DK Science: Comets
Sometimes icy lumps left over from the birth of the Solar System visit our skies. We see them as comets. Although tiny, comets release vast clouds of gas and dust as they approach the Sun and heat up. The clouds form a bright head and long tails, often millions of kilometres long. When Earth passes through the dust from past comets we see METEOR showers.
One of the brightest comets of the 20th century, Hale-Bopp blazed in the night sky for weeks during the spring of 1997. Its bright coma (head) hid a nucleus about 30–40 km (20–30 miles) across. The effects of sunlight and of the solar wind strung out the gas and dust released by the comet into long tails.
Comets may head in towards the Sun from any direction. They have highly elliptical (oval) orbits. Comets may take just a few years or thousands to circle the Sun. Some seem to come from reservoirs of icy bodies in the Kuiper Belt, or from further out in a region called the Oort Cloud. Unseen for most of the time, comets become visible only when they approach the Sun.
Halley was an astronomer and mathematician who became the second Astronomer Royal. He is best known for discovering that some comets are regular visitors to Earth’s skies. He correctly predicted that the comet he had seen in 1682 would return again in 1758. It did, and was named Halley’s comet after him.
On a clear night you may see little streaks of light in the sky, which are often called falling or shooting stars. But these streaks are properly called meteors. They are caused by little rocky specks plunging through the atmosphere towards Earth. Friction with the air makes them so hot that they burn up into dust. As much as 200 tonnes of meteor dust falls to Earth every day.
When Earth crosses the orbit of a comet dozens of meteors per hour may be seen. This is called a meteor shower. Showers are named after the part of the sky they appear to come from. For example, the Leonid shower in mid-November seems to come from the constellation Leo. It takes place when Earth passes through dust from comet Tempel-Tuttle. Every 30 years or so it puts on an exceptional display of hundreds and thousands of meteors per hour.