“What makes a comet from another star system flare twice as fast as expected?” That question now grips astronomers studying 3I/ATLAS, the third confirmed interstellar object to pass through our solar system. Discovered by the Chile-based ATLAS survey in July 2025, this icy wanderer has defied predictions during its October 29 perihelion, when its brightness surged at a rate far exceeding that of typical Oort cloud comets.
The steep rise in luminosity was recorded not by ground-based telescopes blinded by the Sun’s glare but rather by an improvised network of solar-monitoring spacecraft. NASA’s STEREO-A, the ESA’s SOHO, and NOAA’s GOES-19 followed the comet’s approach with coronagraphs that block the Sun’s disk to reveal its outer atmosphere. Their imagery showed that as 3I/ATLAS closed from about 2 astronomical units to 1.36 AU between mid-September and late October, its brightness scaled inversely with heliocentric distance to the power of -7.5 roughly twice the steepness seen in most comets.
This anomaly could be caused by a number of mechanisms. First, sheer velocity: 3I/ATLAS is racing through the solar system at about 137,000 mph, faster than either 1I/‘Oumuamua or 2I/Borisov. Then there’s composition. Spectroscopic data from the Southern African Large Telescope and the Nordic Optical Telescope show a reflectance spectrum with a slope of 22.8 ± 0.1%/µm in the 0.4–0.7 µm range, indicating organic-rich materials akin to outer solar system comets. Yet observations from the James Webb Space Telescope have uncovered an extraordinarily high CO₂-to-H₂O ratio of about 8:1, among the highest ever measured. This enrichment may be the legacy of billions of years of unshielded exposure to galactic cosmic rays, which can convert carbon monoxide ice into carbon dioxide, forming an irradiated crust up to 20 meters deep.
Adding to the intrigue of the event is the colour evolution of the comet. Whereas early observations indicated a reddish-colored dust coma, near perihelion, coronagraph photometry from LASCO and CCOR-1 recorded it as “distinctly bluer than the Sun.” As dust is normally supposed to redden scattered sunlight, such a shift suggests that strong gas emissions-possibly due to cyanogen or ammonia-make important contributions to visible brightness. GOES-19 resolved a gaseous coma extending roughly 300,000 kilometres, comparable to CO₂ plumes traced by NASA’s SPHEREx observatory earlier in August.
With its increased activity, 3I/ATLAS has not developed a bright tail. Its coma remains asymmetric and is mostly filled with large dust grains, which are less affected by solar radiation pressure and tend to cling closer to the nucleus. Photometric estimates for dust mass-loss rates lie in the range between 0.3 and 4.2 kg/s in July, growing toward perihelion, while Hubble observations suggest that bursts up to 60 kg/s are possible. A likely explanation for the absence of fine dust in space could be related to the irradiated crust, which suppresses the ejection of smaller particles until deeper pristine layers are reached.
The comet’s physical shape also comes into focus. Time-series photometry based on a rotation period of 16.16 ± 0.01 hours with an amplitude of the light curve of about 0.3 magnitudes has suggested a moderately elongated nucleus. Hubble constraints place its radius below 2.8 km, with maximum axes around 3.2 by 2.6 km. These dimensions, along with its spin and activity profile, are compatible with weakly active outer solar system comets despite the interstellar origin of this object. For now, 3I/ATLAS remains hidden behind the Sun from Earth’s perspective.
By early December, it will reappear in twilight skies, with its closest approach to Earth on December 19. At that point, coordinated campaigns using ground-based observatories and space telescopes such as Hubble and Webb will probe whether post-perihelion erosion has breached the irradiated shell, revealing the unaltered materials from its home system. The outcome may well refine models of how interstellar objects evolve under cosmic ray bombardment and solar heating, offering a rare laboratory for studying chemistry forged far beyond our Sun’s influence.
