China’s Tianwen-1 orbiter caught a rare glimpse of 3I/ATLAS as it swept past Mars, revealing the hazy coma and tail of a comet born beyond our solar system.
The interstellar object known as 3I/ATLAS just flew past Mars, and China’s Tianwen-1 mission managed to snap some pics with its high-resolution camera. According to the China National Space Agency (CNSA), the orbiter’s high-resolution camera captured images of the comet from a distance of about 30 million km (18.6 million mi). This makes the Tianwen-1 orbiter, which has been orbiting Mars for four years and eight months, one of the closest missions to observe the ISO since it was first detected (on May 7th, 2025).
The image shows 3I/ATLAS’s comet-like features, which include its tail and the gaseous envelope (coma) surrounding it. The images captured by China’s orbiter were also stitched together to create an animation that shows its trajectory and path as it gets closer to making its closest pass to the Sun.
This is the latest in a long line of indications that 3I/ATLAS is an interstellar comet, composed of water and volatiles that have been outgassing since it began to approach the Sun. It also offers a taste of what China’s Tianwen-2 mission will see when it rendezvous with a Near Earth Asteroid (NEA) and a comet in the coming years.
A Difficult Target for a Mars Camera
The team that manages the orbiter’s High-Resolution Imaging Camera (HiRIC) began preparing for the observation opportunity in early September. This included running repeated simulations, theoretical modeling, and extensive instrument checks.
The task of imaging 3I/ATLAS was particularly challenging because of its distance and small size (5.6 km, 3.5 mi in diameter), but also the speed at which the ISO and orbiter are traveling. Whereas 3I/ATLAS is around 58 km/s (36 mi/s), the orbiter has a relative speed of 86 km/s (53.5 mi/s).
In addition, the HiRIC camera was designed to study brightly lit features on the Martian surface, similar to the ESA’s Mars Express and ExoMars Trace Gas Orbiter (TGO) missions. On October 3rd, these orbiters also captured images of 3I/ATLAS using their cameras – the High-Resolution Stereo Camera (HRSC) and Colour and Stereo Surface Imaging System (CaSSIS), respectively. In all cases, 3I/ATLAS is roughly 10,000 to 100,000 times fainter than Mars’ surface features when imaged in daylight.
However, the High-Resolution Stereo Camera (HRSC) on the Mars Express and the Colour and Stereo Surface Imaging System (CaSSIS) have exposure times of 0.5 seconds and 5 seconds, respectively. In Tianwen-1’s case, the images were acquired by its High-Resolution Imaging Camera (HiRIC). While its exposure time is not publicly disclosed, the camera was designed for high time accuracy and fast responses, which enabled the science team to capture this latest (albeit grainy and blurry) image of the ISO.
Why Interstellar Comets Matter
Like its predecessors, 3I/ATLAS is a particularly interesting object due to the opportunities it presents for astronomical and astrobiological research. Since asteroids and comets are essentially leftover material from the formation of solar systems, studying these objects would allow scientists to learn what conditions are present in other star systems.
Essentially, an ISO intercept mission (for which numerous concepts are being explored) could provide information that would otherwise be impossible, short of sending missions to other stars—an extremely time-consuming and expensive approach.
In the coming years, scientists hope to have a mission ready to go that could intercept future ISOs and study them up close. This includes the ESA’s Comet Interceptor, which is expected to be completed by 2029.
Adapted from an article originally published in UniverseToday.
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