ESA’s ExoMars and Mars Express Observe Interstellar Comet 3I/ATLAS
European Space Agency spacecraft are turning their optics toward a rare visitor from beyond our Solar System. Interstellar comet 3I/ATLAS has caught the attention of ESA’s Mars orbiters, with the ExoMars Trace Gas Orbiter (TGO) and the Mars Express mission contributing valuable observations as the object heads toward the Sun. While a complete image of the comet’s nucleus remains out of reach, the shimmering coma surrounding the nucleus offers a surprising glimpse into the behavior of this distant visitor.
CaSSIS Captures a Faint Comet Shadow
ExoMars TGO’s Colour and Stereo Surface Imaging System (CaSSIS) captured a sequence of images showing 3I/ATLAS drifting past the field of view. The comet’s fuzzy white appearance is centered on the image, with the nucleus indistinguishable from the surrounding coma. The reason is simple: at the distance involved, distinguishing a kilometre-scale nucleus from a coma spanning thousands of kilometres is nearly impossible. CaSSIS is designed for high-resolution surfaces, but the brightness and distance here push the instrument to its limits.
Why the Coma Shines
The coma is formed as the comet approaches the Sun. Heat and radiation heat ices inside the nucleus, releasing gas and dust that expand into a glowing envelope around the core. In the observed images, the coma’s glow fades with distance from the nucleus, blending into background noise and making the tail—often hundreds of thousands to millions of kilometres long—too faint to detect in these early frames.
Spectral Observations and the Challenge of Faint Light
In addition to imaging, scientists hoped to obtain spectral data from the coma and tail using Mars Express’s OMEGA and SPICAM spectrometers and ExoMars TGO’s NOMAD spectrometer. However, the faintness of 3I/ATLAS compared to typical targets makes spectral characterization difficult. The exposure time on Mars Express was only 0.5 seconds, the maximum for the spacecraft, whereas ExoMars enjoyed longer exposures of around five seconds. The result is that a definitive spectral fingerprint remains elusive for now, though analysts will continue aggregating data to improve signal detection.
Ongoing Analysis and What Comes Next
Despite the current limitations, data from both orbiters are being combined and reprocessed as scientists search for hidden signals. By stacking multiple images from Mars Express and refining processing techniques, researchers may discern the faint presence of the comet in ways not immediately visible. In parallel, the scientific team is assessing whether the coma and any emerging tail show distinctive spectral features that can illuminate the material making up 3I/ATLAS.
Colin Wilson, a project scientist for Mars Express and ExoMars, underscored the excitement of the encounter: “Though our Mars orbiters continue to make impressive contributions to Mars science, it’s always extra exciting to see them responding to unexpected situations like this one.”
A Rare Interstellar Visitor and a Path Forward
3I/ATLAS stands apart as only the third interstellar comet detected to date, joining 1I/’Oumuamua and 2I/Borisov. Such objects are foreign to the Solar System, offering a tangible connection to the broader galaxy and formative processes beyond our cosmic neighborhood. Discovered on 1 July 2025 by the ATLAS survey in Chile, 3I/ATLAS has sparked a global effort to chart its trajectory and physical properties as it speeds toward the Sun.
What’s Next for ESA and Other Missions
Looking ahead, the juice of interstellar curiosity continues. ESA plans to observe 3I/ATLAS again with the JUICE mission next month, aiming to capture the comet after its perihelion, when activity is expected to peak. While JUICE will be farther away, its vantage just after the Sun’s closest approach could reveal a more active, brighter coma. Data from JUICE is anticipated in early 2026, shedding further light on this extraordinary visitor.
A Mission of Discovery
The broader goal is clear: to understand how interstellar materials differ from those born in our Solar System and to test the limits of rapid-response planetary science. Initiatives like the Comet Interceptor mission, planned for 2029, embody this spirit by preparing to intercept pristine comets from the Oort Cloud or, in rare cases, interstellar objects. As Michael Kueppers notes, these explorations could unlock new knowledge about the cosmos and our place within it.
