“How do you study the invisible?” This was the challenge the scientists at the ESA faced during the early days of December 2025 when the XMM-Newton telescope aboard the space telescope detected a faint but identifiable X-ray flash from the interstellar comet 3I/ATLAS. At a distance of about 282 to 285 million kilometers away from the spacecraft, the comet’s radiation was detected via the EPIC-pn detector on the European Photon Imaging Camera.
X-ray astronomy provides a distinct opportunity to study cometary chemistry in detail. Since visible light is reflected off the dust and gas in a comet’s coma, X-rays are produced as a result of charge exchange between solar wind ions, mainly composed of protons and alpha particles, and neutral atoms or molecules in the coma. Hence, a distinct fingerprint signature emerges in its spectral characteristics that cannot be perceived in visible astronomy observations, specifically in the case of lighter gases such as hydrogen (H₂) and nitrogen (N₂) molecules and atoms, which are barely detectable in UV and visible observations.
XMM-Newton and JAXA’s newly launched XRISM mission have further advanced this capability and enabled the resolution of X-ray-emitting areas up to a distance of 400,000 km in a comet’s nucleus and allowed researchers to detect spectral signs of carbon, nitrogen, and oxygen molecules and atoms within the comet’s coma.
What distinguishes 3I/ATLAS is its background and composition. It is the third confirmed interstellar comet in the galaxy, which dates back billions of years in a distant planetary system when the comet was likely expelled into the galaxy in which we live. The James Webb Space Telescope detected a remarkable ratio of 3I/ATLAS’s CO₂ to H₂O of 8:1, which is one of the highest ever measured in any comet. In addition to this, there is a rich supply of carbon monoxide with detectable amounts of atomic nickel.
The discovery of atomic nickel in the comet by the Very Large Telescope is particularly enigmatic in that there is no measurable iron present in the comet in a way that is consistent with sublimates of metals at quite low temperatures in the presence of ultraviolet light, likely from volatile compounds such as nickel tetracarbonyl, which releases gas in a way that’s triggered by ultraviolet light at temperatures as low
These chemical indicators suggest a cold and chemically different region of origin that may lie beyond a CO₂ ice line in its parent protoplanetary disk. Over a period of several billion years of traveling through interstellar space, 3I/ATLAS would have been irradiated with galactic cosmic rays that would have changed its surface into a CO₂ “cooked shell,” which is now breaking apart due to solar radiation as it releases fresh material from beneath this shell. The high CO₂ and bounded nickel volatile material could remain from its formation environment.
Polarimetric imaging provides a new consideration in this analysis. Through analysis of the scattered sunlight polarization, scientists have inferred properties and distributions of dust grain composition in the coma. Evidences have shown that 3I/ATLAS’ dust grains are unique compared with other solar system comets. In this classification, highly porous aggregates have strong silicate features and higher polarization values. Those with compact aggregates have weaker features and lower polarization values. 3I/ATLAS’ unique coma structure could represent a lack of stellar processing, a result of being isolated in space for a considerably prolonged period.
High-resolution observations have also disclosed gas and dust jets, varying in time with a period of 15-16 hours, already issuing from the nucleus. The jets’ appearance, with no apparent tail, is indicative of large dust, hundreds of microns in size, with a low velocity of about 1 m/s. Their high mass makes it impossible for them to be accelerated by the solar radiation pressure towards the classic antitail, thus justifying the observation of some dust flows towards the Sun.
From a technical standpoint, the multiband observations of 3I/ATLAS show the coming together of different observation methods. The X-ray telescopes identify light gases; the JWST observation instruments analyze volunteer ices; the ground-based observation instruments analyze dust; and the millimeter waves detect molecule concentrations like ALMA. Each of these tools provides data that enables a complete analysis of a body that has been moving at 130,000 miles per hour for possibly seven billion years.
As 3I/ATLAS presses closer to perihelion, there will be increased activity, which could possibly yield further gases, such as methanol, formaldehyde, and hydrogen cyanide. Each discovery will serve to improve models of the origins and evolution of 3I/ATLAS, adding to a growing database of interstellar visitors. With only three examples so far, each one has a disproportionate importance in the scientific community not only an alien chemistry snapshot but a sample of planetary formation in the Milky Way.
