After 28 years of classification as an asteroid, the near-Earth object 1998 SH2 has been identified as a comet. Following an unexpected failure to appear in predicted radar tracking on August 28, 2025, researchers confirmed the object’s true nature by detecting outgassing and a faint, previously invisible tail.
A Case of Mistaken Identity
For nearly three decades, astronomers tracked the object provisionally known as 1998 SH2, operating under the assumption that it was a standard near-Earth asteroid. This classification remained unchallenged as the object completed two solar orbits between 1998 and 2016. However, its behavior during a recent close approach to Earth revealed a significant discrepancy.
On August 28, 2025, the object passed within 2 million miles (3 million kilometers) of Earth. Researchers attempted to observe 1998 SH2 using NASA’s Deep Space Network (DSN) planetary radar system. After calculating the object’s expected path by accounting for the gravitational influence of the Sun and planets, the team found the object was not where it was projected to be.
Confirming Cometary Activity
The deviation from the predicted orbit was the first clue that 1998 SH2 was not a inert rock. Astronomers hypothesized that the object was generating a small, non-gravitational thrust by venting gas into space as the Sun heated internal ice. Because this outgassing occurs in very small quantities, the resulting coma and tail—typical markers of cometary activity—had remained undetected by standard observatories for years.
To test this hypothesis, the research team coordinated a multi-observatory campaign during the 2025 close approach. They utilized the Canada-France-Hawaii Telescope in Hawaii, the European Southern Observatory’s Danish Telescope in Chile, and the 8.2-meter Very Large Telescope on Cerro Paranal.
Following this confirmation, the object has been assigned an additional provisional designation: P/1998 SH2. For the researchers involved, the discovery underscores the scientific process of hypothesis and verification. That’s how science works — you form a hypothesis, and you set out to test it,
Hainaut noted regarding the successful confirmation.
Implications for Planetary Defense and Dark Comets
The reclassification of 1998 SH2 provides new data on the broader category of dark comets
—objects that share the orbital characteristics of comets but lack visible tails or comas. Since the first dark comet was identified in 2016, roughly a dozen more have been cataloged. Scientists now believe that many of these objects may be active comets that simply require more powerful, sensitive imaging to reveal their faint, outgassing signatures.
Understanding these objects is a priority for planetary defense, as the non-gravitational forces exerted by outgassing can significantly alter an object’s trajectory over time. By accurately identifying which near-Earth objects are actually comets, researchers can better predict how these orbits evolve and assess potential impact risks.
“Because of outgassing, the motion of comets is more significantly perturbed than that of asteroids. Detecting these perturbations can be an important diagnostic tool for planetary defense that will help understand which objects may be comets rather than asteroids, how their orbits evolve, and how that influences their Earth impact risks.”
Davide Farnocchia, navigation engineer with NASA’s Center for Near-Earth Object Studies at JPL
Moving forward, the study highlights the necessity of continuous tracking for near-Earth objects. As Farnocchia stated, This work shows the importance of continuously tracking near-Earth objects,
suggesting that future identification of similar “dual personality” objects will rely on the combination of precise astrometry and high-sensitivity optical imaging.
Sources: Gizmodo.
