At the beginning of October, three of NASA's Mars spacecraft had front row seats to view 3I/ATLAS – only the third interstellar object observed in our solar system. Among them, NASA's Mars Reconnaissance Orbiter, or MRO, was in a prime position to snap images of the comet as it sailed by at a distance of 19 million miles (30 million kilometers) – the closest viewing opportunity any of the agency's missions or any Earth-based observatories are expected to get.

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On the evening of Oct. 2, the comet was observed and photographed by the High Resolution Imaging Science Experiment, or HiRISE, which is led by the University of Arizona and has been photographing the Martian surface for nearly two decades.

HiRISE is usually used to spot features as small as boulders (or even rovers) on the Martian surface. In HiRISE's images, 3I/ATLAS is seen at a scale of roughly 19 miles (30 kilometers) per pixel. The pixelated white ball is a cloud of dust and ice called the coma, which is shed as the comet approaches the sun.

"Observations of interstellar objects are still rare enough that we learn something new on every occasion," said Shane Byrne, HiRISE principal investigator and professor at the U of A Lunar and Planetary Laboratory. "We're fortunate, that 3I/ATLAS passed this close to Mars."

The HiRISE imagery is likely to reveal new details that could help scientists place an upper limit on the size of the comet's nucleus, its central core made up of ice and dust. It could also reveal the properties of particles within the atmosphere surrounding the comet, called its coma. Ongoing analysis of the images may even reveal fragments of the nucleus or jets of gas, which are sometimes released as comets break up over time. Throughout October, 3I/ATLAS was too close to the sun from Earth's position to be visible from most telescopes, giving MRO a unique view, according to HiRISE co-investigator James Wray, professor of earth and atmospheric sciences at the Georgia Institute of Technology.

Follow 3I/ATLAS' journey at NASA.gov.

"Thanks to NASA's fleet of capable spacecraft spanning the inner solar system, we can continue to observe this dynamic object, and from unique angles," he said. "All three interstellar objects to date have shown striking differences from each other and from typical solar system comets, so every new observation we make is precious."

The HiRISE camera normally points at the Martian surface, where it has revealed such otherworldly terrain as spider-like shapes formed by gaseous eruptions, frosted sand dunes, and blast-ringed impact craters. The camera's image quality makes it a vital asset for NASA to scout out suitable landing sites for robotic Mars rovers. HiRISE also is critical to preparing for the first astronauts on the Red Planet: The camera has identified safe landing sites and accessible water-ice deposits that will help humans survive in the harsh Martian environment.

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"To capture a glimpse of a visitor from another star system is extraordinary in itself. To do so from a University of Arizona-led instrument orbiting Mars makes it even more remarkable," said Tomás Díaz de la Rubia, senior vice president for research and partnerships at the University of Arizona. "This moment speaks to the ingenuity of our scientists and the enduring impact of this university's leadership in space exploration. HiRISE exemplifies how tools of discovery serve science and the public good."

In addition to its years-long, routine duty of imaging the planet's surface at a high enough resolution to spot details as small as a desk, HiRISE has made headlines for spotting rovers and landers as they studied the planet, including the Curiosity and Perseverance rovers.

Another of NASA's spacecraft orbiting Mars – MAVEN, short for Mars Atmosphere and Volatiles EvolutioN – viewed the comet using its ultraviolet camera, and while that imagery is still being processed, it should provide data for scientists to determine the composition and distribution of gases released by the comet into its coma and tail. On Mars' surface, the Perseverance rover observed the comet on Oct. 4, when it appeared as a faint smudge to the rover's Mastcam-Z camera system; the exposure had to be exceptionally long to resolve such a faint object.

"One of MRO's biggest contributions to NASA's work on Mars has been watching for passing phenomena on the surface, including dust devils the size of skyscrapers and avalanches careening down cliffs," said Leslie Tamppari of NASA's Jet Propulsion Laboratory in Southern California, which leads MRO's mission. "This is one of those rare occasions where we get to study a passing space object, as well."

The mission observed its first comet, called Siding Spring, in 2014. Appearing as a glowing ball in the night sky, it was the first high-resolution image ever taken of an object from the Oort Cloud, a vast, diffuse cloud of icy debris left over from the formation of the solar system.

Comet 3I/ATLAS will be at its minimum distance from Earth around Dec. 19, remaining about 10 times farther away than it got to Mars. 

The U of A operates the HiRISE instrument, which was built by BAE Systems in Boulder, Colorado. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the Mars Reconnaissance Orbiter Project for NASA's Science Mission Directorate in Washington as part of NASA's Mars Exploration Program portfolio.