What is a comet?
A comet is an icy body made of frozen gases, rock and dust that orbits the Sun. When it comes close to the Sun, it heats up and grows a glowing head and long tails.
Comets are sometimes described as dirty snowballs. Their solid core, called the nucleus, is usually a few kilometers wide. As a comet nears the Sun, its ice turns directly into gas, releasing dust and forming a huge cloud around the nucleus called the coma, which can be larger than a planet.
Comets often grow two tails. A curved dust tail reflects sunlight, while a straight, bluish ion tail is pushed by the solar wind. Both tails always point away from the Sun, so a comet leaving the inner solar system actually travels tail-first. Tails can stretch for millions of kilometers.
Where comets come from
Short-period comets, which return within 200 years, mostly come from the Kuiper Belt beyond Neptune. Long-period comets come from the distant Oort Cloud and may take thousands of years to orbit. Halley's Comet returns about every 76 years. It was last seen in 1986 and will return in 2061.
Did You Know?
In 2014, the European Space Agency's Rosetta mission placed the Philae lander on Comet 67P, the first landing on a comet.