The Largest Comet We've Ever Seen Just Delivered a Curious Surprise - ScienceAlert

"These observations are pushing the distances for active comets dramatically farther than we have previously known," says astronomer Tony Farnham, from the University of Maryland (UMD).

The size of the comet – some 100 kilometers or 62 miles across – and its distance from the Sun when it became active are both the main clues that carbon monoxide is present.

"We make the assumption that comet BB was probably active even farther out, but we just didn't see it before this," says Farnham.

"We have the potential of doing this a lot, once a comet is seen, going back through time in the images and finding them while they are at farther distances from the Sun."