Two European Space Agency spacecraft currently orbiting Mars have successfully captured images of 3I/ATLAS, a rare visitor from outside our solar system. The ExoMars Trace Gas Orbiter (TGO) and the Mars Express orbiter monitored the interstellar object between October 1 and 7, providing humanity its closest view yet of the mysterious body as it journeys through the inner solar system. The observations were made as the object passed within 30 million kilometers of the Red Planet.
This encounter marks a significant opportunity for astronomers to study an object that originated from a different star system, offering clues about the formation of worlds far beyond our own. 3I/ATLAS is only the third such interstellar object ever detected, following the discoveries of 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. By turning instruments designed to study Mars toward this distant, faint target, scientists have begun to analyze its composition and behavior, piecing together the story of its multi-billion-year journey through the galaxy.
A Challenging Observation Campaign
Observing 3I/ATLAS was a complex technical challenge for both Mars orbiters. The cameras aboard TGO and Mars Express, the Colour and Stereo Surface Imaging System (CaSSIS) and the High Resolution Stereo Camera (HRSC) respectively, are engineered to photograph the brightly illuminated surface of Mars from just a few hundred kilometers away. In contrast, the interstellar object was millions of kilometers distant and extremely dim. Nick Thomas, the principal investigator for CaSSIS, noted that the comet was between 10,000 and 100,000 times fainter than the typical Martian landscape the instrument usually targets.
To capture the faint visitor, the TGO team used five-second exposures, significantly longer than usual. These images successfully revealed the object’s coma—the glowing cloud of gas and dust that surrounds the nucleus—as a fuzzy, bright dot moving against the backdrop of stars. However, the nucleus itself, estimated to be only a few kilometers wide, was too small and distant to be resolved. The Mars Express orbiter faced even greater difficulty; its camera has a maximum exposure time of just 0.5 seconds, and initial analyses of its images have not yet clearly identified the comet. Project scientists remain hopeful that by combining data from both spacecraft, a more detailed view can be constructed.
The Nature of an Interstellar Visitor
3I/ATLAS was first detected on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Chile. Its trajectory immediately identified it as an object from outside our solar system. Based on its path, astronomers have speculated that 3I/ATLAS could be the oldest comet ever observed, potentially billions of years older than our own sun and planets. Like other comets, it has been observed releasing water vapor as it approaches the sun, which heats its icy surface and creates the visible coma.
While the new images from Mars clearly show the coma, they did not detect a prominent tail. A comet’s tail is formed when solar wind and radiation push gas and dust away from the nucleus. The faintness of the tail in the current observations suggests it may become more visible as 3I/ATLAS gets closer to the sun, an event predicted for late October. Some anomalous behaviors have been noted in its journey, fueling scientific curiosity and underscoring the value of every observation. These interstellar objects are fundamentally different from anything native to our solar system; whereas every planet, asteroid, and local comet shares a common origin in the same protoplanetary disk, visitors like 3I/ATLAS are true outsiders carrying unique information about their home star systems.
Distinguishing Features
- Object Type: Interstellar Comet
- Designation: 3I/ATLAS
- Estimated Size: 5 to 11 kilometers in diameter
- Key Characteristics: Actively releasing water vapor, forming a coma but with a tail too faint to be detected from Mars.
A Coordinated Multi-Spacecraft Effort
The observation campaign involved repurposing missions already in deep space, showcasing the adaptability of modern robotic explorers. Colin Wilson, a project scientist for both ExoMars and Mars Express, expressed excitement over the ability of the Mars orbiters to respond to unexpected scientific opportunities. The successful imaging by TGO provides a crucial data point from a vantage point no other spacecraft could achieve at the time. During its flyby, the comet was traveling at an estimated speed of 210,000 km/h.
While NASA has not made official statements due to a government shutdown, raw images from the Perseverance rover on the Martian surface may also have captured the comet streaking across the sky on October 4. This suggests a broader, multi-agency effort to track the object as it passed through the Mars system. The data gathered from this close pass will be analyzed for months to come, with scientists eager to learn more about the composition of the coma by studying its light spectrum.
Future Observation Plans
The study of 3I/ATLAS is far from over. In November, another ESA mission, the Jupiter Icy Moons Explorer (Juice), will have its chance to observe the comet. Although Juice will be farther away from the object than the Mars orbiters were, its timing is critical. It will see the comet shortly after its closest approach to the sun, a period when cometary activity is expected to peak due to intense solar heating. This could make its tail and other features much more prominent. Data from Juice’s observations are not expected to be received on Earth until February 2026.
The fleeting visits of interstellar objects like 3I/ATLAS highlight the need for rapid-response missions that can be deployed to intercept them. To that end, ESA is developing the Comet Interceptor mission, designed specifically to chase down a long-period or interstellar comet and study it up close. Such a mission would provide an unprecedented connection to the wider galaxy, allowing humanity to move from remote observation to direct exploration of these ancient and mysterious travelers. After its encounter with the inner solar system, 3I/ATLAS will pass Jupiter in March 2026 before continuing its journey back into interstellar space, never to return.
