Decades-Long Observations Reveal Asteroid Is Actually a Dormant Comet

An object near Earth previously labeled as an asteroid has now been confirmed as a faint active comet after nearly 30 years of observation. This revelation regarding P/1998 SH2 suggests that some near-Earth bodies may hide their cometary nature until detailed orbital data exposes subtle anomalies.

Initially identified in 1998, the object was long treated as a typical asteroid. It followed a consistent solar orbit, completing one revolution approximately every 4.5 years. Unlike classic comets, no visible signs such as a coma or tail—usually caused by sunlight vaporizing ice—were detected.

However, during a close Earth flyby in August 2025, NASA scientists utilizing the Deep Space Network planetary radar noticed the object’s position did not perfectly match earlier trajectory predictions. This discrepancy prompted a deeper examination of its path.

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Subtle Orbital Variations Indicate Comet-like Behavior

Decades of high-precision tracking against background stars allowed astronomers to analyze the forces acting on 1998 SH2. They reassessed the object's movement with optical astrometry to check if gravity alone explained its motion.

Results revealed that gravitational interactions were insufficient; minor nongravitational forces were influencing the trajectory, implying material was being released from the object's surface.

According to NASA, these forces align with jets of gas escaping as subsurface ice heats under sunlight. Such emissions act like gentle thrusters, slowly altering the comet’s orbit through space.

“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,” said Davide Farnocchia, lead author of the study and navigation engineer with NASA’s Center for Near-Earth Object Studies at the Jet Propulsion Laboratory.

This discovery was notable because the comet’s activity did not manifest as a prominent tail or coma. Instead, scientists deduced its cometary nature through subtle deviations between predicted and observed positions.

Follow-Up Observations Confirm Cometary Activity

Following these orbital anomalies, astronomers employed powerful telescopes to look for direct evidence of cometary emissions. Observations from the Very Large Telescope in Chile and the Canada-France-Hawaii Telescope revealed a faint yet definitive tail, confirming it was shedding material.

The object was reclassified as P/1998 SH2, officially recognizing it as a comet. Findings published in a July 10 study in Nature Astronomy demonstrate how combining long-term tracking with new observations can unveil hidden characteristics.

This case also emphasizes the difficulty of distinguishing near-Earth objects, as some lack the obvious glowing features that typically reveal comet composition and activity.

Insights Aid Future Planetary Defense Strategies

Recognizing P/1998 SH2 as a comet enriches understanding of near-Earth objects, highlighting differences in their behavior. Comets can experience small but significant forces from outgassing, complicating predictions of their orbital paths.

Detecting such nongravitational influences enables scientists to differentiate between asteroids and comets more accurately, improving trajectory forecasts.

“This work shows the importance of continuously tracking near-Earth objects,” Farnocchia said. He added that: “Because of outgassing, the motion of comets is more significantly perturbed than that of asteroids.”

Researchers point out that other near-Earth objects could be misidentified due to minimal visible activity. Enhanced astrometric precision may uncover more cases where cometary objects have been overlooked.

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