When the interstellar comet 3I/ATLAS passed between ESA’s Jupiter Icy Moons Explorer (Juice) spacecraft and NASA’s Europa Clipper spacecraft in November 2025, scientists seized an once-in-a-lifetime chance: one spacecraft caught the comet’s glowing dayside, the other its dusty night side, simultaneously.
In November 2025, 3I/ATLAS passed between ESA’s Juice spacecraft and NASA’s Europa Clipper spacecraft. Image credit: NASA / ESA / Southwest Research Institute.
“As the comet passed between Juice and Europa Clipper, we were able to informally coordinate observations between the two spacecraft,” said Dr. Kurt Retherford, a planetary researcher at Southwest Research Institute.
“Crucially, we observed hydrogen, oxygen and carbon emissions.”
“These elements are produced when gases escaping the comet’s nucleus break apart into atoms when exposed to sunlight.”
The unique observations were made with the Ultraviolet Spectrograph (UVS) instruments aboard Juice and Europa Clipper spacecraft.
“Observing the interstellar comet was some exciting bonus science,” said Dr. Philippa Molyneux, also from Southwest Research Institute.
“The resulting rare and unique dataset includes gas emissions and scattered dust.”
“This was the first time we’ve had simultaneous direct views of a comet’s coma of escaping gas from two directions.”
“Europa Clipper showed us the night side of the comet, with a great deal of scattered dust, while Juice imaged mostly glowing gas on the day side.”
The researchers found higher levels of carbon emissions from 3I/ATLAS than expected early on, especially in comparison to typical comets from our Solar System, corroborating similar findings through other observations about the interstellar comet’s origin and composition.
Observing the trends of emissions over several days revealed how the ratios of these molecules changed and how the comet evolved during its journey through our Solar System.
“By studying the ratio of water ice and dry ice, we can compare the composition of this interstellar comet to comets native to our Solar System,” Dr. Molyneux said.
“This helps us understand if the Solar System where 3I/ATLAS formed is similar to ours or different.”
“I think the coordination with Europa Clipper was a fun and impactful demonstration that showed how the two projects could coordinate observation plans,” Dr. Retherford said.
“And because SwRI’s team is involved in both missions, it was a special highlight for us in terms of working together.”
