Rising Challenge: Observing a Visitor from Another Star
In October 2025, astronomers faced a familiar problem with an unfamiliar guest. Comet 3I/ATLAS, a rare interstellar visitor entering our Solar System, was about to be swallowed by solar glare as it approached the Sun. From Earth, the glare of the Sun would wash out any view. The logical workaround: turn the cameras of Mars’ own fleet of orbiters toward the inbound visitor.
Between October 1 and 7, 2025, two European Space Agency (ESA) space telescopes circling Mars—the ExoMars Trace Gas Orbiter (TGO) and Mars Express—took on the unlikely task of tracking a faint, far-off comet across the Martian sky. On October 3, the comet passed within about 30 million kilometers of the two spacecraft, providing what ESA researchers described as the closest look the Martian fleet could offer.
The Interstellar Visitor: Not Your Usual Comet
Most comets visible from Earth come from the distant icy outskirts of the Solar System. 3I/ATLAS, however, is believed to have originated outside our solar neighborhood, likely in another star system. It is not bound to the Sun; instead, it is merely sweeping past. Its time in our cosmic backyard is brief, and the opportunity to study it is limited.
Since its discovery in July 2025, ground-based and space telescopes have captured fascinating data about 3I/ATLAS. Analyses suggest the object contains compounds such as carbon dioxide, water ice, water vapor, and carbon monoxide. Some studies even propose that 3I/ATLAS could be one of the oldest comets observed, possibly around 7 billion years old.
What the Mars Missions Saw
The CaSSIS instrument on TGO produced the most publicized images, portraying 3I/ATLAS as a white dot streaking across the Martian sky. In these low-resolution frames, the nucleus and the surrounding coma—the cloud of ice and gas boiling away as the comet nears the Sun—are indistinguishable. A true nucleus detail would be extraordinarily difficult to resolve from Mars with current capabilities, likened by ESA scientists to spotting a mobile phone on the Moon from Earth.
Meanwhile, Mars Express contributed through its other tools, including the OMEGA and SPICAM spectrometers, while TGO provided spectral data with its NOMAD instrument. The team used a technique called spectroscopy to infer composition from light signatures, though early results indicated that the coma and tail were likely too faint for definitive spectral characterization at that stage.
As 3I/ATLAS continues its solar approach, the observed coma grows, and any potential tail becomes more conspicuous. Scientists emphasize that every data point matters for understanding the behavior of an interstellar visitor as it interacts with solar radiation and gravity.
Looking Ahead: Juxtaposing Mars and Jupiter Observations
ESA plans to extend the investigation by turning the spotlight toward the incoming comet with the Jupiter Icy Moons Explorer (Juice). While Juice will be farther from 3I/ATLAS than the Mars orbiters, it will observe the comet near its most active phase, just after perihelion. Data from Juice is expected in early 2026, promising a complementary perspective to the Mars-based measurements.
“This was a very challenging observation for the instrument, but the data will help illuminate how interstellar material interacts with the inner Solar System,” notes Nick Thomas, Principal Investigator for CaSSIS. “We’ll learn more as analyses progress.”
Why This Matters
Tracking 3I/ATLAS from multiple platforms—on Earth, from Mars orbit, and later from Jupiter’s region—helps scientists piece together the mysteries of interstellar material, including its composition and origin. Such observations deepen our understanding of planetary systems beyond our own and refine the methods we use to study faint, distant objects in the presence of bright stars and planets.
Bottom Line
The Mars mission observations mark a milestone in cross-planetary science: rovers and orbiters prioritizing a distant, fleeting visitor over surface-focused tasks. As 3I/ATLAS continues its voyage toward the Sun, future data from Juice and other instruments will enrich our picture of this extraordinary, extraterrestrial traveler.
