Mystery Deepens as Interstellar Comet 3I/ATLAS Brightens Unexpectedly Near the Sun

Interstellar Comet 3I/ATLAS, discovered in July 2025, is only the third known visitor from another star system (after ʻOumuamua and 2I/Borisov). It passed its closest approach to the Sun (perihelion) on Oct. 29, 2025. At perihelion, astronomers expected a typical increase in brightness as solar heat vaporised its ices, but 3I/ATLAS instead brightened far faster than predicted. Scientists at Lowell Observatory and the U.S. Naval Research Laboratory note that this rapid brightening “remains unclear”.

Observations at Perihelion

As per a preprint report, Qicheng Zhang (Lowell Observatory) and Karl Battams (Naval Research Lab) confirm that 3I/ATLAS's brightening “far exceeds” the rate seen in typical long-period comets and say its cause is still “unclear”.

NASA's solar observatories (the twin STEREO probes and ESA's SOHO) and NOAA's GOES-19 satellite tracked 3I/ATLAS as it swooped around the Sun. These spacecraft recorded a sudden surge in the comet's brightness as it neared the Sun. Unusually, the comet's coma appeared “distinctly bluer than the Sun,” indicating that gas emissions (not just reflected dust) were contributing strongly to its glow. Mark Thompson (Universe Today) notes this colour shift suggests emissions from molecules like cyanogen and ammonia in the outgassing material.

Possible Causes and Future Observations

Zhang and Battams suggest several possible explanations. For example, the comet's extremely high speed might heat it differently than normal comets, and its internal composition could differ because it formed around another star. They also note that around 3 astronomical units from the Sun, 3I/ATLAS was shedding mostly carbon dioxide ice, which may have kept its nucleus unusually cool and delayed the normal water-ice sublimation.