The Ulysses spacecraft made an unplanned and unexpected pass through the tail of the comet on May 1, 1996. Based on the Ulysses encounter, the comet's tail is known to have been at least 300 million miles long. Terrestrial observers found ethane and methane in the comet, the first time either of these gases had been detected in a comet.
External link: JPL Comet Hyakutake home page