MRO Imagery Refines Size Estimates for Interstellar Comet 3I/ATLAS Near Mars

New analytical scrutiny of imagery captured by NASA's Mars Reconnaissance Orbiter (MRO) is providing astronomers with refined parameters for the interstellar comet 3I/ATLAS. This celestial body holds the distinction of being the third confirmed object originating from outside our Solar System to traverse its gravitational domain. The critical close encounter occurred as 3I/ATLAS navigated its trajectory in proximity to Mars.

On October 2, 2025, the MRO executed an observation of the comet when it was approximately 30 million kilometers, or 19 million miles, distant from the spacecraft. The orbiter's premier instrument, the High Resolution Imaging Science Experiment (HiRISE), was deliberately maneuvered away from its primary focus on Martian surface phenomena to track the visitor. This specific operational pivot mirrors a successful maneuver performed in 2014 to study the inbound Comet Siding Spring. Dr. Leslie Tamppari noted that the MRO's capacity allows it to study transient space objects alongside its core mission monitoring the Martian surface.

The resulting HiRISE images render 3I/ATLAS as a diffuse, pixelated white sphere, an appearance attributed to the dusty, icy coma the comet expelled during its solar system passage. These specific images were acquired at a spatial scale of roughly 30 kilometers per pixel, a resolution insufficient to resolve the solid nucleus, which Hubble Space Telescope observations estimate to be at most a few kilometers across. Ongoing, meticulous study of this data set is anticipated to establish a more definitive upper boundary for the comet's nucleus size. Dr. Shane Byrne, the HiRISE Principal Investigator, underscored the scarcity of observations involving interstellar objects, asserting that each event guarantees novel scientific acquisition.

Professor James Wray noted that all three interstellar objects cataloged to date—1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS—have exhibited pronounced divergences from one another and from typical comets native to our Solar System. Dr. Tomás Díaz de la Rubia characterized the successful capture of even a fleeting view of an object originating from another star system as extraordinary. The observation campaign was a coordinated effort; the MAVEN orbiter also secured ultraviolet images to probe the comet's composition and water vapor release, while the Perseverance rover managed a faint detection from the Martian surface on October 4, 2025.

Comet 3I/ATLAS was initially cataloged on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Chile. Its closest solar proximity, or perihelion, was registered on October 30, 2025, at a distance of 1.36 astronomical units, positioning it between the orbits of Earth and Mars. The object's closest approach to Mars occurred one day earlier, on October 3, 2025, at approximately 29 million kilometers. Interstellar objects like 3I/ATLAS offer a rare comparative opportunity, as Hubble observations suggest its nucleus diameter is constrained between 320 meters and 5.6 kilometers. The comet's high velocity, recorded at 58 km/s relative to the Sun, is the fastest among the three known interstellar visitors, supporting the hypothesis of an ancient origin.