Asteroid vs. Comet

Comets are made of gas, ice and dust. They orbit around the sun between the planets.

Most comets can only be seen through a telescope.

Some comets have long, shining tails. The force of light from the sun pushes dust and gas away from the comet to make the tail. Light from the sun makes the tail glow. The longest comet tail ever measured was recorded in1843. It was 330 million kilometres long.

The centre of a comet is made of frozen gas and ice mixed with dust. It is called the nucleus.

Around the nucleus is the coma, which is made of dust and gas. The coma evaporates when the comet approaches the sun.

Most comets are seen only once because they are pulled towards the sun by its gravity, and then disappear into outer space. Some comets are trapped by the sun's gravity and stay in orbit around the sun.

Halley's Comet

Halley's comet

Halley's comet

Halley's Comet is the most famous comet, but it can only be seen from Earth every 75 - 76 years.  It is named after an English astronomer, Edmond Halley, who in 1705 worked out that the comet would appear every 75 - 76 years.  It was photographed from spacecraft when it appeared in 1986. It is expected again in 2061.

Read more about Halley's comet

http://www.space.com/19878-halleys-comet.html