“We’re excited that this opportunity to view another target on the way to Jupiter was completely unexpected,” said Dr. Kurt Retherford, principal investigator for the Europa Ultraviolet Spectrograph (Europa-UVS) instrument on NASA’s Europa Clipper mission. Europa Clipper snapped UV photos of comet 3I/ATLAS on Nov. 6, 2025, at a record distance of 102 million miles (164 million kilometers) away. This rare visitor to our solar system is approaching at a speed of 246,000 kilometers per hour (153,000 miles per hour) and is the third confirmed object to travel to our solar system from outside. Mission position helped to fill a critical observation gap since Earth observations would have made it difficult because of Sunlight, while observations from Mars orbit would no longer be in range.
The Europa UVS, originally intended to study the surface and thin atmosphere of the icy moon of Europa, uses ultraviolet spectroscopy to analyze the incident light, breaking it down into its wavelength components. This allows researchers to detect characteristic spectral signals of elements and molecules within the coma, the cloudy atmospheric environs of the frozen nucleus of the comet. By aligning several UV images and adjusting them to visible light, they were able to create a composite picture of the comet that cannot be seen with the naked human eye.
The presence of oxygen, hydrogen, and dust features in the UVS observations indicates that 3I/ATLAS passed through a period of strong outgasing following its perihelion passage. Previous studies at different bands, including the detection by SPHEREx of large amounts of carbon dioxide gas as well as H₂O ice, have revealed the presence of diatomic carbon (C₂), which makes 3I/ATLAS glow green in visible observations, as in solar system comets. The results seem to show both commonalities with solar system comets as well as information on the chemical surroundings in which this star system resides.
But thanks to Europa Clipper’s “behind” viewpoint towards the sun with its unique perspective on the comet, the spacecraft observed the dust and plasma tails of the comet in a manner that revealed their structure in relation to the solar wind. The plasma tails, which are generated when a stream of charged particles emanating from the sun encounters gases in the comet, were probed by X-ray observations using the ESA XMM-Newton and the JAXA XRISM spacecraft. The X-ray observations observed glows extending some 250,000 miles (400,000 km) from the nucleus, tracing the presence of hydrogen and nitrogen gas, which are clouded from view in optical and ultraviolet observations.
The combination of ultraviolet and X-ray observations has great significance. While ultraviolet spectra are very effective for the detection of atomic transitions for gases such as oxygen and carbon, X-ray observations reveal lighter elements, as well as the energetic processes involving the solar wind and the coma of the comet. Such combined results help construct a fuller chemical characterization of the 3I/ATLAS Comet.
The first mission of the Europa Clipper will be the exploration of the subsurface ocean of the Jupiter moon, Europa, which has a potential extraterrestrial environment, according to the mission goals established for April 2030, after the arrival of the spacecraft in the Jovian system. According to NASA calculations, the chemical composition of the ocean beneath the surface of the moon, which is thought to be covered by a kilometers-thick ice crust, could be similar to that of Earth, consisting of chloride, with hydrothermal vents on the floor. The flexibility of reusing the instrumentation on the Europa Clipper spacecraft to investigate comet science highlights the flexibility that exists within deep-space missions.
Furthering the capabilities of ultraviolet observation analyses developed on the icy surface of the European moon to an interstellar comet that proceeds at high speed helps scientists accumulate expertise on how to handle complex ultraviolet data. This not only improves the knowledge base on 3I/ATLAS but also helps in improving future encounters with rare objects. As 3I/ATLAS presses on towards a close approach with Jupiter in March of 2026 and leaves our solar system forever, every observation from ultraviolet to X-rays leaves an imprint on this temporary and precious record. It is an image that space buffs and scientists alike can appreciate not only as revealing of the alien chemistry of the comet itself but of the readiness of the Europa Clipper as it looks towards the most tantalizing world of life outside of our own.
