Interstellar Visitor Passes Near Mars Orbit

Close Encounter with Mars

On October 2, 2025, 3I/ATLAS passed within 0.0179 AU (approximately 2.68 million km or 1.66 million miles) of Mars' orbital path - its closest approach to any planetary orbit during its journey through the Solar System.

While this may sound close, the comet posed absolutely no threat to Mars or its spacecraft. For comparison, Mars is about 6,800 km in diameter, so the comet passed at a distance equivalent to about 394 Mars diameters away from Mars' orbital path.

Observations from Mars Fleet

This close approach provided a unique opportunity for spacecraft currently orbiting Mars to observe the interstellar comet from a different vantage point than Earth-based telescopes:

Mars Reconnaissance Orbiter (NASA)

NASA's Mars Reconnaissance Orbiter, equipped with the HiRISE camera, attempted to capture images of 3I/ATLAS. While the comet was too faint for detailed imaging, the observations provide valuable position data.

Mars Express (ESA)

The European Space Agency's Mars Express used its Visual Monitoring Camera to track the comet's position, contributing to the global observation campaign.

MAVEN (NASA)

NASA's MAVEN orbiter, designed to study Mars' atmosphere, monitored for any potential interaction between the comet's extended CO₂ coma and Mars' atmosphere, though at this distance no interaction was expected or observed.

Ramping Up for Perihelion

As 3I/ATLAS passes Mars' orbit and continues toward its October 29 perihelion, the comet's activity is increasing dramatically:

• Brightness: Magnitude increasing (getting brighter) as it approaches the Sun
• Tail length: Now extending over 200,000 km
• Gas production: CO₂ and H₂O outgassing rates rising
• Dust activity: Coma expanding and brightening visibly

Ground-based observers report the comet is now showing prominent tail structure visible in moderate-sized telescopes, with the dust tail becoming increasingly prominent as solar heating intensifies.

Trajectory and Future Path

After passing Mars' orbit, 3I/ATLAS will continue on its hyperbolic trajectory:

Scientific Value

The Mars close approach provides scientists with several advantages:

• Multi-angle observations: Viewing the comet from both Earth and Mars spacecraft provides 3D perspective on coma structure
• Trajectory refinement: Additional position measurements from Mars orbiters improve orbital accuracy
• Activity monitoring: Continuous observation as the comet rapidly brightens toward perihelion
• Composition studies: Intensifying outgassing reveals more molecular species in spectra

What's Next

With Mars passage complete, all eyes now turn toward the upcoming perihelion on October 29, when 3I/ATLAS will reach its closest point to the Sun. This will be the peak of the comet's activity and brightness, providing the best opportunity yet to study this interstellar visitor in detail.

Observatories worldwide are preparing for intensive observation campaigns around perihelion, when the comet's nucleus will be heated to its maximum temperature, driving the most vigorous outgassing of its entire visit.