The interstellar object known as 3I/ATLAS, detected in July by the Chilean Terrestrial Asteroid Impact Warning System (ATLAS) observatory, has been the subject of intense speculation. A radio signal captured through the MeerKAT radio telescope in South Africa gives us new clues about its nature and ends up disproving speculations, which experts have defined as crazy and spectacular.
The 64-dish radio telescope, 600 kilometers from Cape Town, identified the signal on October 24. This is an absorption of radio waves caused by hydroxyl (OH) molecules, which is the result of the dissociation of water when a comet’s ice sublimates as it approaches the Sun.
According to Michael Küppers, a scientist at the European Space Agency (ESA), this observation confirms that one of the main components of the ice nucleus is water, “normal for a comet”. Küppers adds that similar signals have been detected in numerous stars in the solar system, which strengthens the interpretation that it is a natural object and not something exceptional, as suggested by some speculative hypotheses, which even spoke of an extraterrestrial spaceship.
When the comet disappeared before our eyes behind the Sun, some wanted to hypothesize that it was a spaceship that was hiding. But after reappearing on November 4, that hypothesis disappeared. It was following the predicted trajectory: it was where the calculations predicted it would be. And the signals emitted showed a natural pattern.
The ESA scientist explains that the emissions recorded are spectral lines, a sort of molecular fingerprint. “All molecules have characteristic lines at different wavelengths,” he comments. “In the case of OH emissions, these are very long lengths, which correspond to changes in the rotational state of the molecule.” To detect them, the comet must have sufficient activity to release an appreciable amount of gas.
For researcher Javier Peralta, an expert in planetary atmospheres, it is advisable to carry out this type of detection when the comet is closest to the Sun (at its perihelion). “At those times it is expected that, as it sublimates in water, one of the gases formed is hydroxyl,” he indicates.
It should be noted that this compound had already been previously detected by NASA’s Swift space telescope, which observed the comet in the ultraviolet region of the electromagnetic spectrum, i.e. at wavelengths shorter than visible light. The new detection, made in another part of the spectrum, confirms the same composition and strengthens the interpretation that 3I/ATLAS is, indeed, a natural comet. “The hydroxyl tells us that the comet contains water and that it is still active,” he says.
We remind you that 3I/ATLAS is the third interstellar object recorded and that its study offers a unique window into older environments of our solar system. However, he adds that there will still be new opportunities to observe the phenomenon. In February 2026, ESA’s JUICE mission, currently en route to Jupiter, will carry out new radio measurements to take advantage of the comet’s passage close to the planet. “It is currently taking images of the comet and when it reaches Jupiter it will try to detect radio signals. It will make a new measurement, in this case from a spacecraft,” he comments.
An absorption of radio waves
The MeerKAT observation also revealed that the signal is in the form of absorption and not emission. Küppers explains that, in an emission line, the molecule emits light; while in the absorption one the light comes from the nucleus of the comet.
For astronomer Elena Manjavacas, a specialist in minor bodies, the detection has nothing mysterious about it. Explain that radio signals are simply another form of light and are part of the electromagnetic spectrum. “The comet emits in many wavelengths and is the only one that we can perceive with our eyes. There are other frequencies that we perceive with other senses. For example, heat is an infrared frequency and we feel it”, underlines the astronomer from the Space Telescope Science Institute (STScI) in Maryland (United States). He adds: “Water and nickel had previously been observed at other infrared frequencies. Now hydroxyl has been identified in the radio band.”
Current data also allows astronomers to estimate how much water the comet is releasing and how that production varies with distance from the Sun. However, a single molecule is not enough to reconstruct the object’s origin. According to Küppers, by comparing the proportions of different molecules, we can deduce the temperature or conditions of the environment in which they were born. But his career, in itself, no longer allows us to know where he comes from. “He spent too much time traveling in space,” he says.
In search of interstellar objects
Manjavacas believes that the arrival of comets from other star systems is unlikely – because they have to travel enormous distances and under very specific conditions – and detections like that of 3I/ATLAS show that “we don’t have all the pieces of the puzzle yet”. “In science it is normal for an unexpected discovery to open up new questions,” he underlines. He adds that these celestial bodies may be more common than previously thought and that, until now, we simply haven’t been able to detect them. This, he points out, opens the door to studying in more detail the dynamics of the galaxy and the processes that allow these objects to travel in our environment.
Others believe the 3I/ATLAS case helps prepare us for the next interstellar visitor. “At ESA we are developing the Comet Interceptor mission, which will be launched around 2028 or 2029,” says Küppers. “The idea is to keep the probe at the L2 point, a million and a half kilometers from Earth, ready to go and encounter a comet.”
For this researcher, the fascinating thing about these discoveries is that they open up a completely new field of study. “Less than ten years ago we knew of no interstellar objects,” he reflects in 3I/ATLAS, 1I/’Oumuamua (2017) and 2I/Borisov (2019). Each new object offers us unique clues about how planetary systems form and evolve in other corners of the galaxy. “They bring us surprises in their composition,” he says.
