The interstellar object 3I ATLAS is like an opened bottle of soda: it continuously releases carbon dioxide, wrapping itself in a thick cloud. The SPHEREx telescope split its light into a rainbow — a method called spectroscopy — and spotted the telltale signs of gas and water ice, which absorbs some of the light like tinted glass.

The cloud turned out to be perfectly round, without tails or jets. Precise brightness measurements showed that the gas is so abundant it completely hides the nucleus. Over 99% of the light comes from cosmic dust, not from the body itself. The actual nucleus — a solid chunk no larger than three kilometers across — is lost inside this shimmering shell, like a pea in a cloud of steam. Amazingly, this is only the third such wanderer spotted by humanity.