On October 2, 2025, the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter rotated away from the Martian surface it normally photographs and caught a pixelated white smudge instead: comet 3I/ATLAS, passing close enough to the planet that MRO recorded one of the sharpest views any spacecraft is likely to get of an interstellar visitor. Shane Byrne, HiRISE’s principal investigator at the University of Arizona, called the encounter a fortunate accident of timing, since the comet’s path happened to carry it close to a planet NASA already had multiple spacecraft watching.
Only two other objects like it have ever been confirmed passing through the solar system, and this one is not coming back. Once 3I/ATLAS finishes its current arc past the Sun and the planets, its trajectory carries it back out into interstellar space for good, on a path that never loops around to return.
The comet was first spotted by the NASA-funded ATLAS survey telescope in Rio Hurtado, Chile, which reported the sighting to the Minor Planet Center on July 1, 2025, after archival searches later turned up pre-discovery images dating back to June 14 of that year. Its name follows a simple convention: the letter for interstellar, the number for the third such object ever confirmed, and ATLAS for the telescope network that found it.
Shane Byrne’s camera gets the closest view any spacecraft will manage
MRO observed the comet from about 19 million miles away that day, according to NASA’s account of the Mars-based observations, a distance close enough that the resulting image resolved the coma, the cloud of dust and ice the comet sheds as it moves, at a scale of roughly 19 miles per pixel. Leslie Tamppari, MRO’s project scientist at NASA’s Jet Propulsion Laboratory, described the flyby as an unusual bonus for a spacecraft built primarily to study a different planet, noting that MRO occasionally gets the chance to observe a passing object well beyond its normal mission.
The comet was never a danger to Mars or Earth. NASA’s own overview of the object puts its closest pass by Earth at about 1.8 astronomical units, or roughly 170 million miles, on December 19, 2025, and its closest approach to the Sun at about 1.4 astronomical units on October 30, 2025, a point just inside the orbit Mars itself traces around the Sun. The Mars flyby that MRO captured sat well within that broader path, close enough to make the planet’s own spacecraft useful observation posts rather than distant bystanders.
MAVEN’s ultraviolet eyes and a faint smudge from Perseverance
Two other Mars missions turned toward the comet the same week. Over ten days starting September 27, 2025, the MAVEN orbiter’s Imaging Ultraviolet Spectrograph mapped hydrogen escaping from the comet’s coma, letting scientists estimate how much water vapor the comet released as sunlight warmed it. Shannon Curry, MAVEN’s principal investigator at the University of Colorado Boulder’s Laboratory for Atmospheric and Space Physics, described the detections as significant enough that the team had barely begun working through the data. Justin Deighan, the mission’s deputy principal investigator, called the observing run an adrenaline-filled stretch that opened up a new understanding of how interstellar objects behave up close.
On the surface itself, Perseverance’s Mastcam-Z camera caught 3I/ATLAS as little more than a faint smudge on October 4, 2025, while the rover explored the rim of Jezero Crater roughly 19 million miles from the comet. The exposure had to run far longer than a typical shot, since the camera stays fixed rather than tracking a moving target, which is why the comet barely registers against the streaked star trails around it.
A one-way hyperbolic path with no return trip
What makes a repeat visit impossible is the shape of the orbit itself. NASA’s overview describes 3I/ATLAS as following a hyperbolic trajectory, meaning it moves too fast to be gravitationally bound to the Sun and never settles into a closed loop the way a planet or a typical comet does. Trace its path backward and it clearly originates from outside the solar system entirely; trace it forward and it exits just as permanently, headed back into interstellar space once it finishes the pass astronomers spent the back half of 2025 observing.
It is only the third such object confirmed, following ‘Oumuamua in 2017 and 2I/Borisov in 2019, according to NASA’s facts and FAQ page on the object, which also puts the upper limit on the comet’s nucleus at 3.5 miles across, with a lower bound as small as 1,444 feet. That size estimate traces back to Hubble Space Telescope images from July 21, 2025, processed with contributions from astronomer David Jewitt of UCLA, who has spent much of his career studying small bodies at the solar system’s edges. The James Webb Space Telescope later picked up its own signal from the object, detecting chemical clues to its ancient origin as the comet moved away from the Sun in December 2025, a final round of data collection before the object continues on its way out of the solar system for good.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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