Far out in the solar system, comets are dark and frozen. They are balls of ice, rock, and dust, sometimes called "dirty snowballs." But when a comet's orbit brings it closer to the sun, something amazing happens. The sun's heat causes the ice to turn directly into gas, skipping the liquid stage. This process is called sublimation. The gas and dust stream away from the comet, forming a glowing tail that can stretch millions of miles. On May 18, 1910, Halley's Comet made its closest pass by Earth. People around the world watched as its brilliant tail swept across the night sky. Some were terrified. Newspapers reported that the comet's tail contained a poisonous gas called cyanogen. Scared people bought gas masks and "comet pills." Scientists explained that the gas was far too spread out to cause any harm. Halley's Comet follows a predictable orbit that brings it near Earth roughly every 75 to 76 years. The English astronomer Edmond Halley figured this out in 1705. He studied records of bright comets and realized that sightings from 1531, 1607, and 1682 were all the same object. He predicted it would return in 1758, and it did -- sixteen years after his death. That confirmed his theory and made the comet famous forever. The next visit is expected in 2061.