A space rock that astronomers have tracked for nearly three decades has been unmasked: NASA's JPL found that 1998 SH2 is not an asteroid but a dark comet, with implications for how we understand threats from space.
For twenty-eight years, the object designated 1998 SH2 sat quietly in the asteroid catalog — a near-Earth object with a somewhat unusual orbit, but nothing that raised eyebrows. Then astronomers noticed something odd: its trajectory had shifted in ways that could not be explained by gravity alone. Using some of the most powerful observatories on Earth, NASA's Jet Propulsion Laboratory discovered the truth: 1998 SH2 is not a rocky asteroid at all. It is a dark comet — an object that looks like an asteroid but moves like a comet, complete with a faint coma and tail that had gone undetected for decades.
Dark comets are a recently recognized class of object that blur the line between asteroids and comets. Unlike classical comets, which develop bright, visible tails as they approach the Sun, dark comets are covered in a dark, dusty surface that hides their outgassing activity. Only by tracking their subtle orbital perturbations — the nongravitational acceleration caused by gas and dust jetting off the surface — can astronomers detect their true nature. For 1998 SH2, the telltale sign was an unexplained shift in its orbit that could only be explained by the weak thrust of sublimating ices.
The discovery of 1998 SH2 as a dark comet has practical consequences for planetary defense. Current impact hazard models treat near-Earth objects primarily as asteroids with well-understood orbital mechanics. Comets, however, add nongravitational forces — outgassing — that can alter trajectories in ways that are harder to predict. The more dark comets we find among the asteroid population, the more we need to update our risk calculations. The study also reinforces a broader lesson: our classification of the Solar System's small bodies is only as good as the instruments we use to observe them, and every new wavelength or technique reveals objects we thought we already understood.