Europa Clipper Observes 3I/ATLAS When No Other Telescope Could

In November 2025, interstellar comet 3I/ATLAS disappeared behind the Sun as seen from Earth. Ground-based observatories, Hubble, and JWST all lost their line of sight. But 300 million kilometers away, NASA's Europa Clipper spacecraft had a clear view — and its ultraviolet spectrograph seized the opportunity, capturing data that no other telescope in the solar system could obtain.

The result was a 7-hour observation on November 6, 2025, that confirmed water-ice sublimation in an interstellar comet from a completely new vantage point. Announced by NASA on December 18, 2025, the Europa Clipper observations represent one of the most remarkable feats of opportunistic science in spaceflight history.

Why Earth Lost Sight of 3I/ATLAS

Comet 3I/ATLAS reached perihelion — its closest approach to the Sun — on October 24, 2025. In the weeks that followed, the comet's apparent position moved very close to the Sun as seen from Earth, entering what astronomers call solar conjunction. During this period, the comet was effectively invisible to all Earth-based and Earth-orbiting telescopes.

This was an agonizing gap for scientists. 3I/ATLAS was the third confirmed interstellar object ever detected, and only the second interstellar comet. Every moment of observation was precious. The weeks immediately after perihelion are scientifically critical — peak heating drives maximum outgassing, exposing subsurface volatiles that reveal the comet's primordial composition.

Europa Clipper, launched on October 14, 2024, was cruising through interplanetary space en route to Jupiter. From its position roughly 1 AU (164 million km) from the comet, the spacecraft had a completely unobstructed view. The comet appeared well away from the Sun in Europa Clipper's sky, making observation straightforward.

The Europa-UVS Instrument

The key instrument was the Europa Ultraviolet Spectrograph (Europa-UVS), built by the Southwest Research Institute (SwRI). It is the sixth instrument in the heritage Alice/UVS series that has flown on missions including Rosetta, New Horizons, and Juno.

Europa-UVS operates in the far-ultraviolet band from 55 to 206 nanometers with approximately 0.6 nm spectral resolution and a 7.5-degree field of view. It was originally designed to study the composition and structure of Europa's tenuous atmosphere and to search for plume activity on that icy moon. But its capabilities made it perfectly suited for analyzing cometary emissions.

Dr. Kurt Retherford, the Europa-UVS Principal Investigator at SwRI, described the significance: the instrument provided "a unique and nuanced view of the comet" from a perspective impossible to achieve from Earth.

What Europa Clipper Detected

During approximately 7 hours of continuous observation on November 6, 2025, Europa-UVS detected spectral signatures of hydrogen, oxygen, and dust in the comet's coma. These emissions are the telltale fingerprints of water-ice sublimation — confirming that 3I/ATLAS was actively shedding water as solar heating drove volatile ices from its surface and subsurface.

This was significant for multiple reasons:

• It provided compositional confirmation from a unique geometric perspective — Europa Clipper was viewing the comet's dust and plasma tails from "downstream," looking back toward the nucleus. This geometry is impossible from Earth.
• It demonstrated that comets formed in other planetary systems contain the same fundamental volatile materials as comets in our own solar system. As Dr. Cynthia Phillips of NASA JPL noted, this was among the first direct confirmations of this principle.
• The ultraviolet data complements infrared observations from JWST and visible-light data from ground telescopes, building a multi-wavelength picture of the comet's composition.

Dr. Tracy Becker, Co-Deputy Principal Investigator at SwRI, emphasized that understanding the comet's composition and emission rates "clarifies the comet's origin and galactic transit" — helping scientists trace what happened to this chunk of interstellar ice during its journey between star systems.

The Ion Tail Crossing

Beyond the ultraviolet observations, scientists predicted an even more extraordinary encounter. A paper by Samuel R. Grant of the Finnish Meteorological Institute and Geraint H. Jones of ESA, published on October 15, 2025, calculated that Europa Clipper would be immersed within the ion tail of 3I/ATLAS between October 30 and November 6, 2025.

The ion tail — a stream of charged particles blown away from the comet by the solar wind — extends millions of kilometers behind the nucleus. The minimum miss distance from the ion tail's central axis was estimated at approximately 8 million km, well within the tail's broader extent.

Two Europa Clipper instruments could potentially detect this passage:

• PIMS (Plasma Instrument for Magnetic Sounding) — designed to measure electric and magnetic fields, and capable of detecting charged particles from the comet
• The magnetometer — which could identify magnetic draping structures formed by the solar wind's interaction with the comet's ionized gas

As of early 2026, analysis of the PIMS and magnetometer data is still ongoing. If confirmed, it would mark only the second known instance of a spacecraft passing through an interstellar object's tail — an event of immense scientific value for understanding how interstellar material interacts with our Sun's environment.

