China's Tianwen-1 Captures Interstellar Comet 3I/ATLAS From Mars Orbit

A Visitor From Another Star Comes Into View

China's Tianwen-1 spacecraft photographed interstellar comet 3I/ATLAS from Mars orbit in early October 2025, observing an object about 30 million kilometers away while using a camera built to study the Martian surface. The observation marked the first time an interstellar visitor had been photographed from Martian orbit and tested how far the spacecraft could extend its work beyond its original mission.

The China National Space Administration (CNSA) released images showing the comet's central region and gaseous coma, the cloud surrounding its nucleus. The comet was traveling at about 58 kilometers per second, while its speed relative to the orbiter was approximately 86 kilometers per second. Tracking such a distant, faint and rapidly moving target required the mission team to adapt equipment intended for a very different task.

3I/ATLAS is the third confirmed object discovered passing through our solar system from interstellar space. Its visit offered scientists a chance to examine material formed around another star, using spacecraft already working at Mars as additional observation platforms.

Tianwen-1's high resolution camera, known as HiRIC, gathered multiple exposures that were transmitted to Earth. A ground application system in Beijing received, processed and displayed the data. Specialized algorithms combined the exposures into the sharper images released to the public.

The achievement brought together two kinds of work: a scientific effort to understand an unusual comet and an engineering test of a spacecraft that had been operating stably around Mars for more than four years. Alongside observations by European and American missions, the Chinese images added another perspective on a visitor whose journey began outside our planetary neighborhood.

How Astronomers Identified 3I/ATLAS

The ATLAS telescope system discovered 3I/ATLAS in July 2025 in Rio Hurtado, Chile. ATLAS stands for Asteroid Terrestrial-impact Last Alert System. Astronomers soon identified the object's origin outside the solar system by analyzing its trajectory.

3I/ATLAS follows a hyperbolic path. Unlike the elliptical orbits followed by local comets and asteroids, this trajectory carries an object through the solar system with enough speed to escape the Sun's gravitational pull. After its passage through our neighborhood, the comet can continue into interstellar space.

The discovery placed 3I/ATLAS alongside two earlier confirmed interstellar visitors: 'Oumuamua, detected in 2017, and 2I/Borisov, detected in 2019. Each arrival provides an opportunity to investigate material that formed beyond the solar system, rather than relying entirely on objects born around the Sun.

3I/ATLAS reached its closest point to the Sun in late October 2025. That distance was about 1.4 astronomical units, or 210 million kilometers, placing it just inside the orbit of Mars. An astronomical unit is a way of expressing distances across the solar system; the figures here describe how far the comet remained from the Sun even at its closest approach.

The main dates put the Chinese observation within the comet's wider passage:

• 2017: Astronomers detected 'Oumuamua, the first confirmed interstellar visitor.
• 2019: 2I/Borisov became the second confirmed interstellar object.
• July 2025: ATLAS discovered 3I/ATLAS in Rio Hurtado, Chile.
• Early October 2025: Tianwen-1 photographed the comet from Mars orbit.
• Late October 2025: The comet reached its closest point to the Sun, about 210 million kilometers away.

Why Material From Another Star Matters

CNSA said 3I/ATLAS may have formed around an ancient star near the center of the Milky Way. Its estimated age is about 3 billion to 11 billion years, making it potentially older than the solar system itself. Its birthplace and age remain estimates rather than settled measurements.

Scientists regard interstellar objects as cosmic time capsules because they carry information about the physical and chemical conditions where they formed. Studying 3I/ATLAS can help researchers examine the composition, evolution and early stellar history associated with planetary systems beyond our own.

Gas molecules and dust released by the comet carry traces of that distant environment. Researchers can study those materials indirectly, gathering clues about places light years away without sending a spacecraft to the star system where the comet originated.

Scientists suspect 3I/ATLAS formed in a cold protoplanetary disk, the region of material around a young star where planets develop. In distant, cold parts of such a disk, volatile substances including water ice, carbon dioxide and carbon monoxide can remain preserved for billions of years. These are materials that can turn into gas when conditions change.

Spectral analysis helps researchers identify such substances by examining the light associated with them. Their presence and behavior can provide clues to the temperature and density of the nebula where the comet formed. Those clues can help refine explanations of how planetary systems develop and evolve.

