For nearly thirty years, astronomers were sure that object 1998 SH2 was a perfectly ordinary asteroid. Its orbit was carefully calculated, it was observed at every convenient close approach to Earth, and nothing unusual was found. But in 2025, an event occurred that forced scientists to completely reconsider its nature. It turned out that all this time the object was not a 'dead rock' at all, but a very faint comet that was imperceptibly evaporating and slowly changing its own orbit. The results of the study were published in the journal Nature Astronomy (Original article Eng).

Why this is important

For most people, the difference between an asteroid and a comet comes down to the presence of a beautiful tail. In reality, the difference is much deeper. Asteroids consist mainly of rock and metal, while comets contain a large amount of ice. When a comet approaches the Sun, the ice begins to evaporate, forming jets of gas. These jets act like miniature rocket engines and can gradually change the trajectory of the object.

If such a comet looks like an ordinary asteroid, a serious problem arises. Astronomers calculate its future orbit only according to the laws of gravity, although in reality the object constantly receives additional acceleration. For most celestial bodies, this difference is negligible, but over decades the error gradually accumulates. For objects that periodically pass near Earth, this becomes a matter of planetary security.

In addition, if there are many such hidden comets, scientists will have to revise current ideas about the origin of water on Earth and the evolution of small bodies in the Solar System.

What the scientists did

It all started quite unexpectedly. In August 2025, researchers were preparing to observe 1998 SH2 using the Goldstone radar. The orbit was calculated so accurately that no one expected any surprises. However, the radar simply did not detect the object at the calculated point.

After additional observations, it turned out that the asteroid was approximately 153 arcseconds further from the expected position — an error of almost 19 standard deviations. For modern astronomy, such a thing is practically impossible to explain by chance. The researchers then hypothesized that the object was experiencing additional acceleration not related to gravity. After adding a model of weak gas emission to the calculations, the entire history of observations from 1998 to 2025 unexpectedly perfectly matched the new orbit.

But mathematics alone was not enough. It was necessary to see the process of substance evaporation itself. For this, scientists used several large telescopes at once, including the Canada-France-Hawaii Telescope and the Very Large Telescope of the European Southern Observatory. In ordinary images, the object looked like a completely ordinary asteroid. Only after long exposures and special image processing were researchers able to discern a faint coma and a thin tail more than 20 arcseconds long. After that, the object officially received the comet designation P/1998 SH2.

What the numbers showed

The study is interesting not only for the discovery itself, but also for the number of measurements that confirm it.

The most important figure here is the acceleration. It seems incredibly small, but it acts constantly. That is why, after almost three decades, the object ended up so far from the calculated position that the radar could not detect it at first. It was this small error that led to one of the most interesting discoveries in recent years.

Limitations of the study

Despite the convincing results, the work does not mean that astronomers have suddenly found hundreds of hidden comets. So far, we are talking about only one object. Moreover, its activity is so weak that the tail could only be seen with the largest telescopes in the world after special image processing. Ordinary sky surveys simply do not notice such details.

The authors also emphasize that it is not yet possible to estimate how common such objects are. Perhaps 1998 SH2 is a rare exception. Or perhaps there are many more such 'disguised' comets among known asteroids. The answer to this question can only be provided by new-generation sky surveys, primarily the Vera Rubin observatory, which will begin regularly photographing almost the entire visible sky with unprecedented sensitivity.

What this could change in a year and in five years

In a year, researchers will likely begin mass rechecking known near-Earth asteroids. Instead of looking for a visible tail, they will analyze the slightest orbital deviations, trying to find the same signs of hidden evaporation.

In five years, the consequences could be significantly larger. If it turns out that a significant portion of potentially hazardous asteroids are actually weakly active comets, models for assessing the risk of collision with Earth will have to be revised. In addition, new data could change ideas about the role comets played in delivering water to the young Earth.

Finally, this study unexpectedly returns to the mystery of the interstellar object ʻOumuamua. One of its main features was an unexplained non-gravitational acceleration without a visible tail. Now scientists have a real example that such a thing is indeed possible: a comet can remain practically invisible for decades while continuing to slowly change its trajectory.