For nearly three decades, NASA’s Center for Near-Earth Object Studies classified the object known as (875163) 1998 SH2 as an asteroid. A new study published in Nature Astronomy, led by Davide Farnocchia of CNEOS at the Jet Propulsion Laboratory, found that the body is actually venting gas, behavior that belongs to a comet, not a rock. NASA has confirmed the object will receive the comet designation P/1998 SH2, a change that carries direct consequences for how the agency models impact risk and plans deflection strategies.
Why a 28-year misclassification changes planetary defense math
The distinction between asteroid and comet is not academic. Planetary defense models calculate where an object will be years or decades from now, and those predictions depend on what forces act on it. Asteroids experience the Yarkovsky effect, a gentle push caused by the way they absorb and re-emit sunlight. Comets, by contrast, vent volatile gases as they warm near the Sun, producing thrust that can shift an orbit in ways the Yarkovsky model cannot account for. When orbit projections feed the wrong physics into the calculation, predicted positions drift from reality.
That is exactly what happened with 1998 SH2. The study team detected a statistically significant acceleration acting on the object. The measured force was far too large to be explained by the Yarkovsky effect at the size and reflectivity inferred from infrared data collected by the NEOWISE telescope. The only physical explanation consistent with the data is cometary outgassing, even though no visible coma or tail has been observed.
The finding raises a pointed question for the broader near-Earth object catalog. If one body tracked for nearly three decades slipped through as an asteroid, how many others in the population carry the same hidden identity? The hypothesis is straightforward: cross-referencing objects in JPL’s Small-Body Database that show unexplained non-gravitational accelerations with NEOWISE size and albedo measurements, especially those observed for 20 years or more, could surface additional misclassified comets. The current near-Earth population contains thousands of cataloged objects, and even a handful of reclassifications would alter the risk calculus for close approaches.
Goldstone radar data and the Nature Astronomy findings
The trail of evidence starts with radar. Goldstone observation logs include scheduled entries for 1998 SH2 on 2025-08-26 and 2025-09-02. When the object did not appear at its predicted position during tracking attempts, the discrepancy triggered a deeper review of its orbit. Farnocchia and colleagues at CNEOS/JPL then re-examined the full arc of observations stretching back to 1998 and isolated the transverse acceleration signal.
The key analytical step was ruling out the Yarkovsky effect. That thermal force depends on an object’s diameter and surface reflectivity. NEOWISE provided infrared measurements that allowed researchers to estimate both properties. At the size implied by those measurements, the Yarkovsky effect produces far less acceleration than what was actually observed. The gap between predicted and measured forces pointed squarely to outgassing, the signature behavior of a comet nucleus.
Farnocchia, the study lead from CNEOS at JPL, explained that the perturbations measured on 1998 SH2 are incompatible with an asteroid-only model. The Nature Astronomy paper lays out the statistical case in detail, showing the transverse component of the acceleration is the telltale direction expected from asymmetric gas jets on a rotating nucleus. NASA’s official announcement confirmed the reclassification and stated the object will carry the additional designation P/1998 SH2, following the naming convention for periodic comets.
Open questions about hidden comets in the asteroid catalog
Several gaps in the evidence remain. The raw delay-Doppler data from the 2025 Goldstone sessions have not been publicly released beyond scheduling metadata. No single-epoch flux measurements or specific diameter and albedo values from NEOWISE for 1998 SH2 have been published outside the Nature Astronomy paper. The Small-Body Database has been updated to reflect the new classification, but the change logs lack documented timestamps or author attributions showing exactly when and by whom the switch was made.
No direct statements from radar operators or database curators have described the precise moment the reclassification was triggered. The study establishes that outgassing is the best-fit explanation, but no telescope has yet imaged a visible coma or dust tail around the object. That absence is not unusual for low-activity comets, sometimes called “dark comets,” but it does mean the physical confirmation remains indirect, resting on orbital dynamics rather than direct imaging.
The broader implication is that some fraction of apparently inert near-Earth asteroids may in fact be weakly active comets whose jets are too faint to see but strong enough to alter their paths. Planetary defense analysts now face a dual challenge: identifying which cataloged objects hide this behavior, and updating long-term impact probability calculations to reflect non-gravitational forces that do not fit the standard asteroid models. For a subset of objects, the comfort of highly precise orbit predictions may be replaced by probability ranges that widen more quickly with time.
Revisiting impact risk and deflection strategies
Reclassifying 1998 SH2 as a comet also reshapes how agencies think about mitigation. Deflection concepts such as kinetic impactors or gravity tractors assume a predictable trajectory that can be nudged in a controlled way. If an object’s orbit is being subtly reshaped by outgassing, planners must account for thrust that may change over a mission timeline. A spacecraft that arrives years after launch could face a target that has drifted more than anticipated, complicating navigation and reducing the margin for error.
Cometary material properties further complicate the picture. Compared with rocky asteroids, comet nuclei are often more porous and structurally fragile. That makes their response to an impactor harder to predict: the same collision that nudges a solid asteroid might fragment a weak comet, trading a single threat for a swarm of smaller pieces. For P/1998 SH2, no dedicated mitigation mission is planned, but its new status offers a concrete test case for updating models that underpin future mission designs.
In the shorter term, the reclassification will likely prompt targeted follow-up observations. Optical and infrared telescopes can search for subtle dust or gas features near perihelion, while radar can refine the shape model and spin state during future close approaches. Each new data point tightens the constraints on the non-gravitational acceleration, helping researchers understand whether the jets are steady, seasonal, or episodic. That, in turn, feeds back into more accurate long-range ephemerides.
What comes next for P/1998 SH2 and the NEO catalog
The story of 1998 SH2 underscores how even well-studied near-Earth objects can still surprise. A body tracked for nearly 30 years, with multiple radar passes and infrared measurements, turned out to belong to a different physical class than assumed. That realization emerged only when a failed radar detection forced analysts to question long-standing orbital solutions and search for hidden forces in the data.
Going forward, researchers are likely to perform systematic searches for similar anomalies. Objects with long observational arcs, modest sizes, and unexplained transverse accelerations will be prime candidates for reanalysis. Some may ultimately prove to be asteroids with unusual thermal properties, but others could join P/1998 SH2 as stealth comets whose activity is visible only in the mathematics of their motion.
For planetary defense, the lesson is not that current risk assessments are fundamentally flawed, but that they must remain adaptable. As new observations reveal more about how small bodies behave, models will need to incorporate a broader range of physical effects and a more nuanced understanding of uncertainty. The reclassification of 1998 SH2 to P/1998 SH2 is a reminder that in a dynamic solar system, even the most familiar neighbors can still change category-and that staying safe requires watching not just where they are, but how they move.
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*This article was researched with the help of AI, with human editors creating the final content.
