Meet Phaethon, a weird asteroid that thinks it's a comet—new research may explain what's going on

What's the difference between an asteroid and a comet? A comet is basically a dirty iceball composed of rock and ice. The classic image is of a bright "star" in the night sky with a long curved tail extending into space. This is what happens when they approach the sun and start emitting gases and releasing dust. It normally continues until there's nothing left but rock or until they fragment into dust.