Animation of a swarm of comets passing the Moon and Earth. Comets are small bodies composed of ice and dust that have elongated orbits around the Sun. In the outer solar system they are practically impossible to detect due to their small size, but when they are heated as they approach the Sun, they outgas their volatile components, forming a huge coma and a long tail of reflective dust. This passage may break the comet into numerous smaller pieces, which increases the chances of a collision with Earth. Comet impacts are believed to have occurred several times in Earth's past. Such impacts may have brought much of the planet's water to Earth, but may also have caused destruction and extinction events.