Comets are the solar system's long-distance travelers.
We know a few rather well, such as comet Halley, which reappears regularly every 76 years. Such short-period comets are predictable, making it possible for astronomers to prepare for each apparition. But other comets are unexpected visitors, traveling paths that last took them by Earth before there were human observers capable of recording their visits or predicting their returns for whatever progeny might follow. It was first noticed on January 31 of this year by amateur astronomer Yuki Hyakutake, who systematically searches the sky with giant binoculars. In only two months, this unexpected visitor had grown into one of the most spectacular comets of the 20th century.