Dark comets are often small, just a few metres to a few hundred metres wide. This leaves less surface area for material to escape and form into the beautiful tails we see on typical comets.

And high above, comet Hale-Bopp hung suspended like a feathery fishing lure, its tail curving off a bit, as if blown to the side by the punishing wind. One by one, stars winked on in a darkening sky.

A widely held view is that these vital components were deposited on Earth by a long bombardment of icy comets. Comets are made of dust and ice. Astronomers have examined the water contained in ...