Rare Interstellar Visitor Reaches the Mars Scene
On October 3, a rare interstellar visitor, comet 3I/ATLAS, passed by Mars at a distance of about 30 million kilometers. While comets are common in our solar neighborhood, 3I/ATLAS is extraordinary because it originated outside the Sun’s gravity, offering a priceless glimpse into material from another star system. The event provided a unique test for ESA’s Mars-orbiting fleet and highlighted the capabilities—and limits—of modern planetary observatories in capturing faint, distant objects.
Two Orbiters, One Astounding View
Among all European Space Agency missions observing Mars, the two orbiters offered the best vantage point for this fleeting encounter. The ExoMars Trace Gas Orbiter (TGO) and Mars Express both tracked the comet with their onboard cameras, even though they are primarily tuned to Mars’s bright surface rather than a distant, dim speck in the sky. The challenge was immense: witnessing a faint object hundreds of millions of kilometers away with instruments designed for much closer, brighter targets.
CaSSIS: A Difficult Yet Illuminating Observation
The ExoMars TGO used its Colour and Stereo Surface Imaging System (CaSSIS) to capture a sequence of images. In the resulting animation, the comet appears as a faint white spot near the center—a tiny nucleus enshrouded by a glowing coma. The nucleus itself could not be resolved from the coma because the object was simply too distant; detecting the core would be akin to spotting a mobile phone on the Moon from Earth. Nevertheless, the coma—extending thousands of kilometers—became visible as sunlight warmed the ices, dust, and gas being shed by the comet.
Exposure Limits and Signal Challenges
CaSSIS’s data reveal a delicate balance: while the coma stood out, its brightness faded rapidly with distance from the nucleus, eventually merging with background noise and eluding full measurement. The tail, often a defining feature of comets, remained beyond the current sensitivity. It’s expected to become more detectable as 3I/ATLAS warms and loses more ice closer to the Sun, potentially growing a longer, fainter tail.
Mars Express and Data Processing Efforts
Meanwhile, Mars Express did not detect the comet in any single exposure so far. The mission’s cameras can capture exposures up to 0.5 seconds, whereas CaSSIS on TGO exploited five-second exposures to boost faint signal. Scientists are combining multiple Mars Express images to enhance the signal, hoping to tease out the presence of the comet and extend the science.
Spectral Clues and Ongoing Analysis
Teams also attempted to study 3I/ATLAS’s light spectrum with the OMEGA and SPICAM spectrometers aboard Mars Express, and the NOMAD instrument on ExoMars TGO. Early results remain inconclusive about the coma’s brightness and the detectability of chemical signatures; however, continued data processing over weeks and months aims to reveal the comet’s composition as it responds to solar heating.
What This Means for Interstellar Science
Comet 3I/ATLAS is only the third confirmed interstellar visitor after 1I/ʻOumuamua and 2I/Borisov. These travelers are cosmic messengers from beyond the solar family, carrying clues to planet and comet formation in distant star systems. The observation campaign demonstrates how ESA’s orbiters can contribute to unexpected discoveries, even when targets fall far outside their usual scope. A key takeaway is the need for rapid-response capabilities and cross-instrument collaboration to maximize the scientific return from rare events.
The Road Ahead: Juicy Observations and Interstellar Ambitions
Next month, ESA plans to observe 3I/ATLAS with the Juice mission, which will see the comet after its closest Sun approach. Although Juice will be farther away than the Mars orbiters were, it should provide insights as the body becomes more active. Data from Juice is anticipated in early 2026, with scientists outlining why these observations are crucial to understanding how interstellar objects evolve when exposed to solar radiation.
A Vision for the Future: Comet Interceptor
The broader roadmap includes the Comet Interceptor mission, designed to wait in space for a pristine comet from the distant Oort Cloud or, exceptionally, an interstellar object such as 3I/ATLAS. Although challenging, a successful interception would mark a leap forward in studying these rare visitors directly, offering a tangible link to the broader galaxy and the history of planetary formation across the cosmos.
Conclusion
As ESA’s orbiters continue to analyze the data, 3I/ATLAS remains a symbol of the unexpected. The observations underscore the value of cross-mission science, pushing the boundaries of what instruments optimized for Mars can reveal about objects born outside our Solar System. The coming years hold promise for unveiling more about the materials that comprise interstellar visitors and how they evolve as they travel through the warmth of our Sun.
