The engineering lesson is simple: Sometimes the best observatory is the one already parked at another planet. When an interstellar comet was in a viewing geometry that kept Earth-based telescopes too close to the Sun, Mars unexpectedly became a lookout post close enough to resolve a faint spot into detail and chemistry.
This was accomplished by 3I/ATLAS, the third confirmed interstellar object that has passed through our solar system. During its flyby of Mars, three NASA spacecraft Mars Reconnaissance Orbiter (MRO), MAVEN, and the Perseverance rover swung from their primary missions to build a multi-instrumented portrait from orbit and on the surface. The best images of Mars were taken when the comet was 19 million miles away from the planet, a geometry that provided the best spacecraft view of the mission.
The HiRISE camera on MRO is designed to map Mars at a resolution that is simply breathtaking, but it can also serve as a high-precision tracking telescope if the spacecraft pointing, timing, and stray light problems all come together. To image 3I/ATLAS, the spacecraft pointed to track the comet’s motion a technique that had been successfully tested during the 2014 flyby of comet Siding Spring while taking care to avoid contamination from the thin Martian atmosphere and direct sunlight. The deep space images, with a resolution of about 30 kilometers per pixel, resolved a coma 1,500 kilometers across. Shane Byrne, HiRISE principal investigator, summed up the scientific attitude of these occasional passes: “Observations of interstellar objects are still rare enough that we learn something new on every occasion.”
MAVEN orbited the same target but with a different toolkit: ultraviolet spectroscopy. Over the course of a ten-day observation campaign beginning 27 September 2025, its Imaging Ultraviolet Spectrograph (IUVS) captured multi-wavelength images with the capability to distinguish signals from hydrogen and hydroxyl, which are directly traceable to the comet’s volatile composition. The science isn’t so much about making a pretty picture as it is about separating out the overlapping foregrounds: hydrogen from the comet, hydrogen from Mars, and hydrogen that exists in interplanetary space can all be in the same line of sight, but IUVS has the capability to separate them out by mapping where and how the signal increases. Shannon Curry, the principal investigator for MAVEN, described the findings as “incredible,” saying that the group has “only scratched the surface” of analysis. Justin Deighan, the deputy principal investigator for MAVEN, described the process of getting the first detection signals: “There was a lot of adrenaline when we saw what we’d captured.”
Perhaps the most significant finding from MAVEN is the suggested “upper limit on the deuterium-to-hydrogen (D/H) ratio, a diagnostic used throughout planetary science to assess the formation location and evolution of ices.” At the same time, other telescopes have added to the chemical analysis: James Webb data have been studied for a CO₂-to-H₂O ratio of 8:1, which is remarkably high compared to other comets that have been analyzed, and ground-based observations have detected a 16.16 ± 0.01-hour rotation period and dust mass loss rates of 0.3 to 4.2 kg/s. With all of these findings, the Mars data go from a “cameo appearance” to a set of checks in a distributed lab where morphology, rotation, and chemistry can be compared on different wavelengths and viewpoints.
The contribution of Perseverance came from the surface in a way that only a fixed rover could: in long exposures with Mastcam-Z, trailing the star field sufficiently to image it as trails while the comet was only a bright spot. While the images are not much, they are at least representative of the same object from a ground reference frame on another planet, in conditions that reflect the difficulties of fixed platforms observing rapidly moving objects.
Most of all, it shows what can be done in modern deep space missions when geometry turns against us. When Earth was occulted, comets were still visible from spacecraft around the solar system, and Mars, by virtue of proximity and design, offered the best chance at an interstellar object that would otherwise have been known only by its orbital elements and fading brightness.
