The Asteroid That Wasn't: Inside NASA's 1998 SH2 'Dark Comet' Discovery
For 27 years, 1998 SH2 sat in NASA's near-Earth object catalog as an unremarkable rock, one of tens of thousands of asteroids tracked well enough to know it poses no threat to Earth, and dull enough that almost nobody outside the small community of orbit modelers had reason to think about it twice. Then, in a study published in Nature Astronomy on July 15, 2026, a team led by JPL navigation engineer Davide Farnocchia showed that the object has been quietly drifting off the path gravity alone predicts. It isn't a plain asteroid. It's a comet, one that has been venting gas the entire time without ever showing the tail that would normally give it away.
That's the kind of story that sounds like a footnote until you sit with what it actually implies: an object can be tracked precisely, cataloged, cleared of impact risk, and studied for nearly three decades, and still turn out to be something fundamentally different from what everyone assumed. Here's what NASA's study found, how they caught it, and why it's a genuinely useful data point for anyone who follows near-Earth object tracking, including the same public data feeds that power Gloomia's Asteroid Watch and Orrery wallpapers.
What 1998 SH2 actually is
1998 SH2 is an Apollo-type near-Earth object roughly 380 meters across, first spotted by the Spacewatch survey at Kitt Peak National Observatory in Arizona on September 17, 1998. It orbits the Sun about once every 4.5 years, swinging from 0.79 astronomical units at its closest to 4.7 astronomical units at its farthest, a path that brings it back into Earth's neighborhood repeatedly and made it a natural target for tracking from the start. For almost all of that time, it looked and behaved exactly like the asteroid label suggested: a solid, inert chunk of rock following a predictable, gravity-only path.
The cracks in that picture showed up gradually, in the numbers rather than in any image. Every time astronomers refined 1998 SH2's orbit with a new round of observations, the object was slightly, but persistently, off from where a purely gravitational model said it should be. On its own, a single small discrepancy like that is easy to write off as measurement noise. Piled up across observations spanning most of three decades, it stopped looking like noise and started looking like a pattern.
The close pass that gave it away
The turning point came on August 28, 2025, when 1998 SH2 made its closest approach to Earth in years, passing at a safe distance of roughly 2 million miles, about eight times farther out than the Moon. Researchers pointed NASA's Deep Space Network planetary radar at it, expecting to confirm a position calculated from the object's known orbit and the gravitational pull of the Sun and planets. Instead, the object wasn't quite where the math said it would be.
That mismatch was the signature astronomers had been circling for years: a small, steady, non-gravitational force acting on the object, the kind produced when frozen volatiles trapped just under a rocky, dust-covered surface sublimate in sunlight and vent outward, giving the object a tiny, rocket-like push. The close approach also gave telescopes an unusually good look, and in the deeper images taken during that window, a faint tail turned up, direct visual proof that 1998 SH2 was actively shedding material rather than just drifting for some other, unrelated reason.
Put together, the orbital drift and the faint tail left little room for another explanation. 1998 SH2 is what astronomers now call a dark comet: an object that produces genuine cometary outgassing and thrust, but without the bright, obvious coma that normally makes a comet recognizable at a glance. It had been behaving like a comet the entire time it was cataloged as an asteroid. Nobody noticed because nobody had a reason to look that closely at an object that presented no threat and no visual anomaly.
Why "dark comet" is a real, useful category
Dark comets aren't a brand-new idea invented for this discovery. Astronomers have been identifying them as a distinct class since around 2023, and by 2024 roughly 14 had been confirmed, almost all of them small, fast-rotating objects only tens of meters across. What makes 1998 SH2 stand out is scale: at around 380 meters wide, it's several times larger than the typical dark comet found so far, and it's the rare case where an object sat in a public catalog, fully tracked and considered understood, for 27 years before its true nature surfaced.
The mechanism itself is straightforward once you know to look for it. A body with even a thin layer of ice under a dusty, rocky crust will vent gas when sunlight reaches that ice, and that venting pushes back on the object exactly the way exhaust pushes a rocket, just far more gently. There's no visible plume most of the time because the volatile loss is small and the surface dust hides what little gas does escape. The only way to catch it is to measure the object's position precisely enough, over a long enough baseline, that a gravity-only model stops fitting the data.
What it means for planetary defense
None of this makes 1998 SH2 itself dangerous. Current tracking shows no indication that it poses an impact risk to Earth in any foreseeable pass, and that hasn't changed. The reason the discovery matters is what it implies about the broader catalog of near-Earth objects, not this one entry in it.
Long-range impact-risk forecasting works by projecting an orbit forward, sometimes decades into the future, using the gravitational pull of the Sun, the planets, and occasionally larger moons. A small, steady non-gravitational push is easy to miss across a short observation window, since it barely nudges a position measurement here and there. But projected forward over years, that same small push compounds into a genuinely different predicted path. If an object thought to be a plain asteroid is quietly outgassing the way 1998 SH2 turned out to be, a forecast built on gravity alone will drift further from reality the further out it's projected.
That's the practical takeaway researchers are drawing from this case: if dark comets are more common among the objects currently filed as ordinary asteroids than anyone assumed, a subset of existing long-range trajectory forecasts may be due for a second look, and continuous tracking, rather than a one-time orbit calculation treated as settled, becomes that much more important for any near-Earth object close enough to matter. It's also a reminder that even a well-studied object isn't necessarily a fully understood one. This is the same broader tracking effort behind the much more time-pressured story of 2026 PC6's closest flyby earlier this month, where an object was found with less than a day's warning. 1998 SH2 is the opposite case: an object tracked calmly for nearly three decades that still had a surprise left in it.
