At roughly 250,000 kilometers per hour, comet 3I/ATLAS did not merely pass through the inner solar system it tore through it, fast enough that the Sun could never hope to keep it. That kind of speed is one reason interstellar visitors stand apart from familiar comets, and it set up a rare observing opportunity when the object swept within 19 million miles of Mars.
The most intimate views came from NASA’s Mars Reconnaissance Orbiter, where the HiRISE camera built to pick out boulders and rover tracks was tasked with hitting a moving target well off the Martian surface. In the resulting frames, the comet appears as a compact, pixelated glow at about 19 miles (30 kilometers) per pixel. That modest-looking “ball” is the coma: the dust-and-ice cloud shed as sunlight heats the nucleus. Even at that scale, the imagery helps constrain an upper limit on nucleus size and offers a way to characterize the near-nucleus dust environment that shapes a comet’s brightness and tail.
“Observations of interstellar objects are still rare enough that we learn something new on every occasion,” said Shane Byrne, HiRISE principal investigator. “We’re fortunate, that 3I/ATLAS passed this close to Mars.” HiRISE was only one piece of a wider, multi-spacecraft effort, and the chemistry work is where the campaign becomes unusually distinctive. At Mars, the MAVEN orbiter used its Imaging Ultraviolet Spectrograph to map hydrogen around the comet an approach well matched to tracing water breakdown products and thin gases. Those ultraviolet measurements provide an estimated upper limit on the comet’s deuterium-to-hydrogen ratio, a formation tracer that is seldom obtainable for interstellar objects. “Every measurement we make of this comet helps to open up a new understanding of interstellar objects,” said MAVEN’s deputy principal investigator Dr. Justin Deighan.
While Mars provided proximity, solar-facing spacecraft provided persistence. As 3I/ATLAS moved into geometry that made ground observing difficult, NASA heliophysics missions tracked it near the Sun and through the glare, including imagery that revealed tail structure when other platforms could not. Stacked views from PUNCH observations taken Sept. 20 to Oct. 3 teased out a faint tail as a slight elongation, a reminder that instrument design rarely limits what careful processing and geometry can extract.
The most composition-shaping datapoint came from the James Webb Space Telescope’s infrared spectroscopy: the coma carried nearly eight times as much CO₂ as water vapor, an inversion compared with almost every well-studied solar system comet. “I have never seen such a strong CO2 peak in a comet spectrum,” said Martin Cordiner of NASA Goddard. Because Earth’s atmosphere blocks those wavelengths, Webb’s view turned a fleeting flyby into a diagnostic of alien ice chemistry.
Trajectory work adds still more context. The comet’s hyperbolic orbit confirms an interstellar origin, and analyses using Gaia spacecraft stellar data connect its velocity distribution to objects ejected from the Milky Way thin disk systems that can be long-lived and moderately metal-poor. “gives me goose bumps to think about,” said NASA’s Tom Statler.
As the object recedes, the observing advantage increasingly shifts to platforms that can work under constrained geometry. ESA’s JUICE spacecraft is conducting a dedicated campaign, with data return delayed until February due to spacecraft orientation constraints near the Sun an operational compromise that nonetheless preserves a chance for high-value post-perihelion measurements to join the growing multi-wavelength archive.
