Interstellar comet 3I/ATLAS will fly by Mars 1 month from now — and Europe’s Red Planet orbiters will be ready

The European Space Agency (ESA) is preparing for a unique opportunity to study the interstellar comet 3I/Atlas from what could be the best vantage point in the solar system.

Astronomers made the rare and extraordinary discovery of an interstellar object in our own solar system on July 1, 2025, sparking a scramble to study the mysterious visitor. The object has since been named comet 3I/ATLAS, where 3I stands for “third interstellar,” and designated C/2025 N1 (ATLAS).

As it turns out, it may be observations from Mars, not Earth, that give us our closest look at the comet. 3I/ATLAS will come as close as around 18.6 million miles (30 million kilometers) to Mars, while it will only come within 168 million miles (270 million km) of Earth. That means spacecraft orbiting Mars are in play.

Colin Frank Wilson, Project Scientist for ESA’s Mars orbiters, confirmed that the agency is preparing to attempt observations of the interstellar object 3I/ATLAS during its passage through the solar system.

“We can confirm that we are planning to observe 3I/Atlas around the time of its closest approach to Mars,” Wilson said. “This is predicted to occur on 3 October 2025. On this date, the object will still be approximately 30 million kilometres from Mars.”

ESA will employ both Mars Express and the ExoMars Trace Gas Orbiter (TGO) spacecraft to make observations.

“We will attempt to obtain images of the object using the High Resolution Stereo Camera (HRSC) onboard Mars Express, and also with the Colour and Stereo Surface Imaging System (CaSSIS) onboard TGO,” Wilson explained.

The comet will be too distant for a real close-up look. However, if 3I/ATLAS is bright enough, HRSC may be able to hint at whether it’s elongated, spherical or irregularly shaped. The first discovered interstellar interloper, 1I/’Oumuamua, was unusually elongated and more asteroidal, while 2I/Borisov was more like a typical comet. CaSSIS, meanwhile, could provide insights into 3I/ATLAS’ volatile activity and its composition.

“We will also attempt to measure the spectrum of the object, using the TGO’s NOMAD and MEx’s OMEGA and SPICAM spectrometers,” Wilson said.

These payloads could tease out evidence of molecular signatures such as water vapor or organics in 3I/ATLAS’ coma if it becomes active. “However, we do not have great hopes that the signal will be bright enough for spectral characterization,” Wilson said.

There are other spacecraft around Mars that could be repurposed to take advantage of the 3I/ATLAS close approach as well. These include NASA’s aging Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter and Mars Reconnaissance Orbiter (MRO). China also has its Tianwen-1 orbiter, which carries a high-resolution camera comparable to that of MRO.

Spacecraft studying worlds beyond Mars may be able to get a glimpse of 3I/ATLAS as well. For example, ESA’s Jupiter Icy Moons Explorer (JUICE), which launched in April 2023 and is scheduled to arrive in the Jovian system in July 2031, will give it a go.

“The JUICE spacecraft is also planning observations,” Joern Helbert, head of ESA’s Solar System Section, told Space.com. “Due to thermal constraints on the spacecraft, they are limited to a period in November of this year, and four instruments are planned to be used.”

And a submitted paper by T. Marshall Eubanks et al. notes that spacecraft including Europa Clipper, Hera and even the more distant Lucy probe may pass through 3I’s cometary tail in the period after its closest approach to the sun, or perihelion.

ESA considered utilizing the Hera probe — which launched in October last year to visit the asteroids Didymos and Dimorphos, to study the aftermath of NASA’s DART (Double Asteroid Redirection Test) mission — to look out for 3I/ATLAS. However, the comet was considered to be too faint and poorly illuminated from Hera’s location.

Closer to Earth, NASA’s James Webb Space Telescope (JWST) recently trained its scopes on 3I/ATLAS for the first time, yielding unexpected results.