Comet 3I/ATLAS Caught On Camera From Mars Orbit: “This Was A Challenge”

Over the last week, the European Space Agency’s (ESA) spacecraft Mars Express and ExoMars Trace Gas Orbiter (TGO) were tasked with an unusual request; instead of looking down onto the Red Planet as they usually do, they looked into space. There, they tried to catch a glimpse of interstellar comet 3I/ATLAS.

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Team is still analyzing the data, but ESA was able to share a series of images taken by ExoMars TGO. The Colour and Stereo Surface Imaging System (CaSSIS) spotted the comet moving across its field of view.

What we are seeing is the coma, the fuzzy atmosphere of the comet due to sublimation of its surface material as it gets closer to the Sun. It would be impossible for the orbiters to see the kilometer-sized nucleus of the comet from 29 million kilometers (18 million miles) away. It would be the equivalent of seeing a mobile phone on the surface of the Moon from Earth.

“This was a challenge for our orbiters because, of course, their cameras were designed to look at objects only 1,000 kilometers away or less, and this was 30 million kilometers away,” Colin Wilson, ESA Project Scientist for ExoMars and Mars Express, told IFLScience. “[Comet 3I/ATLAS] was also very, very dim, over 10,000 times less bright than what we are usually looking at!”

Challenge notwithstanding, ExoMars definitely saw the comet and collected data, and the Mars Express observations are getting analyzed. The team did very well.  

“In terms of what the images reveal,  you can see the comet moving through the field of view,” Wilson told IFLScience. “We can certainly see it moving quickly [around 60 kilometers or 37 miles per second], and the other thing is you can see it's not quite symmetrical in the field of view, which again is what we expect from a comet.”

The observations were important because we can’t currently see the comet from Earth, as we are on the other side of the Sun from it. So, these views from Mars are the closest we are going to get of the comet, even though the spacecraft were not designed to comet hunt.

“The exact measurements of how bright it is will get added to the whole series of measurements from other telescopes to see how the brightness evolves along the trajectory of the comet as it approaches the sun. So that analysis is still to come, but we're pleased to be able to contribute to that,” Wilson told IFLScience.

These two orbiters have a history of being able to look beyond Mars. They have conducted observations of the transit of the two moons Deimos and Phobos in front of planets such as Jupiter, Earth, and Saturn. This is not even their first encounter with a comet. In 2013, Mars Express observed Comet Siding Spring, though it was a lot closer to Mars than Comet 3I/ATLAS.

ESA has other planned observations for this interstellar comet with its Jupiter-bound mission JUICE, which will take observations after the comet's closest passage to the Sun in a few weeks. This work is also informing a bold planned mission by the space agency, the Comet Interceptor mission, which will chase an interstellar object or a pristine comet from the distant Oort Cloud from 2029.