NEW YORK, January 4, 2026, 17:01 ET
- Scientists tracked water output from interstellar comet 3I/ATLAS using ultraviolet observations from the SOHO spacecraft.
- The comet’s water production peaked in early November, then fell sharply as it receded from the Sun.
- The results add rare “after-perihelion” data on one of only three confirmed interstellar visitors to the solar system.
Scientists have measured water streaming from interstellar comet 3I/ATLAS after its late-October pass around the Sun, using a solar-monitoring spacecraft to estimate how fast the object was shedding ice into space. Phys
The timing matters because most early work on 3I/ATLAS focused on its approach, when sunlight was ramping up the comet’s activity. Post-perihelion data are harder to capture as objects fade and move farther from Earth, leaving a narrow window to document how a fresh interstellar visitor “winds down.” Phys
Interstellar objects are rare: 3I/ATLAS is only the third confirmed body observed entering the solar system from beyond it, after 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019. Scientists prize them as physical samples of other planetary systems, but they typically do not linger. Phys
The measurements rely on the Solar and Heliosphere Observatory, or SOHO, a spacecraft that has watched the Sun for nearly three decades. A SOHO instrument called SWAN detects ultraviolet light from hydrogen, which can brighten when sunlight breaks apart water molecules flowing from a comet. Phys
In a study posted as a preprint — research shared publicly before formal peer review — researchers used that hydrogen “glow” to back-calculate the comet’s water production. On Nov. 6, when the comet was 1.4 astronomical units from the Sun (one astronomical unit is the Earth-Sun distance), they estimated it was releasing water at a rate of 3.17 x 10^29 molecules per second. Phys
Over the following weeks the output fell steadily, dropping to between 10 and 20 trillion molecules per second by early December, the study said, a decline pattern the authors described as consistent with how ordinary solar-system comets dim as solar heating eases. Phys
The scale of the outgassing also feeds into a central uncertainty: how big the comet’s solid nucleus is, and how much of its surface is active. The analysis cited Hubble-based size estimates ranging from about 440 meters to 5.6 kilometers across, and said the measured water production could imply an unusually high active fraction — around 20% — compared with the roughly 3% to 5% often seen in many solar-system comets. Phys
NASA has said the comet poses no threat to Earth and remains observable from the ground with small telescopes in the pre-dawn sky, with visibility expected to extend into spring 2026 as it continues outward. NASA Science
One risk is that the water estimate is indirect: it depends on modeling how sunlight drives fluorescence and how hydrogen spreads around the comet, along with corrections tied to changing solar ultraviolet output. The work is also a preprint, meaning the methods and assumptions have not yet been vetted through journal peer review. Phys
Even so, researchers said SWAN-based techniques have been refined across decades of comet observations, and the 3I/ATLAS results add a rare post-perihelion checkpoint for comparing an interstellar comet’s behavior with the icy bodies formed closer to home. Phys
