1998 SH2 Identified as Comet After Orbital Shift Defies Asteroid Model

1998 SH2 Identified as Comet After Orbital Shift Defies Asteroid Model

On August 28, 2025, when 1998 SH2 passed within approximately 0.02 astronomical units—or 3 million kilometers—of Earth, NASA’s Deep Space Network (DSN) planetary radar system at the Goldstone complex attempted to observe it. The 70-meter antenna swept the target’s predicted location, but the expected radar echo did not return. It was later determined that the object was 153 arcseconds (190 kilometers) away from its calculated coordinates.

Detecting Nongravitational Perturbations

The research team, led by navigation engineer Davide Farnocchia of NASA’s Center for Near-Earth Object Studies at the Jet Propulsion Laboratory, analyzed the object’s motion across its 1998 to 2025 data arc. They measured a small transverse acceleration of approximately 1.4 hundred-billionths of a meter per second squared.

While sunlight can influence the motion of rotating bodies through the Yarkovsky effect, where absorbed heat is re-radiated unevenly, the team determined this could not account for the observed shift. For an object of this size—estimated at 380 meters across—the Yarkovsky effect would be roughly ten times smaller than the force measured. Instead, the researchers concluded the object was experiencing nongravitational perturbations caused by outgassing. As ice buried deep within the object warmed in sunlight, it turned to gas and vented into space, acting as a gentle, invisible thruster.

Confirmation Through Imaging

To verify the hypothesis that 1998 SH2 was a comet, the team coordinated observations with high-powered optical and infrared telescopes. Between early September and early October 2025, the Asteroid Terrestrial-impact Last Alert System collected 53 images of the object, though it initially appeared as an ordinary, featureless dot.

NASA Study Finds Near-Earth Asteroid Is Actually Comet

However, more powerful instruments provided definitive proof. On September 17, the 3.6-meter Canada-France-Hawaii Telescope on Maunakea recorded a low-surface-brightness tail extending approximately 20 arcseconds from the nucleus. Researchers at the European Southern Observatory’s (ESO) Very Large Telescope in Chile also observed the object. 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 co-author Olivier Hainaut, as reported by Spacedaily.

Implications for “Dark Comets”

The Minor Planet Center has assigned the object the new designation P/1998 SH2. This marks the first time a comet has been identified primarily through perturbations in its motion, followed by successful confirmation via targeted imaging.

The findings shed light on a class of objects known as “dark comets,” which exhibit orbital irregularities characteristic of comets but lack visible evidence of activity like a coma or tail. According to NASA, the discovery suggests that many larger objects currently classified as asteroids may actually be comets that only reveal their true nature when observed under the right conditions.

This reclassification is considered a significant development for planetary defense. Because outgassing alters an object’s trajectory, understanding which near-Earth objects are comets rather than asteroids is crucial for accurately predicting their future paths and assessing potential impact risks. The research team emphasized that as Universe Today noted, there are approximately 285 near-Earth objects currently classified as potentially hazardous asteroids that possess comet-like orbits. Astronomers suggest that continuously tracking these objects and monitoring for subtle nongravitational changes is essential for refining orbital safety models.