On any given night under dark skies you can see a dozen or more bright meteors, heaven s reward for having your eyes on the sky. But sometimes the Earth encounters not just random bits of dust, but a more concentrated swarm. These are called meteor showers, and they are recurring events that take place at the same time each year. The Lyrids appear to come from the constellation Lyra, the Perseids from Perseus, etc. The reason that the dates of meteor showers remain the same from year to year is that on those dates the Earth reaches points in its orbit where there are known concentrations of space dust and debris.
Comets are clumps of dust and ice a few miles across and they are very loosely held together.
As they travel in their orbits about the Sun they are continually scattering dust and debris in their wake, and over time the path of their orbits become one continuous trail of thin dust and debris. The orbits of most comets don t cross the Earth s orbit, but when one does then the Earth, in subsequent years, will cross the dust trail of the comet and a meteor shower will occur at that point each year. The comet Tempel-Tuttle, responsible for the Leonid meteor shower, is seen here as it appeared during its closest approach to the Earth. The glowing head, called a coma, extends at least 30,000 miles from the nucleus, but no tail is evident. The long star trails are due to the rapid motion of the comet as it passes the Earth.