The Southwest Research Institute-led Ultraviolet Spectrograph (UVS) aboard NASA’s Europa Clipper spacecraft made valuable observations of the interstellar comet 3I/ATLAS during a period when it was difficult to observe from Mars- and Earth-based vantage points, viewing its two tails from between their downstream directions. Credit: NASA/JPL-Caltech/APL/SwRI

NASA’s Europa Clipper spacecraft has recorded important new observations of the interstellar comet 3I/ATLAS using its Southwest Research Institute-led Ultraviolet Spectrograph (UVS). In July, 3I/ATLAS became only the third confirmed object from another star system known to pass through our solar system. The UVS instrument observed the comet during a brief period when viewing from Mars and Earth was difficult or not possible.

“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 set to arrive in the Jovian system in 2030. Once there, it will orbit Jupiter and conduct 49 close flybys of its icy moon Europa. The UVS instrument is designed to capture ultraviolet light, which scientists use to study the makeup of Europa’s thin atmosphere and the materials on its frozen surface.

Interstellar comet 3I/ATLAS is seen in this composite image captured on Nov. 6, 2025, by the Europa Ultraviolet Spectrograph instrument on NASA’s Europa Clipper spacecraft, from a distance of around 103 million miles (164 million kilometers). Credit: NASA/JPL-Caltech/SwRI

Timing that made the observations possible

Within days of the comet’s discovery, scientists at NASA’s Jet Propulsion Laboratory (JPL) calculated its path through the solar system. The Europa Clipper team soon realized that their spacecraft would be in a position to observe 3I/ATLAS in November. During that time, the comet’s location near the Sun limited what could be seen from Earth, and viewing conditions from Mars had already passed their most favorable window.

Europa Clipper was able to fill this observational gap, linking earlier Mars-based views from late September with later observations from Earth. Because the comet traveled along a path between Europa Clipper and the Sun, the spacecraft could observe it from an uncommon angle. This geometry is important because comets typically produce two distinct tails: a dust tail that trails behind the comet and a plasma tail that points away from the Sun.

The SwRI-led UVS instrument collects ultraviolet light to assess the composition of Europa’s atmospheric gases and surface materials. It detected oxygen, hydrogen and dust features in the interstellar comet 3I/ATLAS and made unique downstream observations of its tails. Europa-UVS Co-deputy principal investigator Dr. Tracy Becker (left) and Europa-UVS principal investigator Dr. Kurt Retherford (right) are pictured with Europa-UVS during clean cabin testing. Credit: Southwest Research Institute

Looking back at the comet’s twin tails

From its position closer to the Sun, Europa-UVS captured a downstream view of both tails. The instrument observed the comet largely from “behind” the tails, looking back toward the comet’s nucleus and its surrounding coma (cloud of gas surrounding it). At the same time, data from the SwRI-led UVS instrument aboard ESA’s Jupiter Icy Moons Explorer (JUICE) will provide a more traditional anti-sunward view, allowing scientists to compare perspectives taken during the same period.

“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 signatures of oxygen, hydrogen, and dust-related material. These measurements support other evidence showing that comet 3I/ATLAS experienced a phase of intense outgassing shortly after it made its closest pass by the Sun.

This artist’s concept depicts NASA’s Europa Clipper spacecraft in orbit around Jupiter. Credit: NASA/JPL-Caltech

Clues to a comet from another star system

“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.”

By measuring these atomic species in detail, Europa Clipper gives scientists a closer look at the physical and chemical processes shaping the comet and what it is made of.

“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 mission was developed in partnership with the Johns Hopkins University Applied Physics Laboratory (APL), in Laurel, Maryland.

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