Rare Interstellar Visitor Captured by Mars Orbiters
In a landmark moment for planetary science and astronomy, an interstellar comet—3I/ATLAS—passed close to Mars on October 3, offering a rare glimpse from Earth’s neighboring planet. The event marked only the third time humanity has observed a true interstellar object, following 1I/ʻOumuamua (2017) and 2I/Borisov (2019). As the comet traced its path through the inner Solar System, two European Space Agency (ESA) spacecraft studying Mars—the Mars orbiters—provided the best view yet from Mars’ vantage point.
How the View Was Achieved
ExoMars’ Trace Gas Orbiter (TGO), equipped with the Colour and Stereo Surface Imaging System (CaSSIS), successfully captured a sequence of images as 3I/ATLAS approached Mars. In one animation, the faint white spot near the center marks the comet’s core, while a surrounding glow, or coma, reveals the icy, rocky nucleus being heated by the Sun. The works of CaSSIS highlight the challenge: the bright, lively surface of Mars is a far cry from a distant, dim comet. The nucleus itself remained indistinguishable from the coma due to the great distance involved—like trying to spot a mobile phone on the Moon from Earth.
The coma, stretching across several thousand kilometers, formed as sunlight warmed the comet and released gas and dust into a hazy halo. At the observed distance, CaSSIS could not map the entire extent of the coma, whose brightness fades with distance from the nucleus and eventually merges into background noise. A long tail, a signature of many comets as they near the Sun, was not visible in these observations but could emerge in future data as 3I/ATLAS heats up further.
Data Challenges and Early Findings
Nick Thomas, the Principal Investigator for CaSSIS, called the observation exceptionally demanding. The comet’s brightness was 10,000 to 100,000 times fainter than CaSSIS’s typical targets, testing the instrument’s sensitivity and exposure capabilities. Mars Express, another ESA orbiter, provided parallel coverage but faced a different constraint: its maximum exposure time is 0.5 seconds, versus TGO’s five-second exposures. This disparity affected detection prospects for the nucleus and emphasized the need for data combining and signal enhancement in subsequent analyses.
Researchers are currently processing data from both orbiters, aiming to combine Mars Express imagery to boost the faint signal and heighten odds of detecting the nucleus. Spectral studies using OMEGA and SPICAM on Mars Express, as well as NOMAD on TGO, sought to characterize any light signatures from the coma and potential chemical fingerprints. Early results remain inconclusive about the coma’s precise chemistry, underscoring the difficulty of studying such a faint, distant object with in-situ instruments designed for Mars exploration rather than interstellar visitors.
Looking Ahead: What the Data May Reveal
The research teams anticipate possible future visibility of the comet’s tail as 3I/ATLAS warms further while drawing near the Sun. In the weeks and months ahead, more refined analyses may extract details about the materials composing the comet and how those materials evolve with solar heating. ESA scientists emphasize that every data pass brings us closer to understanding the nature of interstellar bodies and their place in our galaxy’s tapestry.
Colin Wilson, Mars Express and ExoMars project scientist, noted the broader significance: these orbiters repeatedly demonstrate their value beyond traditional Mars science, adapting to unexpected phenomena with potential breakthroughs in our understanding of the cosmos.
What’s Next for Interstellar Exploration
Beyond Mars, ESA plans to continue monitoring 3I/ATLAS with future facilities. A notable initiative is the Comet Interceptor mission, proposed to launch in 2029 to wait for a pristine comet or, intriguingly, another interstellar object. The concept envisions a rapid-response platform that could intercept and study such visitors, providing unprecedented insight into materials from distant star systems and the processes that forge planetary bodies across the galaxy.
Bottom Line
The October encounter offered a rare, tangible connection to objects formed far beyond our Solar System. While Mars orbiters may not have captured every detail, their observations amplify our understanding of interstellar visitors and lay groundwork for ambitious missions that could one day rendezvous with an alien comet or asteroid.
