ESA’s Mars Orbiters Turn Their Eyes on a True Interstellar Visitor
In a surprising turn for planetary science, Europe’s ExoMars Trace Gas Orbiter (TGO) and Mars Express have recorded observations of the interstellar comet 3I/ATLAS. While the nucleus remains far too faint to resolve, the coma surrounding the comet’s icy-rocky core becomes visible in CaSSIS imagery, offering a rare glimpse into a visitor from outside our Solar System.
What CaSSIS Saw—and What It Couldn’t
The Colour and Stereo Surface Imaging System (CaSSIS) on TGO captured a series of images showing a small, fuzzy white dot moving downward near the image centre. This dot marks the center of 3I/ATLAS, but the instrument could not distinguish the nucleus from the coma because the comet was too distant. A comparison helps illustrate the challenge: resolving a kilometre-wide nucleus from tens or hundreds of thousands of kilometres away would be like trying to spot a mobile phone on the Moon from Earth.
Despite this, the coma, extending across thousands of kilometres, is clearly visible. The coma forms as the Sun heats the comet during its approach, releasing gas and dust that envelope the nucleus. The brightness and density of the coma fade with distance, limiting the ability to measure its full extent with CaSSIS.
Why the Coma and Tail Matter
Comets develop tails as solar radiation and solar wind push away material from the coma. In most cases, a long, diffuse tail emerges, potentially spanning millions of kilometres. In the current observations, the tail is too faint to be seen in the CaSSIS data, but it may reveal itself in future measurements as 3I/ATLAS warms further.
Instrumental Challenges and Collaborative Efforts
According to Nick Thomas, Principal Investigator of CaSSIS, the observation was exceptionally challenging: the interstellar comet is around 10,000 to 100,000 times fainter than typical targets. Mars Express and ExoMars used different exposure times—Mars Express data employed the maximum exposure of 0.5 seconds, while ExoMars recorded exposures of around five seconds—affecting the detectability of the comet. ESA scientists are now combining multiple images from Mars Express and refining processing methods to uncover any faint signals that may indicate the nucleus or the extended coma.
Spectrum Tryouts and What They Could Reveal
Researchers attempted to measure the spectrum of 3I/ATLAS with Mars Express’s OMEGA and SPICAM spectrometers and ExoMars’s NOMAD instrument. Early results remain inconclusive regarding whether the coma or tail emits enough light for a definitive spectral characterization. Spectral data could reveal the comet’s composition, offering clues about materials from a region outside our Solar System.
A Glimpse Into a Rare Visitor—and a Path Forward
3I/ATLAS is only the third confirmed interstellar object observed in our celestial backyard, joining 1I/’Oumuamua (2017) and 2I/Borisov (2019). The discovery, first announced on 1 July 2025 by the ATLAS telescope in Chile, has sparked renewed interest in how interstellar bodies form and evolve. Scientists hope continued observations—along with future missions like ESA’s Comet Interceptor and the upcoming Jupiter Icy Moons Explorer (Juice)—will shed more light on these mysterious wanderers.
What’s Next for Earth’s Neighbours in the Sky
Next month, the Juice mission will observe 3I/ATLAS after its closest solar approach. Although Juice will be farther away than the Mars orbiters were during their recent observations, it will capture the comet during a more active phase thanks to its proximity to the Sun. Data from Juice is expected around February 2026, with researchers outlining FAQs to explain the timing and expectations.
The Bigger Picture: Interstellar Objects and the Mission Roadmap
Interstellar comets carry unique clues about planetary formation and the processes at work in distant star systems. While the likelihood of intercepting such objects soon remains slim, missions like Comet Interceptor—designed to wait for a pristine comet from the Oort Cloud or, excitingly, an interstellar visitor—could revolutionize our understanding of these travelers. As ESA scientists note, even in the face of uncertainty, these observations push the frontier of solar system science and help interpret future encounters with otherworldly material.
Colin Wilson, ESA’s project scientist for Mars Express and ExoMars, reflects on the broader impact: “Our orbiters continue to make impressive contributions to planetary science, and it’s especially thrilling when they respond to unexpected visitors like 3I/ATLAS. The forthcoming analyses will illuminate what these data reveal about interstellar material and the Sun’s influence on such bodies.”
