A visitor from another star
In late 2025, Earth’s telescopes were chasing light from a visitor that didn’t belong to our solar system. 3I/ATLAS is not a typical comet born in the icy outskirts of the Sun’s domain; it arrived from another star and is currently just a fleeting guest in our celestial neighborhood. Discovered in July 2025, it offered researchers an extraordinary chance to study a body that formed around a different sun and traveled across interstellar space for eons before entering our cosmic vicinity.
As it swept past Mars, astronomers faced a practical challenge: observing something that would normally be lost to solar glare from Earth. A direct view from Earth would be impossible as the Sun would outshine the comet, so scientists turned to the Red Planet itself for help. The European Space Agency’s ExoMars Trace Gas Orbiter (TGO) and Mars Express took on the role of remote observers while orbiting Mars, the planet that had already become a scientific hub for solar system exploration.
How Mars orbiters watched a far-off visitor
Between October 1 and 7, 2025, ESA’s TGO and Mars Express trained their cameras toward 3I/ATLAS. The CaSSIS instrument on TGO rendered a sequence of frames into an animation, depicting a white dot streaking across the Martian sky. In these images, the nucleus and coma—the icy-rocky heart of the comet and its diffuse atmosphere—could not be separately resolved, a reminder that the object was still faint relative to Mars’ own features and light levels.
Mars Express, with instruments including OMEGA and SPICAM, attempted spectroscopic observations to study the lines in the comet’s light. Spectroscopy helps scientists determine the comet’s composition, revealing water ice, carbon dioxide, and other volatiles released as the sun’s heat defrosts the surface. However, there was a catch: the exposure time needed to collect a usable spectrum was limited by the spacecraft’s design and the comet’s faint signal. Despite this, the team remained optimistic that deeper processing could yield useful data later.
The science behind the observation
3I/ATLAS’s brightness and activity provide crucial clues. As the Sun approaches, ices sublimate, creating a coma and a forming tail that stretches far behind the nucleus. The Mars-based observations helped fill in gaps that Earth-based telescopes could not cover, offering a unique vantage point for measuring the comet’s evolution as it neared the Sun. Scientists are particularly interested in whether the coma’s composition includes unusual mixes that might hint at its origin in a distant star system.
“This was a very challenging observation for the instrument,” notes a mission scientist involved in CaSSIS analyses. “The comet is around 10,000 to 100,000 times fainter than our usual targets.” The effort underscores how even well-instrumented spacecraft designed for planetary science can contribute to deep-space discoveries when curiosity overrides mission boundaries.
What the data can tell us—and what comes next
The hope is that the collected data will help answer big questions about interstellar material and planetary formation across the galaxy. Analysts will compare Martian observations with ground-based telescope data and space-borne spectrometers to build a fuller picture of 3I/ATLAS’s makeup and behavior as it treads a solar-facing path. While the nucleus remains unresolved in Mars Express images, ongoing processing and future observations may sharpen those details or reveal subtle spectral features that point to its origin and age.
ESA researchers also plan to extend the hunt beyond Mars. In November 2025, the Jupiter Icy Moons Explorer (Juice) will attempt to observe 3I/ATLAS as it passes by the mission, potentially catching it at a peak in activity just after the Sun’s closest approach. Juice will be farther away than the Mars orbiters but gains by observing the comet when it is most active. Final data are expected in early 2026, contributing to a growing catalog of interstellar visitors and the stories they tell about our galactic neighborhood.
Reflections from the community
ESA scientists emphasize that the value of these observations lies as much in the process as in the results. Observing a faint interstellar visitor with instruments tuned for Mars’ own surface features demonstrates the flexibility and creativity of modern space science. “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,” says a project scientist from the ExoMars program. The collaborative effort across missions and institutions showcases how multiple platforms can converge to illuminate a single, stunning phenomenon.
Looking ahead
As 3I/ATLAS continues toward the inner Solar System, the scientific story is far from complete. Juiced by the data collected from TGO and Mars Express, researchers will refine models of interstellar material and plan further observations. The journey from discovery to understanding hinges on cross-mission data and the patience to parse faint signals—an ode to astronomy’s enduring spirit of exploration.
