Every time an interstellar visitor approaches the vicinity of Earth, a great stir is generated in all kinds of more or less specialized sectors. There are those who see it as a golden opportunity to learn a little more about our neighboring planetary systems and also those who fear them, considering them as a possible extraterrestrial technology.
We saw it in 2017 with Oumuamua, in 2019 with 2I/Borisov and last year with comet 3I/ATLAS. We don’t talk so much about the first and the second, but about the third studies continue to be published. For example, one that has recently been made known, in which, thanks to James Webb, has been achieved for the first time analyze the chemical fingerprint of an interstellar object.
Very rare gases for our solar system. Thanks to the MIRI spectrograph, the James Webb’s specialized mid-infrared instrument, it has been possible to analyze the chemical composition of 3I/ATLAS. This is because different chemicals reflect light differently. By analyzing the resulting spectrum, different gases can be identified.
In this case, water vapor was detected beyond the nucleus, possibly due to the melting of ice grains present in the comet. In addition, both methane and carbon dioxide were identified very close to the core. It is the first time that methane has been identified in an interstellar visitor. The proportion of these last two gases in relation to water is very high. Too much for what is normal for comets in the solar system, which is why the idea that this visitor came from some other very remote place continues to be supported.
MIRI shows the interstellar comet in three different wavelengths of light and illustrates where the different gases were located at the time the comet was observed.
Two key dates. The measurements with the James Webb Space Telescope were carried out on two different dates. On the one hand, between December 15 and 16, 2025 and secondly on December 27 of that same year. At that time, our star visitor began his return journey after going around the Sun.
The Sun is essential. Attempts had been made to identify the gases of this comet before. However, the methane remained hidden until this point. The authors of the study believe that this is because it had remained hidden in its depths, under the ice, and that it took the Sun to melt part of that ice for it to emerge to the surface.
The search continues. When 3I/ATLAS approached our planet, many instruments took the opportunity to land on it and try to obtain as much data as possible. A good example of this is the JUICE probe, of the European Space Agency. While the initial goal of this mission is to study Jupiter’s icy moons, it was in the right place at the right time when we welcomed our mysterious visitor.
The ESA calculation which would be their closest ship at the time of the approach, so they used it to take information about the comet in November 2025. This was sent to Earth in February 2026, so since then there have been scientists working on its analysis. The study that has now been published is possibly just the first of many. And 3I/ATLAS has already left, but the interest in knowing everything possible about it stayed with us.
Image | International Gemini Observatory/NOIRLab/NSF/AURA/B. Bolin | NASA, ESA, CSA, STScI, M. Belyakov (Caltech), I. Wong (STScI), Image Processing: A. Pagan (STScI)
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