In this game of gravity, by far the most dangerous breed of impactor is the long-period comet, which, by convention, are those with periods greater than two hundred years. Representing about one fourth of Earth's total risk of impacts, they fall toward the inner solar system from great distances and achieve speeds in excess of 100,000 miles per hour by the time they reach Earth. Long-period comets thus achieve a much higher impact energy for their size than your run-of-the-mill asteroid. More importantly, they are too dim over most of their orbit to be reliably tracked. By the time a long-period comet is discovered to be heading our way, we might have anywhere from several months to two years to fund, design, build, launch, and intercept it. For example, in 1996, comet Hyakutake was discovered only four months before its closest approach to the Sun because its orbit was tipped strongly out of the plane of our solar system and nobody was looking. While en route, it came within 10 million miles of Earth (a narrow miss) and made for spectacular nighttime viewing.