A quiet reminder that the sky keeps surprising specialists
It's worth sitting with how ordinary 1998 SH2's story is, right up until it isn't. Nothing about its discovery in 1998, its orbit, or its size marked it as unusual. It got the same routine tracking as thousands of other cataloged near-Earth objects, cleared of any impact risk long ago, filed away as understood. The only reason its true nature surfaced now is that one close approach happened to line up with radar precise enough, and a research team patient enough, to notice a discrepancy that had been accumulating the entire time.
That's a genuinely different kind of space story than the ones built around a single dramatic event, a flare-up, a fast flyby, a rare alignment. It's slower and less visual, but arguably more interesting: proof that the tracking infrastructure built to keep an eye on near-Earth objects is good enough to catch something this subtle, even in an object nobody was actively suspicious of. Readers who followed the recent 220P/McNaught outburst already know comets can behave unpredictably in dramatic, visible ways. 1998 SH2 shows the quieter version of that same unpredictability, one that only shows up in the numbers.
For anyone who wants a broader sense of how this kind of live tracking data actually looks outside a research paper, Gloomia's real-data astronomy wallpapers guide walks through exactly what's genuinely live in the Asteroid Watch and Orrery wallpapers, and what isn't, and the best space wallpapers guide covers the rest of Gloomia's space collection for anyone who wants a piece of the night sky on their desktop whether or not there's a dark comet news story to go with it.
The bottom line
1998 SH2 isn't a threat, and it isn't a spectacle you can go outside and look for the way a bright comet or a meteor shower is. It's something quieter and, in its own way, more unsettling: proof that a routinely tracked, thoroughly cataloged near-Earth object can still be fundamentally misclassified for the better part of three decades. The fix wasn't a new telescope or a lucky discovery, it was patient, precise tracking over a long enough baseline to notice that the numbers didn't quite add up. That's exactly the kind of ongoing, unglamorous monitoring that keeps the whole near-Earth object catalog honest, and it's a good reminder that the data feeding wallpapers like Asteroid Watch and Orrery isn't just decoration. It's the same real tracking effort that just found a comet hiding inside an asteroid's file.
Frequently asked questions
What is 1998 SH2, and why is it suddenly in the news?
1998 SH2 is a near-Earth object roughly 380 meters across, discovered by the Spacewatch survey at Kitt Peak National Observatory in September 1998 and catalogued ever since as an ordinary asteroid. A study led by JPL navigation engineer Davide Farnocchia, published in Nature Astronomy on July 15, 2026, showed that its orbit has been quietly drifting off the path gravity alone predicts. That drift is the signature of gas escaping from its surface, which makes 1998 SH2 a comet, not an asteroid, despite showing no visible tail for almost all of the 27 years it was tracked.
What exactly is a dark comet?
A dark comet is an object that behaves like a comet, meaning it vents gas that gives it a tiny rocket-like push away from a purely gravitational path, without producing the bright coma or tail that normally identifies a comet to the eye or a camera. They look like plain rocky asteroids in every image, and the only way to catch one is to measure its position precisely enough, over enough years, to notice that it isn't quite where gravity says it should be.
How did astronomers actually catch 1998 SH2 in the act?
On August 28, 2025, 1998 SH2 passed within about 2 million miles of Earth, its closest approach in years. Researchers aiming NASA's Deep Space Network radar at it found the object wasn't quite where its gravity-only orbit predicted, a mismatch built up from small deviations across nearly three decades of tracking. That close pass also let telescopes gather deeper images, which turned up a faint tail, direct visual confirmation that the object was venting material, not just drifting for some other reason.
Does this mean 1998 SH2 is at risk of hitting Earth?
No. Current tracking shows no indication that 1998 SH2 poses an impact threat to Earth in the foreseeable future. The significance isn't this specific object, it's what its reclassification reveals about a whole category of near-Earth objects that may be quietly accelerating themselves in ways standard gravity-only orbit models don't account for.
Why does one reclassified asteroid matter for planetary defense?
Impact-risk forecasts for near-Earth objects are built on projecting an orbit forward in time, sometimes decades. A small, steady non-gravitational push, the kind dark comets produce, is easy to miss in a short observation window but compounds significantly over years, shifting where a model predicts an object will be. If dark comets turn out to be common among objects currently filed as ordinary asteroids, some existing long-range trajectory forecasts may need a second look, and it strengthens the case for tracking near-Earth objects continuously rather than assuming a one-time orbit calculation stays accurate forever.
How many dark comets are known, and is 1998 SH2 different from the others?
Astronomers had identified around 14 dark comets by 2024, and nearly all of them were small, fast-spinning objects a few tens of meters across. 1998 SH2 stands out because it's roughly 380 meters wide, several times larger than the typical dark comet found so far, and because it was hiding in plain sight in a near-Earth object catalog for 27 years before anyone noticed.
Does Gloomia have a wallpaper that shows real near-Earth object data like this?
Yes. Gloomia's Asteroid Watch wallpaper pulls live flyby data straight from NASA/JPL's CNEOS near-Earth object database and renders it as an Earth-centered scene on your desktop, and the Orrery wallpaper plots real current planetary positions and can layer in the same live asteroid data. Neither wallpaper singles out 1998 SH2 specifically, since it isn't a close-approach flyby event, but both draw from the same public tracking data that flagged its odd orbit in the first place.