An unexpected flyby observation into another planetary system’s distant past was conducted using a spacecraft designed for studying an icy moon in the Jupiter system. The Europa Clipper headed for the Jovian system performed a rare observation in the ultraviolet spectrum of the interstellar comet 3I/ATLAS, becoming only the third definitive object outside the bounds of our solar system to transit this system in space history. The reason this observation was possible was that in November 2025, Earth observations were no longer feasible due to its proximity to the sun, while observations from Mars were now past their peak.
Just one week after being found by the ATLAS survey telescope in Chile, NASA researchers at the Jet Propulsion Laboratory were able to model 3I/ATLAS’s hyperbolic orbit and predict that the comet could be viewed for a short period of time by the Europa Clipper spacecraft. At the time, the geometry of Clipper’s orbit and the comet and the Sun made for a very unusual downstream viewing angle of the comet’s dual tails. Normally, comet tails are seen with a dust tail extended “behind” the comet and orbiting along with it, and a plasma tail pointing straight away from the Sun. However, to Clipper, both tails were seen more or less head-on, or at least from behind the comet and its coma.
The UVS instrument, with a mass of slightly more than 19 kilograms and a power consumption of 7.9 watts, was planned for the exploration of the thin atmosphere and icy surface of Europa, focusing on the detection of far-ultraviolet photons, separating them into individual wavelengths. In the case of the 3I/ATLAS mission, the instrument’s ability to determine the composition of the comet’s outgassing and dust emissions had been utilized for the comet study instead. Signatures of oxygen, hydrogen, and material related to dust were revealed by Europa-UVS, showing that the comet had undergone heavy outgassing action shortly after passing close to the Sun, according to Dr. Retherford: “We can see gases come off the comet, and water molecules break apart into hydrogen and oxygen atoms.”
Ground-based telescope follow-up observations provided supporting data on composition. Spectra obtained in August 2025 revealed the beginning of CN emission with a production rate of 6 × 10²⁴ molecules per second, classifying 3I/ATLAS as a strongly depleted carbon-chain comet. This level of depletion, reflected in the C₂-to-CN ratio, might indicate a formation region poor in certain hydrocarbons or a long history of evolution over millions of gigayears in the interstellar medium. Near infrared spectra spectroscopy confirmed the presence of H₂O, CO, and CO₂. The ratio of CO₂/H₂O was 7.6 ± 0.3 the highest measured value in a comet.
Morphonological investigations of the dust continued to increase the fascination. Initially, there was low speed emission of coarse dust, transitioning to a highly collimated anti-tail jet extending at least 400,000 kilometers in the sunward direction. analysis showed the typical sizes of the dominant population radiating this emission were much larger than for normal cometary dust, ranging from 1 to 100 microns, to be viable under the constraints imposed by the solar radiation pressure vise. Such unusual dust population could be indicative of the presence of local surface activities on the nucleus, radiating material from a small portion of its surface.
The unusual shape of the Europa Clipper also allowed for the possible in situ measurement of the plasma. Modeling showed that pickup ions, and in particular CO⁺ ions photoionized from CO, could be measurable by the Plasma Instrument for Magnetic Sounding on board the spacecraft between November 7 and 11, depending upon the speed of the solar wind. The Magnetometer on board the Clipper orbiter allowed for the measurement of perturbations of the magnetic field due to the comet’s interaction with the solar wind, further establishing the physical characterization.
It’s showing that we can dynamically point our instruments to make observations, said mission scientists about the serendipitous 3I/ATLAS observations. As Dr. Thomas Greathouse 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.” Through combining UV spectroscopy observations with computational models of dust transport and plasma environments, scientists are creating a complex character profile of this body that originated some billions of years ago within a star-forming region far away to provide a comparable entity to comets within our solar system .
