A Cosmic Identity Crisis
For 27 years, the object known as 1998 SH2 was considered just another asteroid in the solar system. But a recent NASA discovery has stunned the astronomical community: it is actually an active comet that has been hiding in plain sight for nearly three decades.
The revelation, published in the journal Nature Astronomy, was led by Davide Farnocchia, a navigation engineer at NASA's Jet Propulsion Laboratory (JPL) Center for Near-Earth Object Studies. The team noticed that the object was experiencing non-gravitational forces—consistent with jets of gas escaping its surface—something that cannot be explained if it were a simple rocky asteroid.
The 27-Year Deception
Object 1998 SH2 was discovered in 1998 and classified as a near-Earth asteroid with an orbit of 4.5 years around the Sun. With no brightness, no coma, and no tail, it looked like an inert rock among thousands.
The deception unraveled in August 2025, when the object passed within 3 million kilometers of our planet. NASA's Deep Space Network radars looked for it where it should have been—and it wasn't there. That tiny orbital discrepancy set off alarm bells for the science team.
The Orbital Clue That Didn't Add Up
Farnocchia's team reviewed decades of precise optical astrometry—the millimeter-level measurement of its position relative to background stars—and found the cause: gravity alone couldn't explain its irregular motion. There were non-gravitational forces acting on the object, consistent with jets of gas escaping its surface as hidden ice warmed in the Sun.
"After measuring the non-gravitational perturbations affecting 1998 SH2's motion and recognizing that they were not compatible with the object being an asteroid, we suspected it might be an active comet," Farnocchia explained.
Telescopes That Revealed the Hidden Tail
To confirm the suspicion, the team pointed two of the most powerful eyes on the planet at the object: the Very Large Telescope of the European Southern Observatory in Chile and the Canada-France-Hawaii Telescope on Mauna Kea. There it was: a faint but unmistakable comet tail, barely visible but enough to change the object's story forever.
With that evidence, the former asteroid was officially reclassified as P/1998 SH2, recognizing it as a comet.
Implications for Planetary Defense
Beyond the cosmic curiosity, this discovery changes the rules of planetary defense. Unlike rocky asteroids, whose orbits are relatively predictable, comets can subtly change course because the escaping gas acts like tiny thrusters. If these variations aren't detected in time, a deflection could alter the calculation of a future impact.
"This work demonstrates the importance of continuous tracking of near-Earth objects. Because of outgassing, comets' motion is more affected than that of asteroids," warned Farnocchia.
How Many 'Asteroids' Are Actually Dormant Comets?
The scientific team believes 1998 SH2 is not an isolated case. With increasingly precise astrometry, many supposed asteroids without tails or brightness could actually be dormant comets, hiding in plain sight in near-Earth object catalogs.
Their subtle orbital deviations could be as revealing as a spectacular tail, forcing us to rewrite risk maps and planetary defense strategies in the coming decades.