Rare Interstellar Visitor Reaches the Red Planet’s Sky
On October 3, Mars-time turned astronomers’ attention to an extraordinary event: an interstellar visitor, comet 3I/ATLAS, swept through the inner Solar System and made a notably close approach to Mars. For the European Space Agency (ESA), this was a rare opportunity to test the limits of their orbital observatories as they watched a body originating from outside our own stellar neighborhood.
The two Mars orbiters with the best vantage—Mars Express and the ExoMars Trace Gas Orbiter (TGO)—turned their gaze outward, using their onboard cameras to track a comet that is typically far too faint to be seen from Mars. At its closest approach, the object was about 30 million kilometers away, a distance that blurs the line between a bright star and a small, active world in space.
CaSSIS Captures the Coma, Not the Nucleus
ExoMars TGO’s Colour and Stereo Surface Imaging System (CaSSIS) attempted to render the comet and achieved a challenging feat: a sequence of images in which the comet appears as a faint, drifting white spot near the frame’s center. The nucleus itself was too small and too distant to separate from the surrounding coma, a cloud of gas and dust that glows as sunlight heats the icy nucleus.
“The nucleus is the heart of the comet, but at this distance it’s as hard to discern as spotting a mobile phone on the Moon from Earth,” explained Nick Thomas, CaSSIS’s Principal Investigator. The coma, however, stretched across thousands of kilometers and became the clearest feature in the observations. It forms when sunlight drives off volatile ices inside the comet, creating a hazy halo around the core.
Limitations and Ongoing Analysis
CaSSIS’s sensitivity to bright sources is calibrated for Mars’ surface, not for faint, distant comets. The brightness of the coma fades as it extends outward, eventually blending into background noise and eluding full measurement. The result is a partial view: a glimpse of the coma but not a full spectral or physical profile of the entire object.
Typically, a visible ion or dust tail would emerge as the object nears the Sun, but in these observations the tail remained too faint. Researchers expect that continued monitoring could reveal the tail if the comet heats up further and sheds more material before it recedes away from Mars’ orbit.
Mars Express and TGO: Complementary Efforts
Mars Express, with a camera exposure limit of around 0.5 seconds, faced stricter constraints than ExoMars TGO, which could extend to about five seconds per frame. Despite this mismatch, scientists are combining data from both spacecraft to enhance the faint signal and improve the chances of detecting the comet’s nucleus and coma more clearly.
In parallel, scientists used spectrometers such as OMEGA and SPICAM on Mars Express and NOMAD on TGO to study the light spectrum of 3I/ATLAS. The goal is to identify chemical signatures within the coma, though it remains uncertain whether the combination of brightness and distance will yield decisive compositional data.
The Bigger Picture: What 3I/ATLAS Means for Science
3I/ATLAS is a landmark object: one of only a handful of interstellar visitors detected so far, joining 1I/ʻOumuamua and 2I/Borisov as genuine outsiders to our Solar System. Its trajectory and composition promise insights into the building blocks of other star systems, offering a rare chance to compare materials formed around other stars with our own.
As ESA scientists continue to process and reprocess the data, the hope is to illuminate how interstellar comets differ from their Solar System cousins and what their presence implies about planetary formation across the galaxy. The work also underscores the value of rapid-response missions that can pivot toward unexpected visitors in the vastness of space.
Looking Ahead: Juices, Interceptors, and a New Era of Discovery
Next month, the Jupiter Icy Moons Explorer (Juice) will study 3I/ATLAS from a greater distance, observing the comet just after it nears the Sun and becomes more active. While direct data may arrive as late as February 2026, these observations will complement Earth-based and orbital datasets, painting a more complete picture of this extraordinary traveler.
ESA’s Comet Interceptor project illustrates a bold vision: a mission designed to pause in space, awaiting the perfect interstellar or pristine comet encounter. By acting as a testbed for rapid-response operations and science-readiness, Comet Interceptor could pave the way for future gains in our understanding of objects that wander the galaxy far beyond our own Sun.
What’s Next?
Researchers will continue to analyze the available data, seeking answers about the comet’s size, activity, and composition. The extraordinary nature of 3I/ATLAS makes every new dataset valuable, and the growing archive of observations from Mars orbiters will feed researchers’ questions for years to come.
Colin Wilson, Mars Express and ExoMars project scientist at ESA, summarized the sentiment: “Our Mars orbiters continue to contribute to Mars science in surprising ways, and watching an interstellar visitor has been an especially exciting moment. We’ll see what the data reveal after further analysis.”
