The new images from the High Resolution Imaging Science Experiment camera onboard NASA’s Mars Reconnaissance Orbiter will allow astronomers to better estimate the size of 3I/ATLAS, the third known interstellar object to pass through the Solar System.
This image of 3I/ATLAS was captured by the HiRISE camera onboard NASA’s Mars Reconnaissance Orbiter on October 2, 2025. Image credit: Image credit: NASA / JPL-Caltech / University of Arizona.
On October 2, 2025, Mars Reconnaissance Orbiter (MRO) observed 3I/ATLAS from 30 million km (19 million miles) away.
The orbiter’s team viewed the comet with the High Resolution Imaging Science Experiment (HiRISE) instrument, which normally points at the Martian surface.
By rotating, the spacecraft can point its camera at celestial objects as well — a technique used in 2014, when HiRISE joined MAVEN in studying another comet, Siding Spring.
“Observations of interstellar objects are still rare enough that we learn something new on every occasion,” said HiRISE principal investigator Dr. Shane Byrne, a researcher at the University of Arizona.
“We’re fortunate that 3I/ATLAS passed this close to Mars.”
Captured at a scale of roughly 30 km (19 miles) per pixel, 3I/ATLAS looks like a pixelated white ball on the HiRISE images.
“That ball is a cloud of dust and ice called the coma, which the comet shed as it continued its trajectory past Mars,” the researchers said.
Further study of the HiRISE images could help the scientists place an upper limit on the size of the comet’s nucleus, its central core made up of ice and dust.
The images could also reveal the properties of particles within the atmosphere surrounding the comet, called 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.
“One of MRO’s biggest contributions to NASA’s work on Mars has been watching surface phenomena that only HiRISE can see,” said MRO’s project scientist Dr. Leslie Tamppari, a researcher at NASA’s Jet Propulsion Laboratory.
“This is one of those occasions where we get to study a passing space object as well.”
“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,” said HiRISE co-investigator Professor James Wray, a researcher at the Georgia Institute of Technology.
“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.”
“To capture a glimpse of a visitor from another star system is extraordinary in itself,” said Dr. Tomás Díaz de la Rubia, senior vice president for research and partnerships at the University of Arizona.
“To do so from a University of Arizona-led instrument orbiting Mars makes it even more remarkable.”
“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.”