"I am a dirty snowball with a tail of fire. I visit the inner solar system occasionally and depart for thousands of years. Ancient people thought I was an omen. What am I?"

Answer: A Comet. Comets are balls of ice, dust, and rock. As they approach the sun, solar radiation vaporises their surface, creating a glowing tail of gas and dust. Before astronomy, their rare appearances were interpreted as omens of disaster.