them closer to Jupiter and the outer solar system. Their paths resemble those of Jupiter-family comets, which makes them easier to imagine as genuine icy comets whose activity has become weak or smothered.

Andrews highlights one especially appealing idea: suffocation. As an object approaches the sun, sublimating ice can loft dust off the surface, but some of that dust may fall back down and accumulate. Over time the comet could end up burying its remaining ice under an insulating blanket. Instead of producing a bright, obvious coma, it would vent only tiny puffs from small exposed patches. In effect, it would be a comet trying to pass as an asteroid.

The “innies” are stranger. They are smaller, often less than 50 meters across, and they stay in the inner solar system, where sunlight should have long since stripped away easy-to-vaporize ice. That makes their continued acceleration hard to account for. Their size is part of the problem too. Active comet nuclei are normally much larger; a body this small should be fragile, thermally battered and vulnerable to breaking apart. Yet the innies persist.

That two-population picture turns dark comets from a curiosity into a deeper physical problem. If the outies are aging, dust-choked comets, the innies might be either their final, desiccated descendants or something else entirely, perhaps related to heavily processed main-belt comets. Either way, the solar system seems to be hiding a population of objects whose life cycles are not yet well understood.

Why astronomers care so much

The article never lets the reader forget that this is not just classification for its own sake. One implication concerns the origin of Earth’s water. If dark comets are common in or near the asteroid belt, they could offer a more practical delivery route for water and other volatiles to the inner solar system than distant comets from the Kuiper belt or Oort cloud. That does not solve the problem outright, but it adds a plausible intermediary population to a long-running debate about how Earth became habitable.

The second implication is more immediate: planetary defense. Farnocchia’s job is to predict the paths of potentially hazardous near-Earth objects. If some small bodies can receive cometlike pushes without displaying obvious cometlike signatures, then orbit forecasts may need to account for that behavior more carefully. Andrews does not turn this into a panic scenario; the point is subtler. The better astronomers understand these hidden accelerations, the better they can project where unusual objects will be in the future.

The article ends on an unusually satisfying note for a mystery story, because the next clues are already on the way. The Vera C. Rubin Observatory should discover many more small bodies and may greatly expand the known dark-comet population. The James Webb Space Telescope could search for the otherwise invisible water vapor that would strongly support the outgassing explanation. And perhaps most tantalizing, Japan’s extended Hayabusa2 mission is already headed toward 1998 KY26, one of the inner dark comets, even though mission planners did not realize it when they chose the target.

That convergence of surveys, infrared measurements and an eventual close-up visit gives the piece its real momentum. Andrews presents dark comets as a reminder that the solar system is not a finished catalog but an active research frontier. The mystery is compelling because it sits at the intersection of celestial mechanics, comet physics, planetary origins and impact risk. These objects are small and dim, but they force astronomers to reconsider how many supposedly ordinary rocks may in fact be something stranger.