Highly anticipated new images of the interstellar comet 3I/ATLAS as it made its nearest pass to the planet Mars last week have been released by the European Space Agency (ESA), showcasing both the opportunities—and the challenges—that viewing the unusual object presents.
The new images, obtained by the ESA’s ExoMars Trace Gas Orbiter, reveal a view of the alien comet similar to those obtained in July by NASA’s Hubble Space Telescope, although at much closer range.
Additional data may have also been obtained by ESA spacecraft as 3I/ATLAS passed Mars, although the agency said it presently remains unclear whether the ExoMars or its sister spacecraft, the Mars Express orbiter, may have collected spectrographic information that could enable chemical analysis.
ESA Spacecraft Spot an Interstellar Visitor
Initially discovered in early July, 3I/ATLAS has captured the fascination of the astronomical community due to its unusual characteristics, which include its size, speed, and a range of behaviors that are not consistent with comets native to our solar system.
During the interstellar comet’s recent flyby past Mars, the ESA trained the cameras aboard its ExoMars and Mars Express orbiter toward the object, although the shorter exposure times of the latter spacecraft prevented it from being able to obtain any imagery of the object.
Fortunately, ExoMars is equipped with the Colour and Stereo Surface Imaging System (CaSSIS) camera, which has a resolution of 11.36 micro-radians (≈2.34 arcseconds) per pixel, translating to 340 km per pixel at 30 million km distance.
However, that resolution exceeds the expected diameter of the comet’s nucleus—somewhere between 5 and 46 km—which meant that the nucleus of the object remained unresolved in the recent images. Due to this, the comet appears as a small, fuzzy point of light in the new ESA imagery, an appearance caused largely by the growing coma that surrounds its main body as it moves closer to the Sun.
“This was a very challenging observation for the instrument,” said Nick Thomas, the ESA’s Principal Investigator for the CaSSIS camera, in a statement issued on Tuesday. “The comet is around 10,000 to 100,000 times fainter than our usual target.”
Size Estimates of an Enigma
Based on current data, the minimum size of 3I/ATLAS, based on the object’s non-gravitational acceleration limits, would have to be at least 5 km, although based on past observations by NASA’s SPHEREx mission, it could be much larger—as much as 46 km—based partly on the brightness it exhibits.
Additionally, the object’s CO₂ gas plume extends far beyond what CaSSIS was able to detect during last week’s observations, which is estimated to clock in at around 348,000 km based again on existing SPHEREx data.
Notably, during Hubble’s observations in July, the comet displayed a roughly 4,000 km-wide glow extending toward the Sun, with an implied length-to-width ratio of around 10. From the vantage point of the ESA’s spacecraft on Mars, this allowed CaSSIS to observe the object perpendicularly with relation to its Sun-oriented axis. Based on this, despite the challenges presented with attempting to view an object like 3I/ATLAS, the perpendicular orientation still offered more detail about the object in the direction of the glow.
3I/ATLAS’s Brightness and Outgassing
In terms of the comet’s brightness, CaSSIS was able to detect the bright inner region, which is roughly 680 km wide, although a faint outer envelope that had been visible during the Hubble Telescope’s observations earlier this year could not be discerned by the ESA’s spacecraft last week.
Another technical factor complicating the ESA’s recent observations of 3I/ATLAS involves the mirror within CaSSIS, which, at 13.5 cm wide, is close to 300 times smaller than the one used by Hubble. While this limits the resolution its imagery can convey, CaSSIS nonetheless relayed indications that the object’s brightness reveals no significant increase in outgassing displayed by the comet over the last three months of observations.
More Images, and More Problems
The ESA’s orbiters weren’t the only spacecraft positioned around Mars as 3I/ATLAS made its recent flyby. China’s Tianwen-1 mission, as well as the UAE’s Hope Orbiter, both may have obtained imagery of the comet, although these are expected to be much lower-resolution.
Right now, the most widely anticipated imagery of 3I/ATLAS is expected to come from the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter (MRO). With HiRISE imagery, we can expect to see 20 km-per-pixel resolution, which could mean that the forthcoming NASA imagery could provide one of the strongest constraints yet seen on 3I/ATLAS’s size.
However, given the current U.S. government shutdown, it remains anybody’s guess as to when the HiRISE images will become available to the public.
Fortunately, there are still more opportunities to observe 3I/ATLAS that will soon be on the horizon. Next month, the ESA’s Jupiter Icy Moons Explorer (JUICE) will have an opportunity to view the comet during what is likely to be its most active state following its closest approach to the Sun. Still, that data likely won’t be in hand until sometime next February.
Exciting New Opportunities Ahead
Altogether, the appearance of 3I/ATLAS has provided an exciting opportunity for astronomers, enough so that current efforts like the ESA’s forthcoming Comet Interceptor project are currently in development with a target launch date sometime in 2029. The mission will aim to locate a comet originating from within the Oort Cloud surrounding our solar system, although another possibility could be that a future detection of an interstellar object could offer a rare opportunity for the Comet Interceptor to get up close and personal with an alien comet like 3I/ATLAS.
While this possibility remains somewhat unlikely, it is far from impossible, according to Comet Interceptor project scientist Michael Kueppers, who says that ongoing future detections of interstellar objects could hold promise for the ESA’s ambitions to intercept comets in the years ahead.
“When Comet Interceptor was selected in 2019, we only knew of one interstellar object – 1I/ʻOumuamua, discovered in 2017,” Kueppers said this week.
“Since then, two more such objects have been discovered, showing large diversity in their appearance,” Kueppers adds. “Visiting one could provide a breakthrough in understanding their nature.”
For now, as we await NASA’s HiRISE data, the newest imagery of 3I/ATLAS offers additional data to the growing wealth of information astronomers hope to acquire about the latest interstellar visitor to our solar system.
“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,” said ESA project scientist Colin Wilson, who works on both the Mars Express and ExoMars missions.
“I look forward to seeing what the data reveals following further analysis,” Wilson said.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.