3I/ATLAS Mars Photos and Non-Gravitational Acceleration: What NASA's Fleet Revealed About the Interstellar Comet

When interstellar comet 3I/ATLAS swept past Mars in October 2025, NASA seized a once-in-a-lifetime opportunity. Three Mars spacecraft — the Perseverance rover, the Mars Reconnaissance Orbiter (MRO), and the MAVEN orbiter — all turned their instruments toward the alien visitor, capturing some of the closest images ever taken of an object from another star system. Meanwhile, ESA's ExoMars Trace Gas Orbiter and Mars Express joined the observation campaign from Martian orbit.

But the Mars encounter was just one chapter in a larger scientific story. As astronomers tracked 3I/ATLAS through perihelion and beyond, they detected something unexpected: the comet was not following the trajectory that gravity alone would predict. It was experiencing non-gravitational acceleration — a phenomenon that would spark intense scientific debate, comparisons to the mysterious 1I/'Oumuamua, and ultimately reveal fundamental truths about this interstellar visitor's composition.

The Mars Flyby: October 2025

On October 3, 2025, comet 3I/ATLAS passed just 0.194 AU (29 million kilometers) from Mars — closer than any other spacecraft observation point during its journey through our solar system. NASA and ESA had multiple assets in position, and every one of them was ready.

Perseverance Rover: A Comet from Mars's Surface

NASA's Perseverance rover paused its exploration of Jezero Crater and aimed its Mastcam-Z camera skyward. The resulting images showed 3I/ATLAS as a bright smudge against the Martian twilight sky — the first time an interstellar object had ever been photographed from the surface of another planet.

The significance went beyond the historic milestone. Perseverance's vantage point on Mars's surface provided a parallax baseline that, when combined with Earth-based observations, helped refine the comet's trajectory with extraordinary precision. This improved orbital solution would later prove critical for detecting the comet's subtle non-gravitational acceleration.

MRO HiRISE: The Sharpest Eye at Mars

On October 2, 2025, NASA's Mars Reconnaissance Orbiter performed one of the most challenging observations in its mission history. The HiRISE camera — designed to photograph features as small as a desk on the Martian surface — turned away from Mars entirely and pointed into deep space.

In a 3.2-second exposure, HiRISE successfully detected 3I/ATLAS at a resolution of approximately 30 kilometers per pixel, from a distance of roughly 30 million kilometers. The comet appeared as a small, fuzzy point of light — but that fuzziness was scientifically valuable, confirming the presence of an extended coma of gas and dust surrounding the nucleus.

This was the first time HiRISE had been used to image an interstellar object, demonstrating the camera's remarkable sensitivity beyond its primary Mars-imaging mission.

MAVEN: The Invisible Coma

While Perseverance and MRO captured visible-light images, NASA's MAVEN orbiter contributed a different perspective. MAVEN's ultraviolet spectrograph detected the extended gas halo surrounding 3I/ATLAS — the vast envelope of sublimated volatile ices that forms the comet's coma.

MAVEN's UV observations were particularly important for understanding the comet's outgassing rate, a measurement that would become central to explaining the non-gravitational acceleration detected weeks later.

ESA's Mars Fleet Joins In

ESA's ExoMars Trace Gas Orbiter (TGO) captured a series of images using its CaSSIS camera between October 1–7, 2025. The comet appeared as a slightly fuzzy white dot — its nucleus and surrounding coma spanning thousands of kilometers. As the Principal Investigator noted, this was exceptionally challenging: the comet was "around 10,000 to 100,000 times fainter" than TGO's usual Mars surface targets.

Mars Express also attempted observations with its 0.5-second maximum exposure time. Scientists planned to stack multiple images to extract the faint cometary signal from the noise.

Non-Gravitational Acceleration: The Mystery

As 3I/ATLAS approached perihelion on December 24, 2025, astronomers noticed something that set the scientific community buzzing. Precise astrometric measurements — tracking the comet's exact position against background stars — revealed that 3I/ATLAS was not following the path predicted by gravitational forces alone. It was accelerating.

The magnitude of this non-gravitational acceleration peaked at (89.3 ± 4.6) × 10⁻⁹ AU/day², approximately one month before perihelion. This was a subtle but unmistakable deviation — the comet was being pushed by something other than the gravity of the Sun and planets.

For anyone who remembered the controversy surrounding 1I/'Oumuamua in 2017, the parallels were immediate and unsettling. 'Oumuamua had also exhibited non-gravitational acceleration, and because no outgassing was directly observed, Harvard astronomer Avi Loeb famously suggested it might be an alien light sail. Would 3I/ATLAS reignite the same debate?

