NASA reclassifies near-Earth asteroid as comet after trajectory anomalies

Researchers from NASA’s Jet Propulsion Laboratory in Southern California have reclassified the near-Earth object 1998 SH2 as a comet, assigning it the provisional designation P/1998 SH2. The decision follows a close approach to Earth on 28 August 2025, when the object passed within 2 million miles (3 million kilometres) of the planet. Scientists identified non-gravitational perturbations in the object’s trajectory that were inconsistent with an asteroid, suggesting outgassing from sublimating ice. Optical observations from the Canada-France-Hawaii Telescope, the European Southern Observatory’s Danish Telescope, and the Very Large Telescope confirmed a weak but clear tail, verifying cometary activity. The findings were published in the journal Nature Astronomy.

The puzzle began when NASA’s Deep Space Network planetary radar system failed to locate the object at its predicted position during the August 2025 close approach. Researchers had calculated the position using data from previous orbits and factored in the gravitational effects of the Sun and planets. When the object did not appear where expected, navigation engineer Davide Farnocchia of NASA’s Center for Near-Earth Object Studies led an investigation that identified the cause. “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,” Farnocchia said.

Although the object’s orbit had been well-tracked from 1998 to 2016, it had completed two solar orbits without additional observations by telescopes until the 2025 attempts. Analyzing all observations collected since its discovery, researchers determined the perturbations were likely caused by the object generating a small thrust by venting gas into space. This venting results from the Sun heating ice mixed with rocky material, turning the ice into a gas. While regular comets form a bright tail and coma, objects producing gas in smaller quantities may not show visible activity to most observatories.

The August 2025 close approach provided the opportunity to gather observational evidence of visible cometary activity. Astronomers at the Canada-France-Hawaii Telescope in Hawaii, the European Southern Observatory’s Danish Telescope in Chile, and the Very Large Telescope on Cerro Paranal tracked 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 Olivier Hainaut, an astronomer with the European Southern Observatory and co-author of the study.

The research also sheds light on the existence of “dark comets,” a class of objects that show trajectory perturbations but lack visible tails or comae. Since the first dark comet was discovered in 2016, approximately a dozen more have been identified. The paper’s authors suggest that many larger dark comets could turn out to be regular comets if observed with powerful telescopes capable of imaging incredibly faint objects. The study underscores the role of NASA’s upcoming NEO Surveyor mission in detecting hard-to-find near-Earth objects, including dark asteroids and comets. JPL is managed by Caltech for NASA, and the Planetary Defense Coordination Office supports these efforts. The Deep Space Network is overseen by NASA’s Space Communications and Navigation program office.