Comets have been observed by humanity for thousands of years, but only in the past few centuries have they been studied as astronomical phenomena. Before modern times, great comets caused worldwide fear, considered bad omens foreboding disaster and turmoil, for example the 1066 passage of Halley's Comet depicted as heralding the Norman conquest of England. As the science of astronomy developed planetary theories, understanding the nature and composition of comets became a challenging mystery and a large area of study.
Halley's comet, reappearing every 75–76 years, was pivotal to the study of comets, especially of their orbits. Thinkers such as Immanuel Kant in the eighteenth century hypothesized about the physical composition of comets. Today, comets are well understood as "dirty snowballs" in eccentric orbits around the Sun, but they continue as objects of scientific and popular fascination. In 1994, comet Shoemaker–Levy crashed spectacularly into the atmosphere of Jupiter. In 1997, a cult committed mass suicide inspired by the passage of comet Hale-Bopp. Since 1985, a total of 8 comets have been visited by spacecraft.