China’s Tianwen-1 captures interstellar comet 3I/ATLAS near Mars

Tianwen-1 has been orbiting Mars for four years and eight months, making it one of the closest spacecraft to observe the interstellar object since its discovery (on May 7^th, 2025).

Tianwen-1 Reveals the Comet's Tail and Coma

The new images show several features that support the conclusion that 3I/ATLAS is a comet. A visible tail extends behind the object, while a cloud of gas and dust (coma) surrounds its nucleus.

Researchers combined multiple images into an animation that traces the comet's movement as it travels toward its closest approach to the Sun. The observations add to growing evidence that 3I/ATLAS contains water and other volatile materials that began escaping as the object warmed during its journey through the inner solar system.

The successful observation also provides a preview of the type of work China hopes to conduct with Tianwen-2. That mission is expected to visit a Near Earth Asteroid (NEA) and later study a comet.

Capturing a Small and Fast-Moving Target

The team responsible for Tianwen-1's High-Resolution Imaging Camera (HiRIC) began preparing for the encounter in early September. Their work included repeated simulations, theoretical modeling, and detailed inspections of the instrument.

Photographing 3I/ATLAS was difficult because the object was both distant and relatively small (5.6 km, 3.5 mi in diameter). Its speed added another major challenge. The comet was moving at around 58 km/s (36 mi/s), while its motion relative to Tianwen-1 reached approximately 86 km/s (53.5 mi/s).

The camera also was not originally built to observe such a dim object. HiRIC was designed primarily to study bright features on the Martian surface, much like instruments aboard the European Space Agency's Mars Express and ExoMars Trace Gas Orbiter (TGO) spacecraft.

Mars Orbiters Turn Their Cameras Toward 3I/ATLAS

On October 3^rd, both European orbiters also photographed 3I/ATLAS. Mars Express used its High Resolution Stereo Camera (HRSC), while the ExoMars spacecraft relied on its Colour and Stereo Surface Imaging System (CaSSIS).

The comet was exceptionally difficult to detect with these instruments. When compared with sunlit features on Mars, 3I/ATLAS appeared roughly 10,000 to 100,000 times fainter.

The High Resolution Stereo Camera (HRSC) aboard Mars Express uses an exposure time of 0.5 seconds, while the Colour and Stereo Surface Imaging System (CaSSIS) can expose an image for 5 seconds. Tianwen-1 collected its observations with the High-Resolution Imaging Camera (HiRIC).

Although HiRIC's exposure time has not been released publicly, the camera was engineered for precise timing and rapid operation. Those capabilities allowed the team to obtain the latest image, even though it appears (albeit grainy and blurry).

A Rare Sample From Another Star System

Like the two interstellar objects identified before it, 3I/ATLAS offers unusual opportunities for both astronomy and astrobiology. Asteroids and comets preserve leftover material from the formation of planetary systems, so an object arriving from interstellar space may contain clues about conditions around another star.

Studying such a visitor could reveal how other planetary systems formed and what kinds of materials they contain. A mission designed to intercept an interstellar object (for which numerous concepts are being explored) could gather information that scientists cannot obtain through ordinary telescopic observations.

The only comparable alternative would involve sending spacecraft directly to another star, an undertaking that would require enormous amounts of time and money.

Future Missions Could Chase Interstellar Visitors

Scientists hope that future spacecraft will be prepared to intercept newly discovered interstellar objects and examine them at close range.

One effort already under development is the European Space Agency's Comet Interceptor. The mission is expected to be completed by 2029 and will wait in space for an opportunity to investigate a suitable comet or other promising target.