Why Comets Accelerate: The Rocket Effect

Non-gravitational acceleration in comets is actually a well-understood phenomenon, even if the phrase sounds exotic. When solar heating causes volatile ices on a comet's surface to sublimate — transitioning directly from solid ice to gas — the escaping vapor acts like a natural rocket thruster.

Because cometary nuclei are irregular in shape and rotate, the outgassing is rarely symmetric. More gas escapes from the Sun-facing side, creating a net thrust that pushes the comet slightly away from the Sun. The effect is tiny — billions of times weaker than the Sun's gravitational pull — but over weeks and months, it accumulates into a measurable orbital deviation.

This "rocket effect" has been observed in dozens of solar system comets. What made 3I/ATLAS special was not the existence of non-gravitational acceleration, but the opportunity to measure it in an interstellar object with volatile composition very different from our local comets.

The Numbers: What the Acceleration Tells Us

If the observed non-gravitational acceleration is attributed entirely to outgassing thrust, the physics constrains the mass of 3I/ATLAS's nucleus to roughly 4.4 × 10¹⁰ kilograms. Combined with a typical cometary density of 200–600 kg/m³, this implies a nucleus diameter between 520 and 748 meters — a result that agrees remarkably well with independent size estimates from Hubble Space Telescope imaging.

This consistency is powerful evidence. Two completely independent methods — direct imaging and dynamical modeling of the non-gravitational acceleration — converge on the same answer. The comet is a natural, icy body roughly half a kilometer to three-quarters of a kilometer across, behaving exactly as physics predicts.

CO and CO₂: The Engines Behind the Push

A key question remained: which volatile ices were responsible for the outgassing that drove the acceleration? James Webb Space Telescope spectroscopy had already revealed that 3I/ATLAS was unusually rich in carbon monoxide (CO) and carbon dioxide (CO₂), while being relatively depleted in water ice compared to most solar system comets.

A 2025 study applied detailed thermophysical models to the problem, incorporating realistic vapor pressure relations for different ices, diurnal temperature variations as the nucleus rotated, and obliquity averaging over the comet's spin axis orientation. The result was clear: CO and CO₂-dominated outgassing, with less than one percent of the surface actively venting, could fully reproduce both the magnitude and direction of the observed non-gravitational acceleration.

No exotic physics was needed. No alien propulsion. Just volatile-rich ice from another star system, sublimating exactly as thermodynamics dictates.

How This Differs from 'Oumuamua

The comparison with 1I/'Oumuamua is instructive. 'Oumuamua also showed non-gravitational acceleration, but no outgassing was ever directly detected — no coma, no dust tail, no spectral signatures of escaping gas. This absence is what allowed Avi Loeb's alien light sail hypothesis to gain traction: if there was no visible outgassing, what was pushing it?

For 3I/ATLAS, the situation is completely different. The comet displays a massive, clearly visible coma extending hundreds of thousands of kilometers. JWST detected specific volatile species in its spectrum. XRISM detected X-ray emissions from solar wind charge exchange with cometary gas. MAVEN measured the UV signature of the extended gas halo. The outgassing is not hypothetical — it is observed across every wavelength from X-ray to radio.

The non-gravitational acceleration of 3I/ATLAS is therefore not a mystery. It is a confirmation — a textbook demonstration of cometary physics operating on an interstellar visitor, proving that the same volatile-driven processes that shape comets in our solar system are universal.

What the Mars Photos and Acceleration Together Reveal

The Mars flyby observations and the non-gravitational acceleration measurements are two pieces of a unified scientific story. The Mars spacecraft provided close-range confirmation of active outgassing (via MAVEN's UV detection and the visible coma in MRO and TGO images). That outgassing, when modeled with the correct volatile composition revealed by JWST, perfectly explains the orbital deviation measured from Earth.

Together, these observations paint a coherent portrait of 3I/ATLAS: a small, volatile-rich icy body, roughly 500–750 meters across, formed in the cold outer reaches of another star's protoplanetary disk. It is dominated by CO and CO₂ ices rather than the water ice that characterizes most of our solar system's comets — a frozen messenger carrying information about the chemistry of a distant, unknown planetary system.

As 3I/ATLAS continues outward past Jupiter's orbit in March 2026, spacecraft like Juno, JUICE, and Psyche may collect additional astrometric data to further refine the non-gravitational acceleration model. Each new measurement adds another constraint, bringing us closer to a complete understanding of this extraordinary interstellar visitor.

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Track 3I/ATLAS's current position in our interactive 3D orbit visualization, review the full observation timeline, or check visibility from your location in the observing guide.