A Fleet of Spacecraft Watched 3I/ATLAS

Europa Clipper was far from alone. Comet 3I/ATLAS became the most-observed interstellar object in history, with at least a dozen spacecraft contributing data:

NASA missions:

• Parker Solar Probe captured roughly 10 images per day from October 18 to November 5, 2025, using its WISPR camera — observing during the same solar conjunction gap
• STEREO-A tracked the comet from September 11 to October 2 using its heliospheric imager
• SOHO spotted 3I/ATLAS with its LASCO coronagraph from October 15 to 26
• PUNCH observed from September 20 to October 3 using visible-light polarimetry
• SPHEREx made infrared observations in August and December 2025, detecting a dramatic post-perihelion eruption
• Hubble imaged the comet with WFC3 and performed UV spectroscopy with STIS
• JWST used NIRSpec and MIRI for detailed compositional analysis

ESA missions:

• JUICE used five instruments from November 2 to 25, 2025
• Mars Express and ExoMars Trace Gas Orbiter observed from Mars orbit
• XMM-Newton performed X-ray observations on December 12, 2025

JAXA:

• XRISM conducted X-ray observations on December 15, 2025

This unprecedented multi-spacecraft campaign provided continuous coverage of 3I/ATLAS across all wavelengths, even during periods when no single observatory had a complete view.

What JWST and SPHEREx Revealed About Composition

While Europa Clipper provided the ultraviolet perspective, other observatories filled in the compositional picture with remarkable detail.

JWST observations using NIRSpec (August 6, 2025) and MIRI (December 2025) revealed that 3I/ATLAS has a CO2-dominated coma, with carbon dioxide accounting for approximately 87% of the comet's gas-phase mass loss. This was a striking finding — most solar system comets are water-dominated.

Even more surprising was the first detection of methane (CH₄) in an interstellar comet, measured at 13.7–27% of the water production rate. JWST also identified water ice, water vapor, carbon monoxide, carbonyl sulfide (OCS), and nickel (Ni) in the coma.

SPHEREx, NASA's infrared survey telescope, observed 3I/ATLAS in both August and December 2025. Its 102-band infrared spectrograph detected methanol, cyanide, methane, CO₂, CO, and water. Most dramatically, SPHEREx caught a surprise eruption in December 2025 — well after perihelion — showing rapid sublimation of subsurface ices that produced a distinctive "pear-shaped dust tail." These results, published in February 2026 in the Research Notes of the AAS, suggest that solar heating continued to penetrate deeper into the nucleus long after the comet's closest solar approach.

Europa Clipper's Journey Continues

The 3I/ATLAS observation was an opportunistic bonus for Europa Clipper, whose primary mission lies years ahead. The spacecraft's trajectory milestones include:

• Mars gravity assist: Completed on March 1, 2025, passing 550 miles above the Martian surface
• Current position: Approximately 329 million km from Earth, in the constellation Scorpius
• Earth gravity assist: Scheduled for December 3–4, 2026, approaching within 163,855 km
• Jupiter arrival: April 2030
• Primary mission: 49 flybys of Europa over 3.5 years, searching for conditions that could support life beneath the moon's icy shell

The fact that Europa Clipper could contribute such valuable science during its cruise phase — years before reaching its primary target — demonstrates the flexibility built into modern space missions and the ingenuity of the teams that operate them.

What 3I/ATLAS Tells Us About Interstellar Chemistry

The combined observations from Europa Clipper, JWST, SPHEREx, and the rest of the spacecraft fleet are painting a picture of 3I/ATLAS as a scientifically extraordinary object. Key takeaways so far:

Volatile composition differs from solar system comets. The dominance of CO₂ over water, the presence of methane, and the specific ratios of carbon-bearing molecules suggest that 3I/ATLAS formed in a colder environment than most comets in our own Oort Cloud — possibly in the outer regions of a different star's protoplanetary disk.

Water is still present. Despite the CO₂ dominance, Europa Clipper's ultraviolet detection of hydrogen and oxygen confirms active water-ice sublimation. This means the building blocks of life as we know it are not unique to our solar system.

Delayed outbursts reveal subsurface complexity. The December 2025 eruption detected by SPHEREx shows that 3I/ATLAS has a layered interior where different volatile species are stored at different depths, similar to solar system comets but with a different compositional balance.

As 3I/ATLAS fades to magnitude 13–15 in the constellation Gemini and heads toward a close approach with Jupiter on March 16, 2026 (at 0.358 AU), it carries with it answers to one of astronomy's fundamental questions: are the chemical ingredients for planets and life universal, or unique to our corner of the galaxy? The evidence increasingly points toward universality.

Track the comet's current position on our Orbit page, find observing tips on our Observing page, and explore the full timeline of 3I/ATLAS discoveries.