Comets: icy visitors
Comets are giant lumps of ice and dust that grow glowing tails.
Far out beyond the planets, there are millions of icy lumps. They are made of frozen water, frozen gases, rock and dust. Some scientists call them dirty snowballs.
Sometimes one of them falls in towards the Sun. As it comes closer, the Sun warms it. Ice turns into gas, and dust is pushed out.
The gas and dust make a big glowing cloud around the comet's icy middle. Then sunlight and a wind of tiny particles from the Sun push them into long tails.
A comet's tails always point away from the Sun, even when the comet is heading back out into space.
The most famous comet is Halley's Comet. It comes back every 75 to 76 years. It last came in 1986, and it will return in 2061.
A spacecraft called Rosetta followed a comet for two years. In 2014 it even sent a small lander called Philae down onto the comet's surface.
The icy middle of a comet, called the nucleus, is often only a few kilometres across, but its tail can be millions of kilometres long.