Anyone can mistake an identity, even astronomers. On August 28, 2025, researchers aimed their equipment towards an area of sky where they expected to find the asteroid 1998 SH2. Based on gravitational data collected during its previous 4.5 year orbits around the sun, the hopeful observers focused NASA’s Deep Space Network (DSN) planetary radar system in the space rock’s assumed location about two million miles from Earth. However, things didn’t go as planned—1998 SH2 was a no-show.
Astronomers knew some unexpected force was affecting its orbit, so they relied on optical telescopes to pinpoint the asteroid’s exact location. Once they actually found 1998 SH2, they quickly realized the mistake.
“After we measured the nongravitational perturbations affecting the motion of 1998 SH2 and recognized they weren’t compatible with the object being an asteroid, we suspected the object could be an active comet,” Davide Farnocchia, a navigation engineer at the NASA Jet Propulsion Laboratory’s Center for Near-Earth Object Studies, explained in a statement.
The differences between asteroids and comets may sound minimal, but they’re crucial. Asteroids are large rocky objects infused with metals orbiting the sun as ancient leftovers from the solar system’s earliest formative stages. By comparison, comets are birthed at the outer edges of the solar system whose mixture of dust and ice vaporizes into a tail.
The last observations of 1998 SH2 took place in 2016, meaning the space rock had since completed two solar orbits. But after reviewing the available analyses once more, Farnocchia’s team determined the object could be generating minor amounts of thrust as it vented gas into space. These vents form when the sun’s radiation heats a comet’s dusty ice, creating a propulsive gas often visible as a bright tail and coma. But when a comet is much smaller, those telltale signs may become harder to find.
Knowing their mystery object’s approach would be its closest in years, Farnocchia connected with astronomers at the Canada-France-Hawaii Telescope near Mauna Kea, Hawaii, as well as the European Southern Observatory’s (ESO) Danish Telescope in La Silla, Chile. Meanwhile, researchers at the ESO’s Very Large Telescope on Chile’s Cerro Paranal mountain also gazed up at 1998 SH2. Their collective results, published in a recent study in the journal Nature Astronomy, proved their suspicions correct.
“The images we collected from these observatories showed a weak but clear tail, thus confirming that 1998 SH2 is, in fact, a comet,” said ESO astronomer and study co-author Olivier Hainaut.
Beyond rectifying the multidecade misclassification, the team’s findings are helping us better understand a subcategory of space objects called dark comets. These entities often have major irregularities in their orbits called pertubations, yet do not offer visible signs like outgassing, tails, or comas. Astronomers discovered the first dark comet in 2016, and have since located about a dozen more across the solar system. The study’s authors suggest that continuing to examine other faint, near-Earth objects with sufficiently powerful telescopes may even force further reclassifications of subjects that astronomers currently believe to be asteroids. Beyond respecting these space rock’s true identities, more accurate observations can also improve the safety of everyone living on this larger rock called Earth.
“Detecting these perturbations can be an important diagnostic tool for planetary defense that will help understand which objects may be comets rather than asteroids, how their orbits evolve, and how that influences their Earth impact risks,” said Farnocchia.
“That’s how science works—you form a hypothesis, and you set out to test it,” added Hainaut. “This data is exactly what was needed to confirm our hypothesis that 1998 SH2 was a comet.”
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