Typically, astronomers identify comets by their distinct atmospheres, or comae, and their tails, which are created as the sun vaporizes a comet’s ice. These features emit a glow when sunlight bounces off dust and water droplets. As gas escapes the body of the comet, it also acts as a tiny thruster, subtly changing the comet’s path.
Yet, a category of objects found in our solar system looks more like asteroids because they lack a visible coma or tail but still show unexplained changes in motion similar to those of a comet. These objects are referred to as “dark comets,” but their true identity remains somewhat mysterious. Now, astronomers have found evidence that one of these objects, previously identified as an asteroid, is actually a comet.
Enigmatic observations
In a new study, published in Nature Astronomy, a group of astronomers focused on an enigmatic asteroid referred to as “1998 SH2.” The object had been tracked since 1998, and astronomers thought they had pinned down its orbital trajectory with 27 years of position measurements. However, in August 2025, during a close approach to Earth, the asteroid wasn’t where models predicted it would be.
By the end of August, the Southern Observatory for Near Earth Asteroids Research Wykrota-Centro de Estudos Astronômicos de Minas Gerais observatory in Serra da Piedade, Brazil, found 1998 SH2 again. The astronomers used a gravity-only prediction—one that would have been accurate for a normal asteroid—but found that the object was 19 standard deviations from its predicted position, explaining the earlier failed radar detection. The study authors realized that the object was experiencing additional acceleration, which they believed was caused by an unaccounted-for mechanism, such as cometary outgassing.
A tail appears
The team then used deep, stacked images from powerful telescopes in Chile and Hawaii to search specifically for very faint dust around 1998 SH2. These images revealed a faint coma and a narrow tail more than 20 arcseconds long. This is the first direct confirmation of cometary activity that had been predicted from an object’s anomalous motion.
The study authors write, “Collectively, the images demonstrate that dust release took place continuously over the period from late August to late September 2025, and do not imply that the activity was confined to that range. Rather, this suggests a sublimation-caused cometary activity process and firmly precludes instantaneous processes (such as a small impact) or repetitive processes (such as rotation-induced shedding), which would produce distinct morphological signatures.”
Questions around past and future Earth impacts
Although the team notes that there is no impact risk from 1998 SH2 in the foreseeable future, they also say that many more of these comets may exist around our solar system—and their tendency for sudden changes in direction is a little ominous. However, the results of this study can help researchers find more of these dark comets, keep an eye on them and carry out better impact-risk calculations.
The study authors write, “Our discovery of 1998 SH2 displaying cometary activity has broader implications for planetary defense. The possibility that a number of potentially hazardous objects currently classified as asteroids could turn out to be comets could increase the relative Earth impact risk from comets with respect to asteroids.
“The different physical properties of comets, for example, composition, directly affect the design of any deflection mission, such as the recent Double Asteroid Redirection Test. Moreover, models of the actual impact effects if an object were to hit Earth can lead to different assessments based on the nature of the object.”
But the implications of a hidden group of comets also reach into Earth’s deep past. The researchers say that if there are many of these hidden comets, there could be important ramifications for how water was delivered to the planet by an analogous population that may have been present when Earth was still forming. Regarding more recent history, the study’s findings may also inform debates about whether hidden outgassing explains the puzzling acceleration of the first confirmed interstellar object to pass through our solar system, which also exhibited dark comet-like characteristics.
Written for you by our author Krystal Kasal, edited by Gaby Clark, and fact-checked and reviewed by Andrew Zinin—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
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Publication details
D. Farnocchia et al, Non-gravitational acceleration indicative of cometary activity of near-Earth object, Nature Astronomy (2026). DOI: 10.1038/s41550-026-02913-7
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Asteroid with unexplained orbital shift turns out to be a ‘dark comet’ (2026, July 15)
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