Anyone can get confused with an identity, even astronomers. On August 28, 2025, researchers pointed their equipment toward an area of ​​the sky where they expected to find asteroid 1998 SH2. Drawing on gravitational data collected during its previous 4.5-year orbits around the Sun, hopeful observers focused NASA’s Deep Space Network (DSN) planetary radar system on the space rock’s presumed location about two million miles from Earth. However, things did not go as planned: 1998 SH2 was not presented.

Astronomers knew that some unexpected force was affecting its orbit, so they relied on optical telescopes to determine the exact location of the asteroid. Once they found the 1998 SH2, they quickly realized the mistake.

“After measuring the non-gravitational perturbations affecting the motion of 1998 SH2 and recognizing that they were not compatible with the object being an asteroid, we suspected that the object could be an active comet,” Davide Farnocchia, a navigation engineer at the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory, explained in a statement.

The differences between asteroids and comets may seem minor, but they are crucial. Asteroids are large metal-infused rocky objects that orbit the sun as ancient remains from the early stages of the solar system’s formation. By comparison, comets are born 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 has since completed two solar orbits. But after once again reviewing the available analyses, Farnocchia’s team determined that the object could be generating smaller amounts of thrust as it ejected gas into space. These vents form when radiation from the sun heats a comet’s dusty ice, creating a propellant gas often visible as a bright tail and coma. But when a comet is much smaller, those telltale signs can be harder to find.

Knowing that his mystery object’s approach would be the 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 ESO’s Very Large Telescope on Chile’s Cerro Paranal mountain also observed 1998 SH2. Their collective results, published in a recent study in the journal Nature Astronomyproved that his suspicions were true.

“The images we collected from these observatories showed a faint but clear tail, thus confirming that 1998 SH2 is, in fact, a comet,” said ESO astronomer and study co-author Olivier Hainaut.

Beyond rectifying decades-old misclassification, the team’s findings are helping us better understand a subcategory of space objects called dark comets. These entities usually have significant irregularities in their orbits called perturbations, but they do not offer visible signs such as outgassing, tails or comas. Astronomers discovered the first dark comet in 2016 and have since located about a dozen more throughout the solar system. The study’s authors suggest that continuing to examine other faint near-Earth objects with sufficiently powerful telescopes may even force new reclassifications of objects that astronomers currently believe to be asteroids. Beyond respecting the true identities of these space rocks, more precise observations can also improve the safety of all who live on this largest rock called Earth.

“Detection of 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 impact risks against Earth,” Farnocchia said.

“That’s how science works: you formulate a hypothesis and you set out to test it,” Hainaut added. “These data are exactly what was needed to confirm our hypothesis that 1998 SH2 was a comet.”

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