For the first time, astronomers have captured X-ray portraits of a comet from beyond our solar system, are providing a glimpse of what happens when gases from a comet collide with charged particles from the Sun. The interstellar comet 3I/ATLAS, found in Chile in July 2025 by the ATLAS survey, is just the third known visitor from outside our solar system. Its passage through our solar system is a once-in-a-lifetime chance to study an object like 3I/ATLAS.
When 3I/ATLAS reached its closest point to Earth on December 19, 2025, when it was still 270 million kilometers away, it was already being scrutinized intensively across the electromagnetic spectrum. Optical, infrared, ultraviolet, submillimeter, and radio telescopes have been monitoring its coma and tail for months. But there were two space-based telescopes that brought a new element to observations: Japan’s XRISM and ESA’s XMM-Newton, which detected X-rays emitted in its encounter with the solar wind.
Between November 26 and 28, XRISM’s Xtend telescope tracked the comet for a total of 17 hours, with the aim being adjusted 14 times to lock onto the slowly moving object. The image that was obtained showcased a faint X-ray halo that extended about 400,000 kilometers from the nucleus, or about the distance between Earth and its satellite, the Moon. The X-ray emission is a result of a process known as charge exchange, whereby highly ionized solar wind interacts with a stream of water vapor, carbon monoxide, and carbon dioxide gas emanating from the comet. The molecules lose electrons and undergo an excited state, emitting X-ray photons as a result. The spectra obtained by XRISM confirmed the presence of carbon, nitrogen, and oxygen.
Just a few days later, on December 3, 3I/ATLAS was imaged for 20 hours with the EPIC-pn camera on board the XMM-Newton satellite, which has the greatest sensitivity among all the X-ray cameras on board any of these satellites. Even at a distance of 282-285 million kilometers, it was able to image a bright spot of low-energy X-ray emission. This method of imaging is very helpful for viewing light gases such as hydrogen and nitrogen, which are hard to image using optical or ultraviolet cameras.
Ground-based spectroscopy follows up these high-energy measurements. In mid-August 2025, scientists measured the beginning of CN (cyanogen) emission, which signified the start of active volcanic release of nitrogen-containing volatiles. The rate of CN production increased from 4.8 × 10²⁴ to 7.2 × 10²⁴ per second in five days, together with an increase in dust production. C₂ and C₃ emissions were not detected, signifying that carbon-chain molecules were heavily depleted a characteristic also exhibited by comet 2I/Borisov and possibly indicative of its formation environment.
Interstellar comets are very scarce objects. The first one, 1I/ʻOumuamua, did not have any observable gas or dust emissions, while the second one, 2I/Borisov, was active, though its chemistry was very different from that of typical solar system comets. The third one, 3I/ATLAS, which has a coma size of 25,000 kilometers and a period of rotation of 16.79 hours, is moving at a speed close to 58 km/s and is thought to have originated from the Milky Way’s thick disk region. Its estimated age is between 3 and 11 billion years, so it may have been a leftover from the “cosmic noon” period when star formation reached its maximum level, and its chemistry may have been characterized by high levels of α-elements and a deficiency of iron during that period. The x-ray glow of 3I/ATLAS is much more than a pretty picture. It is a diagnostic chart of where solar matter interacts with interstellar matter. Comparing these observations with data from solar system comets for the past decades will allow scientists to determine if the physical laws governing solar wind are universal or if they are different for interstellar visitors. As 3I/ATLAS heads deeper into space, these observations will continue to provide a standard for understanding the composition of interstellar wanderers.
