December 18, 2025 — The Southwest Research Institute-led Ultraviolet Spectrograph (UVS) aboard NASA’s Europa Clipper spacecraft has made valuable observations of the interstellar comet 3I/ATLAS, which in July became the third officially recognized interstellar object to cross into our solar system. UVS had a unique view of the object during a period when Mars- and Earth-based observations were impractical or impossible.
“We’re excited that this opportunity to view another target on the way to Jupiter was completely unexpected,” said SwRI’s Dr. Kurt Retherford, the principal investigator for Europa-UVS. “Our observations have allowed for a unique and nuanced view of the comet.”
Europa Clipper launched in 2024 and is scheduled to arrive in the Jovian system in 2030, where it will orbit Jupiter and perform 49 close flybys of its moon Europa. The UVS instrument collects ultraviolet light to assess the composition of Europa’s atmospheric gases and icy surface materials.
Within a week of the comet’s discovery, analysts at NASA’s Jet Propulsion Laboratory (JPL) identified its trajectory through the solar system. The Europa Clipper team quickly realized their spacecraft could observe 3I/ATLAS during November, when Earth-based observations were largely blocked by the Sun’s position and after Mars-based views were optimal.
During this time, Europa Clipper bridged the gap between Mars-based views from late September and later Earth-based observations. With the comet’s trajectory passing between Europa Clipper and the Sun, its vantage point enabled the UVS team to view the comet from a unique perspective. Comets have both dust tails in the trailing direction and plasma tails in the direction away from the Sun.
Europa-UVS’s unusual sunward viewpoint obtained a unique downstream view of the comet’s two tails, viewing largely from “behind” the tails and looking back towards the comet nucleus and coma (cloud of gas surrounding it). Additional data from the SwRI-led UVS instrument aboard ESA’s Jupiter Icy Moons Explorer (JUICE) will complement these insights, providing a more common anti-sunward view at the exact same time.
“We’re hopeful that this new view, along with observations from Earth-based assets and other spacecraft, will help us to piece together a more complete understanding of the tails’ geometries,” said SwRI’s Dr. Thomas Greathouse, co-deputy principal investigator of Europa-UVS.
Europa-UVS detected oxygen, hydrogen and dust-related features, supporting the preponderance of data indicating that comet 3I/ATLAS underwent a period of high outgassing activity during the period just after its closest approach to the Sun.
“Europa-UVS is particularly adept at measuring fundamental transitions from atoms and molecules,” Retherford said. “We can see gases come off the comet, and water molecules break apart into hydrogen and oxygen atoms.”
This capability enables Europa Clipper to closely measure and analyze these atomic species, providing a deeper view into the comet's processes and composition.
“Understanding the composition of the comet and how readily these gases are emitted can give us a clearer view of the comet’s origin and how it may have evolved during transit from elsewhere in the galaxy to our solar system,” SwRI’s Dr. Tracy Becker, co-deputy principal investigator of Europa-UVS said. “What are the chemical processes at play, and how can we unravel the comet’s origin in its own star system? Were those processes similar to how we believe our solar system formed? Those are big questions.”
JPL manages the Europa Clipper mission for NASA’s Science Mission Directorate in Washington, D.C. The Europa Clipper mission was developed in partnership with the Johns Hopkins University Applied Physics Laboratory (APL), in Laurel, Maryland.
For more information, visit Planetary Science or contact Joanna Quintanilla, +1 210 522 2073, Communications Department, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166.