A near-Earth object previously classified as an asteroid has been confirmed as a comet after it failed to appear at its predicted location during a close approach to Earth in August 2025. According to research published in the journal Nature Astronomy, the object, known as 1998 SH2, had been monitored by astronomers since its discovery in 1998. The findings, led by scientists at NASA’s Jet Propulsion Laboratory (JPL), indicate that the object’s trajectory was altered by faint, comet-like outgassing.
Unexpected Orbital Deviation
The puzzle began on August 28, 2025, when 1998 SH2 passed within 2 million miles (3 million kilometers) of Earth. Researchers using NASA’s Deep Space Network (DSN) planetary radar system attempted to observe the object based on orbital calculations that accounted for the gravitational influence of the Sun and planets. However, the object was absent from its expected coordinates.
According to NASA, the object had been well-tracked between 1998 and 2016, completing two solar orbits. Its failure to appear in 2025 prompted a re-evaluation of its motion. Davide Farnocchia, a navigation engineer at NASA’s Center for Near-Earth Object Studies at JPL and lead author of the study, noted that the team measured nongravitational perturbations
that were inconsistent with the behavior of an asteroid.
The team determined the object was experiencing a small transverse acceleration—measured at approximately 1.4 hundred-billionths of a meter per second squared—which they concluded was the result of gas venting into space. While asteroids can be nudged by the Yarkovsky effect, where absorbed sunlight is re-radiated as heat, researchers stated that effect would be ten times smaller than the observed acceleration for an object of this size.
Confirmation of Cometary Activity
Because 1998 SH2 produces gas and dust in very small quantities, it lacked the obvious tail or coma typically used to identify comets. To confirm the hypothesis, the research team coordinated observations with several major observatories:
- The 3.6-meter Canada-France-Hawaii Telescope on Mauna Kea, Hawaii.
- The 1.5-meter European Southern Observatory’s Danish Telescope in La Silla, Chile.
- The 8.2-meter European Southern Observatory’s Very Large Telescope on Cerro Paranal, Chile.
On September 17, 2025, the Canada-France-Hawaii Telescope recorded a low-surface-brightness tail stretching approximately 20 arcseconds, providing visual evidence of cometary activity. Following the identification of the corrected orbit, the Goldstone radar facility successfully re-acquired the object on September 2, 2025. As a result of the investigation, the Minor Planet Center has assigned the object a second designation: P/1998 SH2. According to Spacedaily, this marks the first time a comet has been identified through motion perturbations and subsequently confirmed via targeted imaging.
Implications for Planetary Defense
The study suggests that 1998 SH2 belongs to a class of objects known as “dark comets,” which exhibit trajectory irregularities but lack visible outgassing evidence. At an estimated 380 meters across, 1998 SH2 is the smallest comet ever observed by radar.
Gizmodo reports that these findings have significant implications for planetary defense. Because cometary outgassing causes more substantial orbital perturbations than those experienced by asteroids, accurately identifying these objects is critical for long-term impact forecasting. Researchers noted that other objects currently classified as asteroids might also be “hidden” comets.
While the discovery highlights the complexity of tracking such bodies, Farnocchia emphasized that there is currently no indication that 1998 SH2 poses an impact threat to Earth. The team noted that continued tracking of near-Earth objects remains essential to understanding how these bodies evolve and how their unique motion patterns influence future deflection mission strategies.
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