An interstellar visitor lights up the Red Planet’s sky

Of all ESA spacecraft, the two Mars orbiters had the best view of this rare visitor. During its closest approach to the Red Planet on October 3, the comet was about 30 million km away.

Both orbiters used their onboard cameras to track the comet's motion. These instruments are typically designed to capture Mars's bright surface from just a few hundred to a few thousand kilometers above it, so observing such a faint object at such a great distance presented a major challenge.

ExoMars TGO successfully recorded a series of images using its Colour and Stereo Surface Imaging System (CaSSIS). In the animation below, comet 3I/ATLAS appears as a faint white spot drifting downward near the center. That small blur marks the heart of the comet, made up of its icy, rocky nucleus surrounded by a glowing cloud called the coma.

Because the comet was so distant, CaSSIS could not separate the nucleus from the coma. Detecting the nucleus itself would have been like trying to spot a mobile phone on the Moon from Earth.

The coma, however, is clearly visible. Stretching several thousand kilometers across, it forms as sunlight warms the comet, releasing gas and dust that collect into a hazy halo around the core.

CaSSIS could not measure the coma's full extent because its brightness fades rapidly with distance from the nucleus, eventually disappearing into background noise.

Usually, gas and dust from the coma stream away to create a long tail that can stretch for millions of kilometers as the comet nears the Sun. The tail is much dimmer than the coma, so it does not appear in the current images, but it could become visible in future observations as 3I/ATLAS heats up and sheds more ice.

Nick Thomas, Principal Investigator of the CaSSIS camera, said, "This was a very challenging observation for the instrument. The comet is around 10,000 to 100,000 times fainter than our usual target.

The work continues

So far, 3I/ATLAS has not been detected in the Mars Express images. One reason is that Mars Express can only take exposures of up to 0.5 seconds (its maximum limit), while ExoMars TGO was able to use five-second exposures.

Researchers are continuing to process the data from both orbiters. They plan to combine multiple Mars Express images to enhance the faint signal and improve their chances of detecting the comet.

The teams also attempted to study the comet's light spectrum using the OMEGA and SPICAM spectrometers on Mars Express and the NOMAD instrument on ExoMars TGO. It remains uncertain whether the coma and tail were bright enough to allow scientists to identify the comet's chemical composition.

Over the coming weeks and months, researchers will keep analyzing the data to learn more about the materials that make up 3I/ATLAS and how it changes as it draws closer to the Sun.

Colin Wilson, Mars Express and ExoMars project scientist at ESA, said: "Though our Mars orbiters continue to make impressive contributions to Mars science, it's always extra exciting to see them responding to unexpected situations like this one. I look forward to seeing what the data reveals following further analysis."

A rare visitor

Originating from outside our Solar System, comet 3I/ATLAS is only the third interstellar comet ever seen, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019.

These comets are absolutely foreign. Every planet, moon, asteroid, comet and lifeform in our Solar System share a common origin. But interstellar comets are true outsiders, carrying clues about the formation of worlds far beyond our own.

Comet 3I/ATLAS was first spotted on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile. Since then, astronomers have used ground-based and space telescopes to monitor its progress and discover more about it.

Based on its trajectory, astronomers suspect that 3I/ATLAS could be the oldest comet ever observed. It may be three billion years older than the Solar System, which is itself already 4.6 billion years old.

What's next?

Next month, we will observe the comet with our Jupiter Icy Moons Explorer (Juice). Though Juice will be further from 3I/ATLAS than our Mars orbiters were last week, it will see the comet just after its closest approach to the Sun, meaning that it will be in a more active state. We don't expect to receive data from Juice's observations until February 2026 -- find out why in our FAQs.

Icy wanderers such as 3I/ATLAS offer a rare, tangible connection to the broader galaxy. To actually visit one would connect humankind with the Universe on a far greater scale. To this end, ESA is preparing the Comet Interceptor mission.

Comet Interceptor is due to launch in 2029 into a parking orbit, from where it will lie in wait for a suitable target -- a pristine comet from the distant Oort Cloud that surrounds our Solar System, or, unlikely but highly appealing, an interstellar object like 3I/ATLAS.

Michael Kueppers, Comet Interceptor project scientist expands: "When Comet Interceptor was selected in 2019, we only knew of one interstellar object -- 1I/ʻOumuamua, discovered in 2017. Since then, two more such objects have been discovered, showing large diversity in their appearance. Visiting one could provide a breakthrough in understanding their nature."

While it remains improbable that we will discover an interstellar object that is reachable for Comet Interceptor, as a first demonstration of a rapid response mission that waits in space for its target, it will be a pathfinder for possible future missions to intercept these mysterious visitors.