Who knew a camera built for the study of dusty Martian dunes would, one day, picture a cosmic bullet flying by at 130,000 miles per hour Indeed, it has been a remarkable feat to imagine NASA’s High Resolution Imaging Science Experiment, HiRISE, aboard the Mars Reconnaissance Orbiter, capturing the image of the interstellar comet 3I/Atlas from as close as 19 million miles-the nearest view taken by any spacecraft or telescope while passing through the solar system.
HiRISE wasn’t built to image such a target. Constructed to map Mars in great detail resolving features down to nine inches per pixel it typically scouts out boulders, whirlwinds of dust, and rover landing sites. To turn the instrument toward a fast-moving and faint comet took extraordinary engineering precision. The team had to compensate for the thin but image distorting atmosphere of Mars, suppress stray sunlight reflecting off the Martian surface, and avoid bright background stars that might overwhelm the comet’s dim glow. Resolution at this distance fell to about 19 miles per pixel, but it still yielded scientists a rare data set on the comet’s coma and possibly its jets.
3I/Atlas is the third confirmed interstellar visitor to our solar system, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Its hyperbolic trajectory and extreme velocity about 137,000 mph to start and increasing to 153,000 mph at perihelion prove it is unbound to the Sun’s gravity. Observations from the Hubble Space Telescope constrain its icy nucleus to a diameter between 1,400 feet and 3.5 miles. The coma is the diffuse cloud of dust and gas driven off by solar heating; it could span thousands of kilometers, with jets possibly aimed sunward as the comet’s surface sublimates.
HiRISE imaging could help measure the comet’s rotation rate before and after closest approach to the Sun, which might provide some insight into how solar heating alters spin dynamics of interstellar bodies. Such data are particularly valuable, because 3I/Atlas exhibits behavior very unlike typical solar system comets: its spectral slope, coma chemistry, and jet structures differ markedly; some ground based spectroscopy has a red spectrum with unusual gas composition, dominated by carbon dioxide and cyanogen rather than water vapor. Tracking such a target is a technical challenge that underlines HiRISE’s versatility. The camera’s pointing system can rotate MRO away from Mars onto distant objects a capability which had previously been used in 2014 to image comet Siding Spring.
For 3I/Atlas, timing was everything: Earth-based telescopes were blinded by the Sun’s glare during the comet’s Mars flyby, leaving MRO as the only platform with a clear, close-range view. Other Mars assets joined in. The ultraviolet spectrograph on MAVEN mapped hydrogen and hydroxyl in the coma, constraining the comet’s water content and deuterium tohydrogen ratio key tracers of its origin. On the Martian surface, Perseverance’s Mastcam-Z captured a faint smudge in a long exposure, a testament to the comet’s dimness even at relatively close range.
Every observation is precious since such sightings are so rare; it is clear from current detection rates that many interstellar visitors pass through unobserved. The Vera C. Rubin Observatory, which was recently commissioned, will use its 3.2‑gigapixel LSST camera to scan the southern sky every few nights. It may find dozens of faint, fast-moving interstellar objects in the next decade. Estimates vary widely from fewer than two per year to as many as seventy-depending on assumptions about their size distribution and reflectivity.
For the moment, 3I/Atlas is a unique opportunity: its close passage offers a brief but deep view of material formed around another star, possibly billions of years ago. As Shane Byrne, HiRISE’s principal investigator, explained, “Observations of interstellar objects are still rare enough that we learn something new on every occasion.” In capturing this cosmic traveler, HiRISE has extended its legacy far beyond Mars, proving that a camera built for one world can, with a little bit of ingenuity, open a window to the galaxy.
