When the first interstellar comet ever seen in our solar system was discovered in 2017, one characteristic — an unexplained acceleration away from the sun — sparked wild speculation, including that it was an alien spacecraft. An astrochemist found a simpler explanation and tested it with an astronomer: in interstellar space, cosmic rays converted water to hydrogen in the comet’s outer layers. Nearing the sun, outgassed hydrogen gave the tiny comet a kick.