NASA unmasks 1998 SH2 as a comet after 30 years, sharpening planetary defense modeling

For nearly 30 years, astronomers tracked a near-Earth object called 1998 SH2 as an asteroid. Then a close pass about 2 million miles (3 million kilometers) from Earth in August 2025 exposed a problem: the object was not where orbital predictions said it should be.

NASA says the discrepancy showed up when researchers used the agency's Deep Space Network planetary radar system. Instead of matching gravity-only motion, 1998 SH2 behaved like something leaking momentum, prompting a deeper look that ultimately revealed comet activity. Optical astrometry over decades showed tiny nongravitational forces consistent with jets of gas escaping from hidden ice warmed by sunlight. Observations with the European Southern Observatory's Very Large Telescope in Chile and the Canada-France-Hawaii Telescope atop Mauna Kea then spotted a faint but unmistakable comet tail, confirming the true identity and earning the object its second designation: P/1998 SH2.

That’s the cosmic mistaken identity part. Here’s why it matters to decision-makers, not just skywatchers. In planetary defense, the core job is forecasting where an object will be in the future and assessing any potential impact risk. That forecast is only as good as the physics you build into the models. For rocky asteroids, gravity typically provides the dominant influence. For comets, subtle outgassing acts like tiny thrusters, changing the orbit in ways that gravity-only predictions cannot capture. When a comet’s path is modeled incorrectly, the error doesn’t stay small. Over time and across orbital uncertainties, it can meaningfully alter risk estimates and response planning.

NASA’s statement highlights exactly what changed in this case: researchers measured nongravitational perturbations affecting the motion of 1998 SH2 and found they were not compatible with it being an asteroid. Davide Farnocchia, lead author of the study and navigation engineer with NASA's Center for Near-Earth Object Studies at the Jet Propulsion Laboratory, described the pivot from “asteroid-looking” to “active comet suspected” after the nongravitational signatures appeared. The subsequent telescope observations delivered the confirmation, not just a stronger guess.

From a governance and operational perspective, this is a reminder that near-Earth object work is a continuous tracking system, not a one-time cataloging exercise. Farnocchia also emphasized the importance of continuously tracking near-Earth objects. The reason is simple: outgassing can perturb comet motion more significantly than asteroid motion. In other words, the “rules of the road” differ, and your navigation system has to notice when the vehicle is not behaving like you expected.

This discovery also suggests an important second-order effect for how institutions structure observation and analysis workflows. The study notes that as astronomers apply increasingly precise astrometry to near-Earth objects, they could uncover more hidden comets that have been classified as asteroids because they lack obvious tails or glowing comas. That implies a pipeline problem that boards and program leaders should care about: detection alone is not enough. If the early classification is wrong, downstream orbit determination and impact monitoring can inherit the wrong assumptions. The more precise your measurement loop becomes, the more you can treat classification as something that gets refined over time, not something decided once.

There is also a subtle systems point about technology and corroboration. In this case, the chain went from radar anomaly to optical astrometry to telescopes capable of picking out faint comet features. NASA’s Deep Space Network planetary radar system flagged that the object was not where it should be. Optical astrometry over decades quantified that gravity could not explain the irregular motion, revealing nongravitational forces consistent with jets of gas. Then the European Southern Observatory’s Very Large Telescope and the Canada-France-Hawaii Telescope confirmed the comet tail. That layered approach is what prevents a single-instrument mistake from becoming a long-term modeling error.

Finally, the timeline matters. The study’s findings were published July 10 in the journal Nature Astronomy, and the reveal followed a close pass in August 2025 at roughly 2 million miles (3 million kilometers) from Earth. That sequencing underscores that planetary defense readiness is built on observations that might look routine until a specific geometry or measurement turns the key. When it does, the difference between “gravity-only” and “gravity plus outgassing” is the difference between a clean prediction and a potentially misleading one.

For peers leading space, science, or risk-mitigation programs, the takeaway is blunt: the universe keeps updating the dataset, and comets update it differently than asteroids. Identifying nongravitational perturbations can be an important diagnostic tool for planetary defense, helping teams understand which objects may be comets rather than asteroids, how their orbits evolve, and how that influences Earth impact risks. The more accurately you can separate “rock” from “rock with hidden ice,” the better your models will be, the more defensible your risk assessments become, and the faster you can respond when the sky decides to play cosmic clerical errors.