Scientists using an ultraviolet spectrograph (UVS) aboard NASA's Europa Clipper spacecraft observed 3I/ATLASonly the third confirmed interstellar object ever discovered to have entered the Solar System from beyond. While most Martian and ground-based telescopes were poorly positioned to observe the interstellar visitor due to its proximity to the Sun, Europa Clipper was able to collect data from a unique vantage point as the space probe headed toward Jupiter.
This composite image of interstellar comet 3I/ATLAS was taken on November 6, 2025 by the UVS instrument aboard NASA's Europa Clipper spacecraft from a distance of about 164 million kilometers (103 million miles). Image credit: NASA/JPL-Caltech/SwRI.
Europa Clipper was launched in 2024 and is scheduled to arrive in the Jupiter system in 2030, where it will enter Jupiter orbit and make 49 close flybys of its icy moon Europa.
The UVS instrument collects ultraviolet light to assess the composition of Europa's atmospheric gases and icy surface materials.
“We are thrilled that this opportunity to see another target on the way to Jupiter was completely unexpected,” said Europa-UVS principal investigator Dr. Kurt Retherford, a postdoctoral fellow at the Southwest Research Institute.
“Our observations provided a unique and detailed view of the comet.”
3I/ATLAS was discovered by the NASA-funded ATLAS Survey Telescope in Rio Hurtado, Chile, on July 1, 2025.
At that time, the interstellar comet was at a heliocentric distance of 4.51 astronomical units (AU) with an eccentricity of 6.13.
Within a week of discovery, NASA Jet Propulsion Laboratory analysts determined its trajectory through the solar system.
The Europa Clipper team quickly realized that their spacecraft would be able to observe 3I/ATLAS in November 2025, when observations from Earth were largely blocked by the position of the Sun and after images from Mars were optimal.
During this time, the spacecraft bridged the gap between September images from Mars and more recent ground-based observations.
Because the comet's trajectory passed between Europa Clipper and the Sun, its vantage point allowed the UVS team to view the comet from a unique perspective.
Comets have both dust tails in the direction they are traveling and plasma tails in the direction away from the Sun.
Europa-UVS's unusual vantage point facing the Sun provided a unique view of the comet's two tails downstream, looking mostly “behind” the tails and looking back toward the comet's core and coma.
“We hope that this new perspective, along with observations from ground-based sites and other spacecraft, will help us piece together a more complete understanding of the geometry of the tails,” said Europa-UVS Deputy Principal Investigator Dr. Thomas Greathouse, also from the Southwest Research Institute.
The UVS instrument detected features associated with oxygen, hydrogen and dust, confirming the preponderance of data indicating that 3I/ATLAS experienced a period of high outgassing activity in the period immediately after its closest approach to the Sun.
“Europa-UVS is particularly good at measuring fundamental transitions of atoms and molecules,” said Dr Retherford.
“We can see gases escaping from the comet and water molecules breaking apart into hydrogen and oxygen atoms.”
This capability allows Europa Clipper to carefully measure and analyze these atomic species, providing greater insight into the comet's processes and composition.
“Understanding the comet's composition and how easily these gases are released can give us a clearer understanding of the comet's origins and how it may have evolved during its transition from elsewhere in the galaxy to our solar system,” said Europa-UVS Deputy Principal Investigator Dr. Tracy Becker, also from the Southwest Research Institute.
“What chemical processes are happening here and how can we unravel the origin of the comet in its own solar system?”
“Were these processes similar to how we think our solar system formed? These are big questions.”
