Comet passing near Earth, computer artwork. Comets are small astronomical objects composed of ice and dust. Comets have eccentric orbits. The cloud (coma) surrounding a comet is subliminated ice that forms when the comet approaches the Sun. The solar wind blows the coma into a tail, pointing away from the Sun. It is rare for a cometary nucleus to hit Earth, but Earth has passed harmlessly through comet tails Cometary impacts were more frequent in the early history of Earth, and are thought to have contributed the water that formed Earth's oceans.