Comets have been observed for millennia, and their appearances were long thought to be harbingers of floods, famines, and other disasters. This illustration from the 16th-century Book of Miracles shows a comet that appeared in 1401 and was followed by a plague in the German region of Swabia. Though not actually omens, comets do carry a wealth of information. On page 34 , Kathrin Altwegg describes how in situ cometary measurements have uncovered details about the bodies’ compositions and the solar system’s history. (Image from steeve-x-art/Alamy Stock Photo.)