3I/ATLAS therefore offers a practical comparison for theories built largely from our own solar system. Its value lies in the chance to investigate a physical sample from a different stellar environment, through the material it releases during its passage.

A Mars Camera Given a Different Job

Tianwen-1's HiRIC camera was designed to photograph Martian topography with high precision. Imaging bright features on a nearby planetary surface is very different from following a small, faint object against the darkness of space. The team needed a new observation strategy to make that change possible.

Preparations included completely recalibrating the spacecraft's targeting systems and carrying out precise orbital maneuvers to align the equipment with the comet's predicted trajectory. The camera had to point toward where the moving comet would be during the observation, while the orbiter itself continued traveling around Mars.

Wang Yanan, chief editor of Aerospace Knowledge magazine, explained that the observation showed how spacecraft launched to study planets could also become platforms for examining more distant celestial bodies. Tianwen-1 had been designed for Mars exploration, yet it successfully captured an object far beyond the surface it usually observes.

Wang described accurate attitude control as central to that achievement. Attitude control means managing the direction a spacecraft faces. In this case, optical sensors had to be aligned with an extremely distant point and then aimed again as required during orbital operations.

The work could involve coordination across multiple axes, Wang said, making it a comprehensive test of both orbital observation and spacecraft orientation. The result demonstrated that Tianwen-1's navigation and pointing systems could handle an observation outside the mission's usual routine.

The mission team also ran extensive simulations to select suitable exposure times. An exposure had to be short enough to limit blurring from movement, yet long enough to collect the faint light reflected by the nucleus and coma. A setting that worked well for the bright Martian surface would not automatically work for this target.

Faint Light, Fast Motion and a Brief Window

CNSA described the observation as Tianwen-1's first attempt to photograph a target so distant and relatively dim. The comet appeared between 10,000 and 100,000 times fainter than targets on the Martian surface, pushing HiRIC's ability to detect faint objects to its limit.

The comet's reported diameter was approximately 5.6 kilometers. At a distance of about 30 million kilometers, its small size added to the difficulty of obtaining useful images. The camera had to collect enough light from a compact target while following its motion.

The two speed figures describe different parts of the same challenge. 3I/ATLAS was traveling at around 58 kilometers per second, while its motion relative to Tianwen-1 was about 86 kilometers per second. The relative speed mattered for tracking because it described how quickly the target moved in relation to the observing spacecraft.

The instruments also needed stable temperatures. The team made adjustments so that the camera's lenses and sensors could operate within their optimal temperature range during an observation window lasting only a few minutes.

The successful images resulted from this combination of precise pointing, carefully chosen exposures, temperature management and processing on Earth. The operation tested the spacecraft's hardware and the team's ability to adapt its procedures to a target that was much fainter and farther away than Mars.

European spacecraft faced similar problems. The European Space Agency's Mars Express used its High Resolution Stereo Camera, while the ExoMars Trace Gas Orbiter used its Colour and Stereo Surface Imaging System. Both also captured images of 3I/ATLAS and had to work within the sensitivity and exposure limits of cameras designed for planetary observations.

What the Images and Chemical Clues Suggest

Tianwen-1's images and the initial spectral findings offered clues about the comet's structure and composition. The photographs showed a dense, well defined central region, described as likely containing a mixture of rock and different kinds of ice.

Analysis of reflected light pointed to reddish organic dust, a feature associated with bodies formed in cold regions far from their host star. A preliminary paper published in late October, however, described 3I/ATLAS as appearing distinctly bluer than the Sun. The reddish dust indication and the blue appearance left researchers with a puzzle about the comet's dust composition.

Spectrometers aboard other missions detected signatures of water ice and carbon dioxide turning into gas from the surface, together with traces of carbon monoxide. This process, called sublimation, occurs when frozen material changes directly into gas. The detections showed the kind of activity expected from a comet.

3I/ATLAS also developed a rare anti-tail and displayed an unusual chemical makeup. The anti-tail was another feature of its appearance that researchers could examine alongside the gases and dust surrounding the central body.

By January 17, 2026, the comet had apparently changed course slightly and may have lost a substantial amount of mass. These observations were described as signs of outgassing and a resulting change in acceleration. As material escapes from a comet, it can affect the comet's motion as well as reduce the amount of material left in the body.

