Unveiling a cosmic traveler: 3I/ATLAS
The interstellar visitor known as 3I/ATLAS has surprised scientists by showing signs of water activity while still far from the warmth of the Sun. As the third confirmed object to journey into our solar system from another star, 3I/ATLAS is offering a rare glimpse into the chemistry of worlds beyond our own. Two orbiting Mars spacecraft and a suite of ground-based observatories have come together to capture data that challenge assumptions about where and how water can exist in small bodies traveling through interstellar space.
Close approach and capturing the comet’s glow
During its October 3 flyby, 3I/ATLAS passed within about 30 million kilometers (18.6 million miles) of Mars. The ExoMars Trace Gas Orbiter (TGO) and the European Space Agency’s Mars Express provided complementary views, with TGO recording a sequence of images showing the comet streaking past at an estimated 130,000 mph (210,000 km/h). While Mars Express could not capture the faint visitor, the data from TGO offered humanity its clearest look yet at the interstellar cometing act.
Evidence of water: hydroxyl signatures
Astronomers at Auburn University, leveraging NASA’s Swift Observatory, detected ultraviolet signatures consistent with hydroxyl, a byproduct of water vapor. This detection marks a pivotal confirmation: 3I/ATLAS is actively releasing water as it travels through interstellar space. By mapping hydroxyl within a 10-arcsecond radius, researchers estimated the molecule counts to be in the low 10^30 range in July and rising to about 4.2 × 10^30 by mid-August, implying a coma that ejects roughly 40 kilograms of water per second.
How much surface is actively venting?
Analyses indicate that at least 8 percent of 3I/ATLAS’s surface—around 7.8 square kilometers—may be actively releasing vapor. That level of activity dwarfs the 3–5 percent typically observed in many comets, suggesting either an ice-rich surface or ongoing emission of volatile dust grains that continue releasing water after leaving the nucleus.
Why this matters for astrobiology and planet formation
The discovery is meaningful beyond pure curiosity. The timing is notable: the water activity was detected when the comet was roughly three times farther from the Sun than Earth, a region ordinarily too cold for water to vaporize. The leading explanation points to water being released from tiny ice grains that sublimate under even weak sunlight, a mechanism that could help distribute water and possibly organic materials across vast distances in the galaxy.
As researchers discuss the implications, 3I/ATLAS appears to be an unusually active interstellar visitor. Its chemical fingerprint—rich in water relative to certain molecules like cyanogen, and possibly carbon-poor—could reflect formation conditions near a metal-deficient, young star. Such traits add to the growing realization that comets and other small bodies serve as long-range couriers of the ingredients for life, potentially delivering water and organics from one star system to another.
Lead researcher Zexi Xing emphasized the broader significance: each interstellar visitor reshapes our understanding of how planetary systems form and evolve. “Oumuamua dehydrated, Borisov was carbon monoxide-rich, and now ATLAS is losing water at a distance where we didn’t expect it,” Xing notes. “Each one is rewriting what we thought we knew about the formation of planets and comets around stars.”
Looking ahead: what we can learn next
3I/ATLAS’s ongoing activity will guide future missions and observations. By refining measurements of water production, scientists hope to constrain the comet’s origin, its nucleus properties, and the processes that sustain vapor release so far from the Sun. In turn, this research informs models of how life’s building blocks—water and organic molecules—could be shared among distant star systems, potentially shaping the story of life across the cosmos.
