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.
A Europa Clipper instrument has uniquely observed the interstellar comet 3I/ATLAS in ways other instruments couldn’t.
The SwRI-led Ultraviolet Spectrograph viewed 3I/ATLAS at points that most other assets could not.
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 recognised 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.
SwRI’s Dr Kurt Retherford, the principal investigator for Europa-UVS, said: “We’re excited that this opportunity to view another target on the way to Jupiter was completely unexpected.
“Our observations have allowed for a unique and nuanced view of the comet.”
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 instrument, from a distance of around 103 million miles (164 million km). Credit: NASA/ JPL-Caltech/ SwRI.
Europa Clipper
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 realised 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.
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.
Unique view
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.
SwRI’s Dr Thomas Greathouse, co-deputy principal investigator of Europa-UVS, said: “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.”
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.
Retherford said: “Europa-UVS is particularly adept at measuring fundamental transitions from atoms and molecules.
“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 analyse these atomic species, providing a deeper view into the comet’s processes and composition.
SwRI’s Dr Tracy Becker, co-deputy principal investigator of Europa-UVS, said: “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.
“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.”
