What do you do when you want to observe something that's on the other side of the Sun?
That's the problem astronomers faced as October 2025 rolled around and comet 3I/ATLAS, the mysterious interstellar visitor, became lost in solar glare, as seen from Earth.
More on 3I/ATLAS
They knew the deep-space interloper would be lost from our view just as it was making its closest approach to Mars.
The solution? Get some of the many rovers and spacecraft we have on Mars to try and take a look at it.
And that's exactly what's just happened.
Between 1 and 7 October, the European Space Agency's ExoMars Trace Gas Orbiter and Mars Express spacecraft turned their cameras towards comet 3I/ATLAS as it passed close to Mars.
These two spacecraft, which are orbiting the Red Planet, had the closest view of the comet of all the space agency's Martian fleet.
On 3 October, 3I/ATLAS passed just 30 million km from them.
A visitor from another star
The vast, vast majority of comets that we see in the night sky originate from an icy ring of rocks and space debris on the edge of our Solar System.
But not 3I/ATLAS. It's one of only three interstellar comets ever discovered in our Solar System.
That means it originated from a different star system in our Galaxy.
And it's not orbiting the Sun, it's passing by the Sun, meaning its presence in our cosmic neighbourhood is only fleeting, and will soon be lost from view forever.
Since it was discovered in July 2025, astronomers have been pointing our most powerful telescopes at the comet, leading to some amazing images of 3I/ATLAS that show exactly what a deep-space visitor looks like.
They've been able to learn about its composition, including that it contains carbon dioxide, water ice, water vapour and carbon monoxide.
One study even concluded that comet 3I/ATLAS is likely the oldest comet we've ever seen. It could be at least 7 billion years old, making it almost twice as old as our Sun.
What ESA's Mars missions saw
The European Space Agency's ExoMars Trace Gas Orbiter and Mars Express spacecraft used their cameras to observe 3I/ATLAS in the Martian sky.
However, these cameras were designed to photograph the surface of Mars just a few thousand km below them, not to observe dim, distant comets in the sky.
Hence, scientists weren't sure exactly what they'd see.
ExoMars TGO used its Colour and Stereo Surface Imaging System (CaSSIS) to capture 3I/ATLAS, which appears as a white dot, tracing across Mars's sky in an animation produced by stitching together individual frames.
The dot is the centre of the comet: its icy-rocky nucleus and its surrounding coma.
The nucleus and coma are indistinguishable in the TGO images.
ESA scientists say capturing a distinctive image of the nucleus would have been like trying to see mobile phone on the Moon from Earth.
The comet's coma stretches a few thousand km across, and is generated as frozen ice and gas defrost, due to 3I/ATLAS getting closer to the Sun.
That's why these images, as low-res as they may appear, are so important.
3I/ATLAS is getting more active as it approaches the Sun, so every piece of data, every observation, is vital.
As it continues to get closer to the Sun, this defrosting material is swept behind the comet in a huge tail.
The tail is too dim to be seen here, but ESA says it may be visible in future images as it continues to grow.
"This was a very challenging observation for the instrument.," says Nick Thomas, Principal Investigator of CaSSIS.
"The comet is around 10,000 to 100,000 times fainter than our usual target."
Mars Express images do not yet show 3I/ATLAS, ESA says, because its camera's exposure time – its light-gathering limit – is too short at just 0.5 seconds, compared to TGO's 5 seconds.
But further analysis and image processing techniques may yet bring 3I/ATLAS to light in Mars Express images.
Seeing the light
Scientists were able to use Mars Express’s OMEGA and SPICAM spectrometers, along with ExoMars TGO’s NOMAD spectrometer, to measure and study the light coming from 3I/ATLAS.
This technique, where astronomers analyse an object's spectrum, can tell them a lot about what the object is made of.
"At this point, it is uncertain whether the coma and tail were bright enough for a spectral characterisation," ESA says.
"Scientists will keep analysing the data over the next weeks and months to try to figure out more about what 3I/ATLAS is made of and how it is behaving as it approaches the Sun."
"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," says Colin Wilson, Mars Express and ExoMars project scientist at ESA.
"I look forward to seeing what the data reveals following further analysis."
Onwards to Jupiter
ESA isn't finished with 3I/ATLAS yet.
In November 2025, the space agency plans to use the Jupiter Icy Moons Explorer (Juice) mission to study it, as it passes close by the spacecraft.
Juice is on its way to Jupiter, where it will study the planet's icy moons to search for signs of life.
Though Juice will be further from 3I/ATLAS than the Mars orbiters were, it will have the advantage of seeing the comet at its most active, just after its close approach to the Sun.
However, ESA won't get data from Juice's observations until February 2026.
