Interstellar Comet 3I/ATLAS Reveals Secrets of Water Production

When the interstellar comet 3I/ATLAS passed close to the Sun on October 30, 2025, it marked a significant event in astronomical research. This comet became only the third confirmed visitor from interstellar space detected by scientists. Unlike its predecessors, the mysterious ‘Oumuamua’ and 2I/Borisov, which provided limited data during their flybys, 3I/ATLAS presented an opportunity for extensive observation.

An unexpected ally in this research was the Solar and Heliosphere Observatory (SOHO), which has been monitoring the Sun from its position 1.5 million kilometers from Earth for nearly three decades. Notably, SOHO houses a camera called SWAN, designed not to observe the Sun but to detect specific wavelengths of ultraviolet light emitted by hydrogen atoms. This capability allowed for a unique study of the comet’s hydrogen emissions shortly after its closest approach.

Nine days after reaching perihelion, SWAN began to record a distinctive glow of hydrogen around the comet. This emission occurs when sunlight interacts with water molecules released from the comet’s nucleus, breaking them apart and producing hydrogen atoms that emit ultraviolet light. By analyzing this glow, astronomers could estimate the comet’s water production.

The findings were astounding. On November 6, when the comet was about 1.4 astronomical units from the Sun, it was releasing water at a staggering rate of 3.17 × 10²⁹ molecules per second. To visualize this, the rate is akin to filling an Olympic swimming pool every few seconds, showcasing the remarkable activity of this relatively small icy body.

What makes these observations particularly valuable is their timing. Most research on 3I/ATLAS took place before perihelion, as the comet approached the Sun and began to heat up. The SWAN observations captured the subsequent decline in activity as the comet moved away from the Sun. Over the following weeks, water production decreased steadily, dropping to between 10 and 20 trillion trillion molecules per second by early December, approximately 40 days post-perihelion.

This decrease follows a well-known pattern observed in other comets within our Solar System. As a comet travels farther from the Sun, reduced heating leads to less ice sublimating from its nucleus, resulting in diminished activity. The behavior of 3I/ATLAS suggests that despite its long journey through interstellar space, it has not fundamentally altered from the icy bodies formed in the early Solar System.

The method used to measure this water production has a proven track record. Developed over two decades ago and refined through observations of more than 90 different comet apparitions, it combines SWAN’s hydrogen measurements with daily readings of solar ultraviolet output and adjustments for the Sun’s rotation. Each element of the calculation is critical, as the fluorescence rate directly correlates to the Sun’s ultraviolet emissions at any moment.

The significance of 3I/ATLAS extends beyond mere curiosity about a single comet. Formed potentially billions of years ago in a planetary system around another star, studying its composition and behavior offers insights into conditions in that distant stellar neighborhood. This comparison can enhance our understanding of the diversity of planetary systems across the Galaxy and their similarities to our own Solar System.

Additionally, the substantial water production raises intriguing questions regarding the size and surface activity of the comet’s nucleus. Based on observations from the Hubble Space Telescope, the nucleus diameter is estimated to be between 440 meters and 5.6 kilometers. If water is indeed sublimating directly from the surface, a significant portion of it—potentially around 20 percent—would need to be active, which is substantially higher than the typical 3 to 5 percent seen in most Solar System comets.

As 3I/ATLAS continues its journey away from our Solar System, traveling for millennia before approaching another star, the detailed observations captured by SWAN and other instruments provide a remarkable snapshot of this interstellar traveler. Scientists now have the opportunity to analyze this comet’s secrets, enriching our understanding of the formation and evolution of celestial bodies both within and beyond our own Solar System.