New images from High resolution imaging science experiment camera on board NASA 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 3I/ATLAS image was captured by the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter on October 2, 2025. Image credit: Image credit: NASA/JPL-Caltech/University of Arizona.
On October 2, 2025, the Mars Reconnaissance Orbiter (MRO) observed 3I/ATLAS at a distance of 30 million kilometers (19 million miles).
The orbiter team observed the comet using the High Resolution Imaging Science Experiment (HiRISE), which is typically aimed at the surface of Mars.
While rotating, the spacecraft can also point its camera at celestial objects. 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 every time,” said HiRISE principal investigator Dr. Shane Byrne, a researcher at the University of Arizona.
“We are lucky that 3I/ATLAS passed so close to Mars.”
Shot at a scale of about 30 km (19 miles) per pixel, the 3I/ATLAS image appears as a pixelated white ball in HiRISE images.
“This ball is a cloud of dust and ice, called a coma, that the comet shed as it continued its path past Mars,” the researchers said.
Further study of the HiRISE images could help scientists establish an upper limit on the size of the comet's nucleus, its central core made of ice and dust.
The images can also reveal the properties of particles in the atmosphere surrounding the comet, called the coma.
Continued analysis of the images may even reveal nuclear fragments or plumes of gas that are sometimes released as comets break up over time.
“One of MRO's biggest contributions to NASA's work on Mars has been observing surface phenomena that only HiRISE can see,” said MRO project scientist Dr. Leslie Tamppari, a researcher at NASA's Jet Propulsion Laboratory.
“This is one of those cases where we can also study a passing space object.”
“Thanks to NASA's fleet of efficient spacecraft spanning the inner solar system, we can continue to observe this dynamic object from unique angles,” said HiRISE co-investigator Professor James Ray, a researcher at the Georgia Institute of Technology.
“To date, all three interstellar objects have demonstrated striking differences from each other and from typical solar system comets, so every new observation we make is precious.”
“Seeing a visitor from another star system is unusual in itself,” said Dr. Thomas Diaz de la Rubia, senior vice president for research and partnerships at the University of Arizona.
“The fact that this can be done with an instrument led by the University of Arizona 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 demonstrates how discovery tools serve science and the public good.”
