A rare cosmic visitor from beyond our Solar System has drawn the attention of European scientists, offering a unique chance to study material older than the Sun itself.
What did ESA’s Mars orbiters see?
Between 1 and 7 October, ESA’s ExoMars Trace Gas Orbiter and Mars Express spacecraft turned their cameras towards comet 3I/ATLAS as it passed near Mars. During its closest approach on 3 October, the comet was around 30 million kilometres from the orbiters, making them the closest ESA missions to observe it.
The ExoMars orbiter’s Colour and Stereo Surface Imaging System captured faint images of the comet’s coma, the glowing cloud of gas and dust surrounding its icy nucleus. Scientists say the nucleus itself was too distant to be resolved, comparing the challenge to spotting a mobile phone on the Moon from Earth. The coma appeared clearly despite being thousands of times fainter than Mars’ surface, which the camera was originally designed to photograph.
Why is this observation significant?
Comet 3I/ATLAS is only the third interstellar comet ever detected, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Unlike objects born within our Solar System, interstellar comets originate from distant star systems and carry material that may be billions of years older than the planets around the Sun.
Astronomers believe 3I/ATLAS could predate the Solar System by around three billion years, making it potentially the oldest comet ever studied. First discovered on 1 July 2025 by the ATLAS telescope in Chile, it has since been tracked by ground and space telescopes.
Scientists are now analysing data from Mars Express and ExoMars, including spectral information that could reveal the comet’s composition. Colin Wilson, ESA project scientist, said the orbiters’ response to such an unexpected opportunity highlights their versatility and scientific value.
What comes next for interstellar research?
Further observations are planned as the comet continues its journey. ESA’s Jupiter Icy Moons Explorer (Juice) will observe 3I/ATLAS after it passes closest to the Sun, when its activity is expected to peak. The results from this encounter are not expected until February 2026.
Looking ahead, ESA’s Comet Interceptor mission, launching in 2029, aims to wait in space for a pristine comet or another interstellar visitor. Michael Kueppers, project scientist for the mission, said visiting such an object could transform understanding of how planetary systems form elsewhere in the galaxy.
While reaching 3I/ATLAS is unlikely, its fleeting visit offers a rare glimpse into material from beyond our Solar System, connecting humanity with the wider universe and deepening our understanding of how worlds beyond our own are formed.
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