Eyes on the Sky: ExoMars TGO Documents an Interstellar Intruder
The European Space Agency’s (ESA) Mars orbiters were on high alert last week as interstellar comet 3I/ATLAS passed near the Red Planet. NASA and ESA observers have long anticipated the chance to study these rare visitors up close, and the ExoMars Trace Gas Orbiter (TGO) delivered, snapping imagery that offers a rare glimpse of a body born outside our solar system.
Close Approach and a Challenging Observation
On October 3, 3I/ATLAS came within approximately 19 million miles (30 million kilometers) of Mars. The observation was notably challenging: the comet’s brightness is far fainter than the typical targets observed by the instrument and required careful data handling. CaSSIS, the Colour and Stereo Surface Imaging System aboard TGO, captured the closeup despite the difficulty, according to Nick Thomas, CaSSIS’s Principal Investigator. “This was a very challenging observation for the instrument,” Thomas said in a statement released by ESA. He noted the comet’s faintness—“around 10,000 to 100,000 times fainter than our usual target”—which tested the limits of the imaging system.
What Is 3I/ATLAS?
3I/ATLAS is only the third confirmed interstellar object observed in our solar system. It was discovered this past July by the ATLAS telescope in Río Hurtado, Chile. The “3I” designation marks its place as the third confirmed interstellar visitor, following 1I/’Oumuamua (2017) and 2I/Borisov (2019). Interstellar comets like 3I/ATLAS carry material from other star systems, offering scientists a rare opportunity to study the building blocks of distant worlds without traveling beyond the solar system.
Why Interstellar Comets Matter
ESA officials emphasized the scientific value of such visitors. “Every planet, moon, asteroid, comet and lifeform in our solar system share a common origin,” the agency noted, but interstellar comets are “true outsiders, carrying clues about the formation of worlds far beyond our own.” In the case of 3I/ATLAS, researchers believe the comet’s lineage may be ancient—potentially older than our own solar system by several billion years—making it a time capsule from a distant region of the galaxy.
TGO’s Role in a Concerted Campaign
ExoMars Trace Gas Orbiter, which reached Mars in 2016 to study atmospheric methane and trace gases, formed part of a broader ESA effort to observe 3I/ATLAS from multiple angles. Mars Express, ESA’s older orbiter, also attempted to image the comet on October 3. While Mars Express imagery has not yet clearly resolved 3I/ATLAS, ESA officials explained that the shorter exposure time (0.5 seconds for Mars Express, versus 5 seconds for TGO) contributed to the difference in results.
Looking Ahead: Other Voyages and Future Observations
ESA’s Mars science mission lineup is just getting started on this front. Mars Express and ExoMars will likely continue analyzing data from this encounter, with the agency hoping for further insights as data are processed. In addition, the JUICE mission—ESA’s Jupiter Icy Moons Explorer—will also seek 3I/ATLAS as it journeys outward, aiming to observe the comet during its travel through the inner solar system. JUICE is set to begin its targeted observations on November 2, just days after 3I/ATLAS’s close solar approach, when the comet will be passing the inner solar system at about 130 million miles (210 million kilometers) from the Sun.
Why This Matters for Mars and Beyond
While the immediate scientific payoff of photographing an interstellar comet may seem modest, the long-term implications are profound. Each observation helps calibrate techniques for studying faint objects against bright planetary backdrops and improves our ability to detect and characterize interstellar material. The event also underscores the collaborative spirit of planetary science, with multiple ESA missions sharing data to maximize what we can learn from a single, fleeting visitor from another star system.
As the solar system continues to reveal its surprises, missions like ExoMars TGO and Mars Express remind us that even established observatories can pivot to chase the extraordinary—turning a chance encounter with a spacefaring stranger into a window on the broader cosmos.
