New milestone in interstellar exploration
The European Space Agency (ESA) has released the closest images yet of interstellar comet 3I/ATLAS, captured during its pass by Mars on October 3. The images were taken by ESA’s ExoMars Trace Gas Orbiter (TGO), with Mars Express contributing observations that, together, provide a unique glimpse of the mysterious visitor from beyond our solar system.
How the encounter unfolded
3I/ATLAS arrived in the inner solar system after traveling through interstellar space for millions or even billions of Earth years. On Friday, October 3, it reached a distance of about 30 million kilometers (roughly 18.6 million miles) from Mars. The comet’s rapid movement, estimated at about 210,000 kilometers per hour (130,000 mph), meant it was a fleeting but potent target for Mars-bound spacecraft.
ESA noted that the TGO was well positioned to capture the event, while Mars Express, designed for high-resolution mapping of the Martian surface, faced challenges due to the comet’s faintness from that vantage point. The result is an animated GIF-like sequence created by ESA from multiple TGO observations, showing a bright, fuzzy dot descending toward the planet before speeding away again.
What the images reveal about 3I/ATLAS
The bright point in the sequence represents the comet’s nucleus, surrounded by a halo of gas known as the coma. As comets draw closer to the Sun, surface ices sublimate into gas, thickening the coma and sometimes forming a tail as solar wind pushes dust and gas away. In these Mars-based images, the coma is visible as a diffuse glow, but a tail is not, likely because the comet was too faint from this distant vantage.
ESA’s Nick Thomas, lead investigator for ExoMars’ Colour and Stereo Surface Imaging System, explained that the comet’s extreme faintness—10,000 to 100,000 times dimmer than their usual targets—made tail detection unlikely with TGO’s instruments. The lack of a visible tail does not preclude one; as 3I/ATLAS approaches the Sun in coming weeks, the coma and potential tail may brighten and become detectable with improved conditions and vantage points.
What comes next for 3I/ATLAS and Earth-based observers
While the latest Mars-based data do not yet shed definitive new details about the interstellar visitor’s size or composition, ESA scientists remain hopeful. Ongoing analyses of Mars orbiters’ imagery aim to extract clues about the comet’s nucleus size, shape, and what its material can reveal about its origin in another star system.
The interstellar visitor is currently on the far side of the Sun, which limits Earth-based visibility. Ground-based telescopes are expected to regain sight of 3I/ATLAS in early December as it travels through the inner solar system, with a future close approach to Jupiter in March 2026 before it ventures back into interstellar space.
Context: why 3I/ATLAS is extraordinary
3I/ATLAS is only the third interstellar object confirmed visiting our solar system, following 1I/’Oumuamua and 2I/Borisov. Early estimates place its size between 3 and 7 miles (5 to 11 kilometers) across, suggesting a sizable body that has likely wandered through the galaxy for eons. Its speed and trajectory imply a long, interstellar voyage far surpassing the age of our Sun.
As researchers continue to study these rare visitors, missions in Mars orbit and Earth-based observatories together paint a broader picture of how interstellar material interacts with planetary systems. The ongoing analysis will help scientists better understand the diversity of objects that travel through our cosmic neighborhood.
