An object that spent nearly three decades cataloged as a near-Earth asteroid has been reclassified as a comet after NASA scientists caught it drifting off its predicted course. The body, known since its 1998 discovery as 1998 SH2, betrayed its true nature when it failed to appear where astronomers expected during a 2025 pass by Earth. Researchers led by NASA’s Jet Propulsion Laboratory traced the discrepancy to gas quietly venting from its surface, the signature of a comet, and confirmed it with a faint tail spotted from three mountaintop observatories. NASA announced the result on July 16, 2026, adding the object to a small but growing list of “asteroids” that have turned out to be something else.
A radar target that never showed up
The puzzle began on Aug. 28, 2025, when 1998 SH2 passed within about 2 million miles (3 million kilometers) of Earth during its four-and-a-half-year loop around the Sun. A team preparing to bounce radar off the object with the Deep Space Network had calculated its position from earlier orbits, folding in the gravitational tug of the Sun and the planets. When the object was not where the math placed it, the researchers concluded that some additional force had been nudging it along. Its orbit had been tracked closely from 1998 to 2016, but it then completed two full circuits of the Sun unobserved before the 2025 radar attempt, leaving a gap that hid its behavior.
Nongravitational nudges pointed to venting ice
Precise optical astrometry, which measures an object’s exact position against background stars, revealed the culprit. Davide Farnocchia, a navigation engineer with NASA’s Center for Near-Earth Object Studies at JPL and the study’s lead author, said the measured nongravitational perturbations were incompatible with an asteroid and raised suspicion that the object was an active comet. The suspected mechanism is simple: sunlight heats ice mixed into the rocky body, turning it to gas that jets into space and gives the object a slight thrust. In a typical comet, that outgassing builds a bright tail and a glowing coma. When the gas and dust escape in small enough quantities, those features can stay invisible to most telescopes.
A weak tail from three mountaintops
The 2025 close approach gave astronomers a rare chance to look for the missing evidence directly. The team enlisted the Canada-France-Hawaii Telescope on Mauna Kea, the European Southern Observatory’s Danish telescope at La Silla, Chile, and ESO’s 8.2-meter Very Large Telescope at Cerro Paranal to observe the object. ESO astronomer and study coauthor Olivier Hainaut said imagery from the observatories revealed a weak but clear tail, confirming the object’s cometary nature. As an outcome of the investigation, the object received an additional comet provisional designation, P/1998 SH2. The full analysis was published in the journal Nature Astronomy.
What the switch means for planetary defense
The reclassification feeds a broader effort to understand “dark comets,” objects that swerve like comets but show none of the usual coma, tail, or visible outgassing. Since the first dark comet was identified in 2016, roughly a dozen more have been cataloged, splitting into two groups: larger ones on orbits like 1998 SH2’s and smaller ones circling nearer the Sun. The study’s authors argue that many of the larger dark comets may simply be ordinary comets waiting for a powerful enough telescope to catch their faint activity. Sorting comets from asteroids is not academic bookkeeping. Because outgassing perturbs a comet’s path more than gravity alone, tracking those nudges helps forecast how an object’s orbit will evolve, and how that bears on any Earth-impact risk.
A new telescope to catch the hidden ones
Finding more of these disguised objects is a job for hardware still on the way. NASA’s NEO Surveyor, the first space telescope built specifically for planetary defense, is designed to hunt down some of the hardest targets: dark asteroids and dim comets that reflect little visible light. Paired with precision tracking of the objects already on the books, it could reveal that other cataloged asteroids are, like 1998 SH2, comets in disguise. The Center for Near-Earth Object Studies, the Goldstone radar group, and NEO Surveyor are all managed by JPL under NASA’s Planetary Defense Coordination Office, part of an infrastructure devoted to spotting and characterizing whatever crosses Earth’s neighborhood before it becomes a surprise.
The discovery also shows why a long observational gap matters. An orbit derived from older positions can appear secure while a tiny repeated push accumulates into a measurable displacement. Recovery images after each return let researchers separate a one-time measurement error from a persistent nongravitational force and update future predictions accordingly. The faint tail supplied the physical explanation that the orbit alone could only suggest. Combining precise positions with deeper imaging is therefore central to identifying weakly active comets: motion reveals that an extra force exists, while sensitive telescopes show the gas or dust responsible for it.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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