The astronomical unit (AU) is a basic unit of distance used by astronomers. It is the average distance from the Earth to the sun and is about 93-million miles (150-million km).
What is a Comet?
Comets are small members of the solar system, usually a few miles or kilometers in diameter. They have been described as "dirty snowballs" by astronomer Fred Whipple and are thought to be made of:
- ice (water, ammonia, methane, carbon dioxide)
- some carbon-containing (organic) materials (e.g., tar)
- a rocky center (some comets)
Comets are thought to be made from the earliest materials of the solar system. When the sun first formed, it blew lighter material (gases, dust) out into space. Some of this material (mainly gas) condensed to form the outer planets (Jupiter, Saturn, Uranus, Neptune) and some remains in orbit far from the sun in two areas:
- Oort Cloud - a sphere about 50,000 AUs from the sun; named after the Dutch astronomer Jan Oort who proposed it
- Kuiper Belt - an area within the plane of the solar system outside the orbit of Pluto