As they heat up, they give off gas and dust, which form the tails. Pressure from the sun and solar wind always blows a comet's tail away from the sun. As comets go, Hyakutake was unusual. It wasn't particularly large or bright, but it did give off the gases ethane and methane as it warmed up in its approach to the sun. Ethane had never before been detected in a comet, nor is it present in interstellar matter.
Hydrocarbons are part of the necessary ingredients for life.
This new information about Comet Hyakutake and the fact that a satellite can fly through its tail are a far cry from early theories about comet tails. Johannes Kepler, the mathematician and astronomer who formulated the laws of planetary motion, theorized that comets were spawned by the sun and were guided by a special spirit. He also speculated that if a comet's tail were to come into contact with our atmosphere, our air would be poisoned and many would die. Ulysses, launched in 1990, is a joint project of NASA and the European Space Agency. It is studying the sun from an orbit that takes it around our star's poles, away from the equatorial region where the planets orbit.