For as long as humans have watched the night sky, they have been fascinated—and puzzled—by the stars that left brilliant trails of light in their path. The Chinese called these apparitions “broom stars” and the Greeks’ equally descriptive term, kometes (“long-haired stars”), provided the root of our name for them. Ancient peoples feared comets, whose sudden appearance was seen as a portent of disaster. The Bayeux Tapestry’s depiction of King Harold’s defeat at the Battle of Hastings in 1066 includes a comet streaking across the fabric sky.

Nearly 700 years later, English astronomer Edmund Halley reviewed historical descriptions of great comets and concluded that three of them (including the one that signaled William’s conquest) may represent a single celestial object orbiting the sun in a distinctive and predictable elliptical path. In subsequent centuries, the rapid development of telescopes greatly expanded investigations in astronomy, but even the most powerful telescopes could not resolve the question of what lay at the icy heart of a comet.

The era of space exploration triggered significant advances in our understanding of comets. Jim Zimbelman, a planetary geologist at the Air and Space Museum, examines how a new explosion of insights expanded and deepened our earlier knowledge of these ancient wanderers through space—which are now seen as frozen remnants from the formation of the solar system. He covers significant modern investigations into comets, including the international armada of spacecraft that met the 1986 appearance of Halley’s Comet, and the European Space Agency’s mission of the Rosetta spacecraft, which in 2014 released a robotic lander that became the first to achieve a successful soft landing on a comet.