The object that caused the Russian fireball was either a small asteroid or a chunk of a comet. When such space rocks hit the atmosphere, they become known as meteors. Any meteor fragments that reach the ground are then called meteorites.
Meteor showers occur when the Earth passes through loose fields of debris from comets and thousands of fragments -- from paint-chip sized flecks up to boulder-sized pieces or bigger -- penetrate the atmosphere, setting off sometimes spectacular sky shows. These showers, such as the Perseids and Leonids, occur at regular times of the year as the Earth swings through these debris fields.
- Brian Vastag