Two spacecraft on their way to Jupiter observed and imaged both sides of the interstellar comet 3I/ATLAS and provided chemical analysis of the gases vaporising off it.
The European Space Agency's Juice spacecraft and NASA's Europa Clipper spacecraft were able to simultaneously photograph both hemispheres of the comet as it flew through the Solar System in late 2025.
More on 3I/ATLAS
A comet from beyond the Solar System
Comet 3I/ATLAS is an interstellar comet, meaning it originated from a distant corner of our Galaxy, far beyond our Solar System.
It could be as old as 10 billion years old, making it more than twice the age of our Solar System, and the oldest comet ever observed.
The vast majority of comets we know of formed on the edges of our Solar System. 3I/ATLAS is one of only three interstellar objects ever found.
it's currently exiting our Solar System and once it's gone, it will be gone from view forever.
That's why astronomers are trying to learn as much about 3I/ATLAS as they can.
A double spacecraft encounter
In late 2025, scientists turned the Ultraviolet Spectrograph instruments on both Juice and Europa Clipper towards comet 3I/ATLAS.
These spacecraft are currently on their way to Jupiter to learn more about the planet's icy moons and their potential to host life.
Their positions deep in the Solar System gave them the opportunity to observe 3I/ATLAS from a new angle and use their science instruments to learn more about the interstellar visitor.
The instruments are led by the Southwest Research Institute in San Antonio, Texas and were used to simultaneously image both hemispheres of the comet and detect its ultraviolet emissions.
What they found
"As the comet passed between Juice and Europa Clipper, we were able to informally coordinate observations between the two spacecraft," says Dr. Kurt Retherford, principal investigator of Juice-UVS and Europa-UVS.
"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."
'Observing the interstellar comet was some exciting bonus science. The resulting rare and unique dataset includes gas emissions and scattered dust," says SwRI’s Dr. Philippa Molyneux, co-deputy principal investigator for the Juice-UVS instrument.
"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 reason 3I/ATLAS was giving off so much gas is because, like all comets, it's composed of water ice and rock.
As a comet approaches the Sun, it heats up and that ice sublimates into gas, which forms the fuzzy head and long tail visible on active comets.
In this case, the science team detected higher levels of carbon emissions from 3I/ATLAS than expected, especially in comparison to comets from our Solar System.
They were also able to track emissions over several days and discover how the ratios of molecules changed, as well as how 3I/ATLAS was evolving as it made its way 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," Molyneux says.
"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," Retherford says.
"And because SwRI’s team is involved in both missions, it was a special highlight for us in terms of working together."
