Interstellar Object 3I/ATLAS Observed from Mars by China’s Tianwen-1 Spacecraft

Abstract China’s Tianwen-1 Mars orbiter successfully imaged the third interstellar object, 3I/ATLAS, during its close encounter with Mars using the onboard HiRIC CMOS camera. This is China’s first deep-space observation of an astronomical object. These observations constitute the first imaging of this object from a vantage point significantly out of its orbital plane, providing a unique constraint on dust dynamics. Three observing epochs between 2025 September 30 and October 3 reveal clear changes in coma and tail morphology driven by the rapidly evolving viewing geometry. Comparison with Finson–Probstein dust dynamical models indicates that the coma is dominated by large grains with solar radiation pressure parameter β ≈ 10 −3 –10 −2 , corresponding to grain sizes of a few hundreds of microns. The extent of the sunward coma implies dust ejection velocities of 3–10 m s −1 . Despite the morphological evolution, the azimuthally averaged surface brightness profile remains nearly unchanged through the three epochs, transitioning from a radial slope near −1 close to the nucleus to slightly steeper than −1.5 at larger cometocentric distances, consistent with steady-state dust outflow accelerated by solar radiation pressure. Photometry yields an average Afρ ∼ (2.0 ± 0.2) × 10 4 cm and a corresponding dust mass loss rate of M ̇ ∼ 1 0 3 kg s − 1 .