NASA researchers have identified the near-Earth object 1998 SH2 as a comet, confirming its identity after decades of misclassification as an asteroid. The discovery, detailed in Nature Astronomy, marks the first time a comet has been identified through orbital perturbations—deviations in its expected path—before being visually confirmed by targeted imaging of its faint tail.
The Failure of Radar and the Shift in Orbit
The reclassification began with a failed observation. In August 2025, NASA’s Goldstone complex attempted to track 1998 SH2 using radar, but the antenna received no echo. According to data from the Jet Propulsion Laboratory, the object had drifted 153 arcseconds from its gravity-only predicted path—a significant 19-sigma offset. Navigation engineer Davide Farnocchia led the team in analyzing the object’s motion from 1998 to 2025, identifying a small transverse acceleration of approximately 1.4 hundred-billionths of a meter per second squared. This movement could not be explained by the Yarkovsky effect, where sunlight heats a rotating body, as the force was ten times greater than what the effect could plausibly generate for an object of that size.
Did you know?
1998 SH2 is the smallest comet ever observed by radar, measuring approximately 380 meters across. Its surface reflects radio waves weakly, a characteristic more common in comet nuclei than in standard asteroids.
Confirming the Comet: Imaging the Tail
Following the orbital analysis, astronomers used high-powered telescopes to search for physical evidence of outgassing. While the Asteroid Terrestrial-impact Last Alert System (ATLAS) initially recorded the object as an inert dot, larger instruments captured a different view. The Canada-France-Hawaii Telescope on Maunakea and the European Southern Observatory’s Very Large Telescope in Chile both recorded a low-surface-brightness tail. Olivier Hainaut, an astronomer with the European Southern Observatory, confirmed that these images proved 1998 SH2 is a comet, noting that the data showed a “weak but clear tail.”
The “Dark Comet” Challenge and Future Detection
The discovery of 1998 SH2 highlights a growing trend in space observation: the identification of “dark comets.” These are objects that appear to be inert rock but exhibit non-gravitational acceleration similar to comets. Since 2016, 14 such near-Earth objects have been identified. These objects are often categorized into two groups: smaller bodies in Earth-like orbits and larger objects, like 1998 SH2, with elongated orbits similar to Jupiter-family comets.
Implications for Planetary Defense
The presence of hidden comets in asteroid catalogs complicates long-term trajectory modeling. As of late 2025, there are 2,009 known near-Earth asteroids on comet-like orbits, with 285 of those classified as potentially hazardous. While the paper confirms that the probability of an Earth impact for 1998 SH2 remains zero, the classification matters for mission planning. According to the study, cometary outgassing creates more complex orbital perturbations than the solar radiation pressure affecting asteroids, which would significantly alter the requirements for a kinetic impactor mission like NASA’s DART.
Researchers suggest that the key to identifying these hidden comets is “tracking everything.” By fitting orbits precisely and monitoring for unexpected deviations, astronomers can use the “misbehavior” of light to distinguish between inert asteroids and active comets.
Frequently Asked Questions
- Why was 1998 SH2 previously classified as an asteroid?
For 27 years, it appeared as an ordinary dot in survey telescopes. It lacked a visible tail until targeted imaging was performed in 2025. - Is 1998 SH2 a threat to Earth?
No. The study explicitly states that the probability of an Earth impact for this specific object in the foreseeable future is zero. - How do scientists tell a comet from an asteroid?
Beyond visual tails, scientists look for non-gravitational acceleration caused by outgassing. If an object moves in a way that gravity and solar radiation cannot explain, it is often a sign of volatile ice venting into space.
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