Comet Hyakutake formally designated C/1996 B2) is a long-period comet. It was discovered on 31 January 1996 and passed very close to Earth in March of that year. It was dubbed The Great Comet of 1996; its passage near the Earth was one of the closest cometary approaches of the previous 200 years. Hyakutake appeared very bright in the night sky and was widely seen around the world. The comet temporarily upstaged the much anticipated Comet Hale–Bopp, which was approaching the inner Solar System at the time. Scientific observations of the comet led to several discoveries. Most surprising to cometary scientists was the first discovery of X-ray emission from a comet, believed to have been caused by ionised solar wind particles interacting with neutral atoms in the coma of the comet. The Ulysses spacecraft unexpectedly crossed the comet's tail at a distance of more than 500 million kilometres (3.3 AU or 3×108 mi) from the nucleus, showing that Hyakutake had the longest tail ever known for a comet. Before its most recent passage through the Solar System, its orbital period was about 17,000 years, but the gravitational perturbation of the giant planets has increased this period to 70,000 years.