NASA researchers have confirmed that 1998 SH2, an object tracked for nearly three decades as an asteroid, is actually a dark comet
. Following a close Earth approach in August 2025, scientists observed unexpected orbital deviations and a faint tail, proving the object vents gas as it orbits the Sun.
From Asteroid to Comet: The 2025 Orbital Surprise
For 27 years, 1998 SH2 was classified as a standard near-Earth asteroid. Astronomers had meticulously tracked its path from its initial discovery in 1998 through 2016, building a predictive model that left little room for uncertainty. That expectation vanished in August 2025, when the object passed within 2 million miles (3 million kilometers) of Earth.
When researchers attempted to observe the object using the NASA Deep Space Network (DSN) planetary radar system, they encountered a significant problem: the object was not where their calculations predicted it would be. This shift was far too large to be a measurement error, indicating that an unknown force was nudging the object off course.
Identifying the Invisible Thrusters
The solution to the mystery lay in nongravitational perturbations
—small, consistent changes in velocity that gravity alone cannot explain. Scientists hypothesized that the object was acting like a comet, venting gas into space as solar heat caused surface ice to sublimate. This venting creates a subtle thrust, effectively turning the object into its own propulsion system.
“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.”
Davide Farnocchia, navigation engineer with NASA’s Center for Near-Earth Object Studies at JPL, via NASA
While standard comets display prominent, glowing tails, 1998 SH2 belongs to a rare category known as dark comets
. These bodies produce such minimal amounts of gas and dust that they remain largely invisible to most telescopes. To confirm the hypothesis, the team turned to high-powered observatories, including the European Southern Observatory’s Very Large Telescope in Chile and the Canada-France-Hawaii Telescope. These instruments finally captured evidence of a weak, clear tail, leading to the object’s new provisional designation: P/1998 SH2.
Planetary Defense and the Hidden Population
The reclassification of 1998 SH2 carries significant weight for planetary defense. Identifying whether a near-Earth object is a rock or a comet is vital for long-term orbit forecasting. Because cometary outgassing can cause unpredictable trajectory shifts, misidentifying these objects could lead to inaccurate collision-risk assessments.
As KTLA reported, this discovery highlights the necessity of continuous tracking. 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,
Farnocchia noted. NASA intends to address this identification gap with its upcoming Near-Earth Object Surveyor mission, which is designed to better detect these difficult-to-spot dark objects.
