Meteoroids are rocky or metallic objects that can range in size from small grains to several-kilogram bodies. Most meteoroids are pieces that have been blown or knocked off larger objects, such as comets and asteroids. Therefore, it is not uncommon for comets and asteroids to share their orbits with “streams” of meteoric material. Meteor showers occur when Earth passes through one of these streams.
The brief streaks of luminescence we call meteors are caused by the bits of meteoric material burning up as they pass through the upper atmosphere. These streaks all appear to originate in the same place because all the meteors in the stream are passing Earth in the same direction at the same speed. The number of meteors visible during a particular shower depends on the density of the meteor stream.