Astronomers have reclassified the near-Earth object (875163) 1998 SH2 as a comet following observations that revealed outgassing, a process previously masked by its asteroid-like appearance. According to research published in Nature Astronomy by Dr. Davide Farnocchia and his team at NASA’s Jet Propulsion Laboratory, the object’s unexpected orbital deviations confirmed it is venting gas, leading to its new designation, P/1998 SH2.
Why Orbital Deviations Reveal Hidden Comets
The discovery began when the object failed to appear at its predicted location during a close approach to Earth in August 2025. Scientists expected the object to follow a trajectory dictated solely by gravity. Instead, Dr. Farnocchia’s team observed non-gravitational perturbations, which suggested an additional force was acting upon the object.
This force is the result of outgassing—sunlight warming internal ice, which then escapes as a jet-like thrust. While traditional comets display prominent, visible tails, P/1998 SH2 exhibited such weak venting that it remained undetected by standard surveys for decades. The discrepancy between the predicted and actual path served as a diagnostic tool, revealing the object’s true composition.
Did you know?
Not all near-Earth objects are solid rocks. Some “asteroids” are actually dormant comets that have lost their most volatile surface materials, leaving behind a rocky crust that hides an icy interior.
Confirming the Identity of P/1998 SH2
To verify the hypothesis, researchers utilized high-powered observatories, including the Canada-France-Hawaii Telescope, ESO’s Danish Telescope, and the Very Large Telescope. These instruments captured images showing a faint tail, which confirmed the object was active.
Dr. Olivier Hainaut of the European Southern Observatory (ESO) noted that this process highlights the scientific method in practice: forming a hypothesis based on orbital data and then testing it through targeted observation.
Implications for Planetary Defense
The reclassification of P/1998 SH2 carries significant weight for planetary defense strategies. According to Dr. Farnocchia, identifying objects that are actually comets is essential for accurately predicting their future paths. Because outgassing creates unpredictable acceleration, the motion of comets is more complex to model than that of purely rocky asteroids.
By detecting these subtle perturbations, scientists can better determine how an object’s orbit might evolve over time. This data is critical for assessing Earth impact risks, as the non-gravitational forces can shift an object’s path in ways that gravity-only models fail to capture.
Frequently Asked Questions
- Why was 1998 SH2 originally classified as an asteroid?
It lacked a visible coma or tail, and its appearance was indistinguishable from rocky asteroids until scientists measured its non-gravitational orbital shifts. - What is “outgassing”?
It is the process where solar heat causes ice trapped inside a celestial body to turn into gas, creating a jet-like thrust that pushes the object off its expected path. - Are all asteroids potentially comets?
Not all of them, but this study suggests that some objects currently designated as asteroids may actually be active or dormant comets that require closer monitoring.
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