Comets usually fade quietly once they swing past the Sun. Interstellar comet 3I/ATLAS did not follow the same path. Instead, as it began to exit the solar system, it flared up dramatically. NASA’s SPHEREx space telescope caught the outburst in December 2025, spotting water vapour, carbon dioxide, and complex organic compounds streaming into space. For astronomers, this was not just a pretty display but something rare. A chemical snapshot of material formed around another star. Only the third confirmed interstellar visitor ever seen here, 3I/ATLAS is offering what might be the clearest look yet at alien ice from beyond the Sun’s family.
NASA’s SPHEREx observes comet 3I/ATLAS still glowing
By December 2025, comet 3I/ATLAS was already on its way out of the inner solar system. It had passed its closest approach to the Sun two months earlier. Normally, activity peaks near that point. Heat rises. Ice vaporises. Gas and dust spill outwardNASA’s SPHEREx, short for Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer, observed the comet in infrared light. The comet was erupting, brightening. Throwing off a surge of gas and dust. Phil Korngut, a co-author of the study, explained that the comet likely formed a radiation-processed shell during its long journey between stars. Beneath that shell, pristine ices remained locked away.Carey Lisse, lead author of the study, Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, said the comet was “full-on erupting” after its close flyby of the Sun. Even water ice was sublimating into gas out in interplanetary space.
What SPHEREx actually detected
SPHEREx detected water vapour, carbon dioxide, methane, methanol, and traces of cyanide streaming from comet 3I/ATLAS. Organic molecules, too. It seems the comet developed a glowing coma rich in these ingredients. A pear-shaped dust tail formed as rocky fragments were ejected. And it is only the third confirmed interstellar object ever spotted, after 1I/’Oumuamua in 2017 and 2I/Borisov in 2019.