Traveling at a blistering 130,000 miles per hour through the solar system, interstellar comet 3I/ATLAS has become the focus of one of NASA’s most technically challenging multi-platform observation campaigns to date. In early October 2025, the comet made its closest pass to Mars-just 30 million kilometers away-offering a rare opportunity for spacecraft already in orbit and on the surface to capture unprecedented optical and ultraviolet data from another star system’s relic.
The Mars Reconnaissance Orbiter (MRO) provided some of the sharpest visible-light images yet made of an interstellar object. Using its High-Resolution Imaging Science Experiment (HiRISE) camera usually employed to map Martian terrain the spacecraft pivoted to track the comet, a move duplicated during the 2014 flyby of comet Siding Spring. With resolution of about 30 kilometers per pixel, the images show the diffuse coma, the 1,500-kilometer-wide halo of dust and gas created by solar heating. These frames will enable scientists to refine size estimates for the nucleus, believed from other observations to be between 320 meters and 5.6 kilometers across, and to study coma particle color and distribution.
Meanwhile, the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter used its Imaging Ultraviolet Spectrograph (IUVS) to conduct a 10-day chemical survey from September 27 to October 7. MAVEN’s multi-wavelength UV imaging and high-resolution spectroscopy mapped hydrogen emissions from three distinct sources: Mars’ own atmosphere, the interplanetary medium, and water vapor outgassing from 3I/ATLAS. This dataset also provided an upper limit on the comet’s hydrogen-to-deuterium ratio a key isotopic fingerprint that can constrain its formation zone in its parent system. “The images MAVEN captured truly are incredible,” said Shannon Curry, MAVEN’s principal investigator at the Laboratory for Atmospheric and Space Physics. “The detections we are seeing are significant, and we have only scraped the surface of our analysis.”
Complementary ground-based and infrared observations have been made, from which spectroscopic modeling has revealed that the reflectance spectrum of the coma closely matches that of D-type asteroids mixed with about 30% large-grain water ice. The near-infrared slope flattens beyond 1.5 microns, consistent with the optical properties of ~10-micron ice grains particles sufficiently massive to withstand solar radiation pressure and remain concentrated sunward. The implication is that water ice sublimation is the main driver of activity, although super-volatiles like CO or CO₂ may also contribute, based on this composition and on the lack of strong CN gas emissions at large heliocentric distances.
On October 4, the Mastcam-Z camera on Perseverance made a technically challenging detection from the Martian surface. The comet was a faint point amid star trails in the long-exposure frames: star trails are inevitable as it is not possible for the rover to track a moving target. These images are thus scientifically valuable in constraining brightness from the fixed vantage point of a planet despite their visual subtlety, very rarely done for such a distant, fast-moving target.
On its inbound trajectory, 3I/ATLAS had already shown signs of unusual chemistry. James Webb Space Telescope spectroscopy measured an exceptionally high CO₂-to-H₂O ratio of about 8:1, one of the largest ever recorded in any comet, indicating that it might have formed near a CO₂ ice line in a cold, possibly radiation-rich region of its natal system. The comet’s properties, including its D-type-like dust and large water ice grains, thus blur the compositional distinctions formerly assumed to exist between interstellar and solar system comets.
Trajectory modeling has improved by an order of magnitude in precision regarding its outbound path due to parallax measurements from ESA’s ExoMars Trace Gas Orbiter at Mars. This is a crucial improvement for scheduling follow-up observations as the comet recedes, especially with assets such as JWST and possible serendipitous encounters by deep-space probes.
As 3I/ATLAS emerges from solar conjunction, it is showing a significant brightening, increased ultraviolet and X-ray activity, and color changes in its coma indications of its changing outgassing as it comes to perihelion. It will pass by Earth on December 19 at 2 astronomical units, posing no threat but offering one of the last clear windows to study an object forged in another planetary system before it disappears into interstellar space once more.
