In late October 2025, the interstellar comet 3I/ATLAS made its remarkable journey past the Sun, marking its status as only the third confirmed object from interstellar space. This event provided a unique opportunity for detailed study, contrasting sharply with previous visitors like ‘Oumuamua and 2I/Borisov, which offered limited insights during their respective flybys.
An interesting element of the observations came from the Solar and Heliosphere Observatory (SOHO), which has been monitoring the Sun from its orbit approximately 1.5 million kilometers away for nearly three decades. SOHO is equipped with a special camera called SWAN, designed not for solar observation but for detecting ultraviolet light emitted by hydrogen atoms, allowing it to create an all-sky map of hydrogen scattered throughout the Solar System.
Nine days after 3I/ATLAS reached its closest point to the Sun on October 30, SWAN began to detect a significant hydrogen glow around the comet. This phenomenon occurred as sunlight struck water molecules emitted from the comet’s nucleus, breaking them apart and releasing glowing hydrogen atoms in ultraviolet light. By measuring this glow, astronomers could deduce the comet’s water output.
On November 6, at a distance of 1.4 astronomical units from the Sun, 3I/ATLAS exhibited an impressive water production rate of 3.17 × 10²⁹ molecules per second, equivalent to rapidly filling an Olympic swimming pool. The timing of these observations was crucial, as much of the earlier research focused on the comet as it approached the Sun. The SWAN data provided insight into the comet’s behavior as it retreated, illustrating a decline in its water production rate, which decreased to between 10 and 20 trillion trillion molecules per second within about 40 days post-perihelion.
This reduction in activity aligns with patterns observed in Solar System comets, as diminished solar heating results in less ice sublimating from the comet’s nucleus. This finding suggests that 3I/ATLAS, despite its long journey through interstellar space, has not undergone significant changes from the icy bodies formed in the early solar system.
Developed over the past two decades, the technique utilized to measure the comet’s water production relies on SWAN’s hydrogen observations combined with daily solar ultraviolet output readings and adjustments for the Sun’s rotation. This meticulous approach is essential, as the fluorescence rate is contingent upon solar emissions at any given time.
Beyond mere curiosity, understanding 3I/ATLAS serves as a window into the conditions of its distant stellar neighborhood, potentially revealing insights into how its environment compares to our own Solar System. The comet’s substantial water production prompts further inquiries into the nucleus’s size and surface activity. Hubble Space Telescope data suggest the nucleus could range between 440 meters and 5.6 kilometers in diameter, indicating that a significant portion—possibly around 20 percent—of its surface is actively producing water, which is notably higher than the typical 3 to 5 percent observed in most Solar System comets.
As 3I/ATLAS departs from our Solar System, likely traveling for millennia before encountering another star, scientists have captured a significant snapshot of this celestial messenger. The insights garnered from the SWAN observations not only deepen our understanding of this specific interstellar visitor but also broaden our perspective on the diversity of planetary systems that exist throughout the Galaxy. This research underscores the vital role of international collaboration in space observation and the continuous quest for knowledge beyond our own planet.