The chemical findings and changes in appearance give researchers several connected lines of investigation: what the comet contains, how it releases that material and how the release affects its path. Together, they provide a more detailed picture than a photograph alone.

Mars Became a Shared Observation Platform

The Chinese observation was part of a wider international effort using spacecraft at Mars. The presence of several probes in orbit allowed a coordinated campaign, expanding the quantity and range of data collected on 3I/ATLAS.

ESA and NASA directed their orbiters to investigate the gaseous composition of the comet's coma and to try to obtain images of its nucleus at higher resolution. Observations from multiple instruments gave researchers different ways to examine the same object during its passage.

Avi Loeb, head of the Galileo Project and former chair of Harvard University's astronomy department, said the highest resolution images of 3I/ATLAS came from the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter. HiRISE has an aperture of 50 centimeters and achieved a spatial resolution of about 30 kilometers per pixel.

A pixel is one element of a digital image. A resolution of 30 kilometers per pixel describes the physical scale represented by each element, helping readers understand how much detail the camera could record at the comet's distance.

NASA's Perseverance and Curiosity rovers were also programmed to try to locate and observe 3I/ATLAS in the Martian sky. Their planned observations added the possibility of a view from the surface, alongside the images taken from orbit.

The combined campaign connected instruments with different designs, positions and capabilities. Mars orbiters could study the coma and central region, while the rover attempts offered a distinct observing position on the planet itself.

What Tianwen-1 Gained Beyond Its Main Mission

CNSA said the opportunity to observe a faint celestial body allowed the Tianwen-1 team to carry out useful technical tests and build experience. The comet tracking operation tested the flexibility and robustness of navigation and attitude control systems beyond the spacecraft's main task of studying Mars.

The successful observation showed that the orbiter could undertake future observation opportunities when unusual objects come within reach. A mission focused on one planet had produced an additional scientific result by adapting its existing instruments and operating methods.

Wang Yanan said precise adjustments in orbit, including aligning sensors toward a designated direction, are essential skills for future asteroid exploration missions. Following 3I/ATLAS gave the team experience with the kind of accurate aiming needed when studying other small bodies in deep space.

Wang also said China's space program has consistently supported sharing achievements with the international community. Bringing together observations and information from major spacefaring nations, he said, can help humanity understand the unusual visitor.

For Tianwen-1, the comet observation was described as an "unexpected harvest" with value for the study of planets beyond the solar system. Its contribution came from both the data it collected and the demonstration that a spacecraft designed for Mars could respond to a rare event elsewhere in space.

From Mars Mapping to Tianwen-2

Tianwen-1 launched in July 2020 and reached Mars orbit in February 2021. The mission successfully landed the Zhurong rover in May 2021, establishing a major milestone in China's exploration of Mars.

The orbiter continues its mapping work, adding to scientific knowledge of the planet. More than four years of stable operation in Mars orbit provided the platform from which the team could attempt the unusual comet observation.

The experience is also considered valuable for Tianwen-2, which launched in May 2025. That mission aims to collect samples from an asteroid near Earth and study a comet in the main asteroid belt.

The observation protocols developed for 3I/ATLAS are intended for direct use in those more complex exploration tasks. The technical tests on Tianwen-1 provided experience relevant to the advanced maneuvering required for Tianwen-2's future encounters, connecting an unexpected observation at Mars with China's next work among asteroids and comets.

The Bottom Line

• Tianwen-1 photographed 3I/ATLAS from Mars orbit in early October 2025, at a distance of about 30 million kilometers.
• ATLAS discovered the comet in Chile in July 2025. It is the third confirmed interstellar visitor after 'Oumuamua and 2I/Borisov.
• The comet traveled at about 58 kilometers per second, with a speed relative to Tianwen-1 of approximately 86 kilometers per second.
• The target appeared 10,000 to 100,000 times fainter than features on Mars, testing the limits of the HiRIC camera.
• CNSA said the comet may have originated near the Milky Way's center and could be about 3 billion to 11 billion years old.
• Observations identified cometary activity involving water ice, carbon dioxide and carbon monoxide, while dust findings presented a color puzzle.
• ESA and NASA spacecraft joined the observation effort, and Perseverance and Curiosity were programmed to attempt views from Mars.
• The pointing and observation experience gained by Tianwen-1 supports preparations for Tianwen-2's asteroid sampling and comet studies.