What does it mean when a comet racing at about 130,000 miles per hour shows hydrogen signatures unlike anything seen in our solar system?
NASA’s MAVEN spacecraft has provided an unexpected vantage point on the interstellar comet 3I/ATLAS, using ultraviolet imaging to extract hydrogen composition data that now anchors a deeper investigation into chemistry from beyond the Sun’s influence. Though originally designed to study Mars’ upper atmosphere, MAVEN was retasked during a ten‑day window in late September 2025, when 3I/ATLAS slipped behind the Sun from Earth’s view. The Imaging Ultraviolet Spectrograph collected multi‑wavelength datasets capable of distinguishing hydrogen from the comet, from Mars, and from the interplanetary medium. “The images MAVEN captured truly are incredible.” said Shannon Curry, principal investigator for MAVEN. The precision was made possible by a special observing mode that separated emissions by relative velocity, allowing the spacecraft to show hydrogen confined tightly around the comet’s position.
This hydrogen mapping now serves as a chemical tracer for reconstructing the comet’s environmental history. Previous detections of hydroxyl-an indicator of water breakdown-had been made by NASA’s Neil Gehrels Swift Observatory, but MAVEN’s ultraviolet capability allowed the first estimate of the upper limit of the comet’s deuterium‑to‑hydrogen ratio. That is a key measurement for evaluating where the comet formed relative to the volatile ice lines in its natal disk. Justin Deighan, the deputy principal investigator, said, “There was a lot of adrenaline when we saw what we’d captured,” noting that such interstellar sampling is extremely rare.
Chemical context from ground‑based and spaceborne assets deepens the chemical narrative. Very Large Telescope observations showed nickel in the coma without iron, suggesting an unusual sublimation route. Simultaneously, JWST measurements indicated a surprisingly high abundance of carbon dioxide relative to water vapor in the coma, consistent with surface layers processed due to long exposure to cosmic rays. Scientists suggest that a “cooked shell” may form on its surface with the accumulation of CO2 ice during billions of years in interstellar space. Such chemically distinct layers could be related to the very cold regions of its birth environment, analogous to the distant CO2 ice line in the disks around young stars.
Tracing 3I/ATLAS’s path serves to highlight the vastness of that interstellar journey. Utilizing star‑motion data from Gaia, scientists modeled encounters out to distances over 100 million astronomical units, but none significantly changed its trajectory-an affirmation of its true birthplace within the Milky Way’s thin disk and its truly remarkable age. The comet might be older than seven billion years, which would make it one of the oldest macroscopic objects ever seen to enter the solar system.
Spectroscopy remains central to decoding its structure. MAVEN’s ultraviolet imaging complements optical and infrared data, allowing the identification of hydrogen production mechanisms, quantification of outgassing rates, and discrimination between sublimation driven both by solar heating and photodissociation of trapped molecules. These methods parallel approaches applied to earlier interstellar objects, including 2I/Borisov, which showed extreme carbon‑monoxide enrichment in observations reported by Hubble and ALMA. Where Borisov acted as a volatile‑rich messenger from a cold disk environment, 3I/ATLAS appears to carry signatures not only of primordial formation but also of prolonged exposure to interstellar chemistry. In equal measure, the contrast with 1I/‘Oumuamua is instructive: ‘Oumuamua lacked detectable outgassing, and its non‑gravitational acceleration still stands without a confirmed mechanism, whereas 3I/ATLAS exhibits classic comet activity-multiple jets, a broad coma, and persistent hydrogen emission.
Fleet-wide monitoring continues as 3I/ATLAS moves outward following its 2025 perihelion. Missions such as Psyche, Lucy, STEREO, and ESA’s SOHO have contributed views from widely distributed vantage points, while JWST is expected to deliver deeper spectral inventories as the object dims. Each dataset has refined constraints on its nucleus size, internal composition, and the balance of gases such as methanol, hydrogen cyanide, and methane that may emerge at differing solar distances.
Every signature of hydrogen that MAVEN isolates, every fragment of the spectrum logged through telescopes across the solar system, and every chemical anomaly that can be detected in its coma serves to help humankind understand interstellar planetesimals, objects carrying the ancient blueprint of planetary system formation. As Deighan said, “Every measurement we make of this comet helps to open up a new understanding of interstellar objects.”
