ESA’s ExoMars and Mars Express observe interstellar comet 3I/ATLAS
NASA/ESA’s ExoMars Trace Gas Orbiter (TGO) and the Mars Express mission have joined a historic moment in planetary science by observing the interstellar comet 3I/ATLAS. The observations, captured with the Colour and Stereo Surface Imaging System (CaSSIS) on TGO, show a faint but telling picture of a world visitors rarely bring: an icy-rocky nucleus surrounded by a luminous coma, set against the Martian sky as the spacecraft glided through its orbit.
What CaSSIS reveals and what it can’t
In the images and a glowing GIF released by ESA, 3I/ATLAS appears as a slightly fuzzy white dot near the image centre. CaSSIS is powerful enough to detect the coma, a hazy envelope of gas and dust released as the comet approaches the Sun, but it cannot resolve the nucleus itself. The nucleus of 3I/ATLAS is thought to be a kilometre-sized body, an object so small in cosmic terms that resolving it from Mars’ distance would be as hard as spotting a mobile phone on the Moon.
The coma, however, stretches across thousands of kilometres. As solar heat loosens ices inside the nucleus, gas and dust expand into space, forming a visible halo around the nucleus. The bright inner coma fades with distance, blending into the image noise and preventing a full measurement of the coma’s true extent with CaSSIS.
Why the coma matters for science
The appearance of the coma is a snapshot of the comet’s activity as it warms. While the tail—often extending millions of kilometres—usually lags behind, it doesn’t always show up in CaSSIS images. The brightness of the dust and gas declines rapidly with distance from the nucleus, complicating direct observation of the tail in these early exposures. Still, the data lay the groundwork for future observations as the comet approaches the Sun.
Nick Thomas, the CaSSIS principal investigator, notes the challenge: the comet is 10,000 to 100,000 times fainter than CaSSIS’s usual targets. Such faintness makes every pixel of data precious and pushes scientists to combine multiple observations and spectroscopic data to glean insights.
Spectral attempts and future observations
Scientists have tried to characterize 3I/ATLAS spectrally using Mars Express’s OMEGA and SPICAM instruments, along with ExoMars TGO’s NOMAD spectrometer. Early results remain inconclusive about spectral features, largely because the coma’s brightness was not sufficient for a clear spectral signature at the observed exposure times. The teams will continue co-adding images from Mars Express and refining exposure strategies to improve the chances of a spectral fingerprint.
In addition to ongoing Martian observations, ESA plans to study 3I/ATLAS with the Jupiter Icy Moons Explorer (Juice) next month. Juice will observe the comet just after its perihelion, when activity should be higher. Although Juice will be farther away from the comet, it is expected to provide valuable context about the comet’s activity as it heats up, with data anticipated in early 2026.
Looking ahead: from comet to interception
Interstellar objects like 3I/ATLAS offer extraordinary clues about planetary formation beyond our Solar System. ESA continues to expand its high-profile investigations, including the planned Comet Interceptor mission, designed to respond rapidly to discoveries of pristine comets, potentially including interstellar visitors. This mission would wait in space for a suitable target, providing one of the first real “tickets” to visit a fresh interstellar body if such an object becomes reachable.
Colin Wilson, ESA’s project scientist for Mars Express and ExoMars, emphasizes the broader significance: even when data tease rather than reveal, the response of orbiters to unexpected visitors showcases the flexibility and resilience of space science missions in deepening our understanding of the cosmos.
What happens next?
Over the coming weeks and months, analysts will combine datasets from ExoMars, Mars Express, and ground-based observatories to refine the picture of 3I/ATLAS’s composition and activity. The ongoing work not only enriches our knowledge of this interstellar traveler but also demonstrates the potential for future rapid-response missions to study far-flung visitors to our Solar System.
For space enthusiasts and researchers alike, 3I/ATLAS remains a rare, instructive guest—one that connects us to a galaxy far beyond our own and hints at what discoveries still lie beyond the next bend of interstellar space.
