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\approx 10^{-3} $ - $10^{-2}$, corresponding to grain sizes of a few 100s $μ$m. 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 $Afρ\sim (2.0\pm0.2)\times10^4$ cm and a corresponding dust mass loss rate of $\dot{M} \sim 10^3$ kg s$^{-1}$. The dominance of large grains in both interstellar comets discovered to date, 2I/Borisov and 3I/ATLAS, together with their high supervolatile contents, may indicate that these objects originate from the outer regions of their parent planetary disks.

Figures (5)

• Figure 1: The sky coordinate of 3I from Earth between 2025 July 1 and early 2026 January (blue line) and from Mars during the three epochs of Tianwen-1 observations (filled orange squares). The thin dashed curve marks the ecliptic plane. The vertical dotted line marks the RA of 3I from Earth at perihelion, and the shaded area marks the range of RA with solar elongation <45°. While no ground-based optical observations are possible near perihelion and solar conjunction, monitoring can still be continued with space-based facilities.
• Figure 2: Stacked images of 3I/ATLAS acquired by HiRIC CMOS camera onboard Tianwen-1 spacecraft (upper row) and the corresponding 1/$\rho$ divided images (lower row) from the three epochs. All images are displayed north up and east to the left. The stacked images are displayed with logarithmic brightness stretch, and the enhanced images are displayed with a linear brightness stretch. The arrows in the upper panels mark the projected directions of the Sun and the heliocentric velocity vector. The scalebars are 10000km. The bright star trails are visible in each stack due to the apparent movement of 3I in the sky background during each 30-second observation epoch.
• Figure 3: The syndynes (solid curves) and synchrones (dashed lines) system of 3I/ATLAS corresponding to Tianwen-1 observations overlaid with the stacked images (upper row) and the 1/$\rho$ ratioed images (lower row). The image orientations and brightness stretch are the same as Fig. \ref{['fig:image']}, but displayed with different color tables. The syndynes correspond to $\beta = 1.0e-4$ to 1, and the synchrones correspond to a release time of 50 – 300 days before the observations, as shown in the legend in the lower row.
• Figure 4: (left) Surface brightness scan of the coma along the sun-antisun direction, normalized to the peak brightness. Left is the sunward direction, and right is the anti-sunward direction. The profile is averaged over a $\sim$10 wide strip. An obvious asymmetry is shown between the sunward (left) and anti-sunward (right) direction, consistent with the effect of SRP. (right) Azimuthally averaged surface brightness profile with respect to cometocentric distance, derived from the photometry measured in a series of concentric annulus apertures centered on the centroid of the comet. Also shown are the lines representing exponential slopes of -1 and -2.
• Figure 5: Pre-perihelion heliocentric light curve of 3I/ATLAS. The measurements from this work (red squares, $\rho=5000km$) are compared with space-based measurements from Zhang:2025, ground-based measurements from Jewitt:2025, and CCD data from the COBS database (gray dots). Dashed lines indicate the brightening trends, which follow an $r_{\rm h}^{-3.8}$ power law at larger distances before transitioning to a steeper $r_{\rm h}^{-7.5}$ slope near $r_{\rm h} \sim 2.5\au$.