Could Mars have just stolen Earth’s front-row seat to one of the rarest cosmic events in decades? In early October, NASA’s Mars Reconnaissance Orbiter (MRO), the MAVEN spacecraft, and the Perseverance rover captured unprecedented views of comet 3I/ATLAS-only the third confirmed interstellar object to visit our solar system. While Earth-based telescopes were blocked by the Sun’s glare during the comet’s perihelion, Mars was perfectly positioned to intercept this fleeting traveler from another star system.
Starting from 19 million miles away, MRO’s HiRISE camera locked onto the comet, repurposing an instrument normally tasked with mapping Martian terrain. That required precise rotation of the spacecraft to point HiRISE at a moving target in deep space, a technique first tested in 2014 when imaging comet Siding Spring. At about 19 miles per pixel, the resulting image shows a bright, pixelated sphere-the coma, a cloud of dust and ice driven off the nucleus by solar heating. Scientists expect this dataset will help constrain the nucleus size, analyze coma particle color and distribution, and possibly detect jets or fragments breaking away.
The view from MAVEN’s Imaging Ultraviolet Spectrograph was decidedly different, capturing multi-wavelength ultraviolet images for more than ten days beginning September 27, then transitioning to a high-resolution UV mode to isolate the hydrogen emissions from the comet. That allowed identification of such molecules as hydroxyl and quantification of water vapor release rates as the comet warmed. IUVS also yielded an upper limit on the D/H ratio in the comet’s water – a key isotopic fingerprint for tracing its birthplace. In planetary science, D/H ratios vary with the temperature and chemistry of the environment where icy bodies form; values close to those of Earth’s oceans suggest formation in warmer inner regions of a protoplanetary disk, whereas higher ratios point to colder, more distant origins. As MAVEN’s deputy principal investigator Justin Deighan noted, “Every measurement we make of this comet helps to open up a new understanding of interstellar objects.”
These isotopic studies complement decades of research on comets, including ESA’s Rosetta mission, which showed how dust in a coma can locally change the measured D/H values by releasing water enriched or depleted in deuterium. Laboratory experiments performed on dust grain surfaces show that they can preferentially adsorb heavy water, HDO, creating complex fractionation patterns. In the case of 3I/ATLAS, MAVEN’s capability to differentiate between the sources of hydrogen-Mars, interplanetary space, and the comet itself-means the isotopic readings will be as uncontaminated as possible.
Even at the same brightness and viewing geometry, this very faint smudge on the Martian surface required an exceptionally long exposure to capture with Perseverance’s Mastcam-Z on Oct. 4. Since the camera remains fixed during such exposures, stars appear as streaks and the comet itself is hard to pick out among them. While less visually striking, the technique results in a very useful confirmation of the comet’s brightness and position from Mars’ vantage point.
These Mars-based observations complement a solar system-wide campaign involving NASA’s heliophysics missions, asteroid-bound spacecraft such as Lucy and Psyche, and even ESA’s ExoMars Trace Gas Orbiter, which improved trajectory predictions by a factor of ten through triangulation with Earth-based data. The refined path allows astronomers to more precisely target instruments before the comet speeds back into interstellar space at over 130,000 mph.
3I/ATLAS is good practice for planetary defense specialists in how to track fast-moving, non-returning objects from multiple vantage points. This comet is no threat, but the ability to combine data from orbiters around other planets-making allowance for their rapid motion and unique geometry-could be crucial if a hazardous object were to approach from a challenging angle, for instance near the Sun.
As comet 3I/ATLAS approaches its closest point to Earth on December 19, still a distant 170 million miles away, the combined optical and ultraviolet datasets from Mars orbit and surface stand as the most detailed ever obtained for an interstellar comet. They not only capture its fleeting physical presence but also preserve chemical clues to its origin, offering a rare opportunity to compare the building blocks of distant planetary systems with our own.
