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Two European spacecraft orbiting Mars have captured rare images of an interstellar comet passing nearby. On October 3, ESA’s ExoMars Trace Gas Orbiter and Mars Express turned their cameras toward comet 3I/ATLAS as it came within about 30 million kilometers of the red planet. The close approach gave scientists an unusual chance to study material from beyond our solar system.
Comet 3I/ATLAS was first spotted in July 2025 by the ATLAS telescope in Chile. It is only the third interstellar comet ever confirmed, following ‘Oumuamua in 2017 and Borisov in 2019. These comets travel on hyperbolic paths, meaning they are not bound to the sun and will not return once they pass through.
Astronomers believe 3I/ATLAS may be far older than our solar system. Models suggest it formed about 7.6 billion years ago, roughly three billion years before the sun and planets. Its orbit shows that it likely escaped from a distant star system long ago and has been drifting through space ever since, carrying frozen material from another corner of the galaxy.
The ExoMars orbiter used its CaSSIS camera to take a series of long exposures, capturing the comet as a faint white spot moving across the sky. That blur represents the nucleus and its coma, the cloud of dust and gas released as sunlight warms the surface.
The nucleus itself is estimated to be only about a kilometer wide, too small to be resolved directly from such a distance. By comparison, scientists described it as like trying to see a phone from the surface of the moon.
The coma already measures thousands of kilometers across, though its edges are hard to see against the background. No tail has yet been detected, but astronomers expect one to form as the comet moves closer to the sun in the coming weeks.
Mars Express also attempted to record the event, but its shorter exposures made the comet more difficult to pick out. Teams are combining and processing multiple images to improve visibility. Both orbiters also tried to collect spectral data to identify gases escaping from the comet.
Capturing these images was a technical challenge. The orbiters are built to photograph the Martian surface, which is only hundreds of kilometers away. The comet, by contrast, was tens of millions of kilometers distant and tens of thousands of times dimmer. According to CaSSIS team leader Nick Thomas, the task pushed the limits of the spacecraft’s instruments.
Studying interstellar comets is important because they are made of material that formed around other stars. Small differences in their chemistry, such as isotope ratios or unusual ice content, help researchers compare how planetary systems form across the galaxy.
When astronomers studied 2I/Borisov, for example, they found unusually high levels of carbon monoxide, suggesting it came from a colder environment than our solar system.
Analysis of the Mars images will continue for several weeks, as scientists measure the coma’s size and look for chemical signatures. The comet will also be observed by ESA’s Juice spacecraft, which is currently on its way to Jupiter. Juice is expected to capture new data after 3I/ATLAS makes its closest approach to the sun. Because of the spacecraft’s distance, that information will not reach Earth until February 2026.
Looking ahead, ESA plans to launch the Comet Interceptor mission in 2029. The spacecraft will wait in space for a target, which could be an untouched comet from the Oort Cloud or even another interstellar visitor. Unlike current observations from afar, a direct encounter would allow scientists to study the surface and structure of such an object up close.
Source: ESA’s ExoMars and Mars Express observe comet 3I/ATLAS
