When meteoroids enter our atmosphere, they travel at enormous velocities. Meteoroids that overtake Earth travel at about 6 miles per second (10 km/s). Those that plunge head-on into our atmosphere, however, can reach g speeds of 45 miles per second (75 km/s), more than 225 times the speed of sound.
The surface of the meteoroid, rapidly heated by friction to more than 2,000 degrees Fahrenheit (1,100° C), begins to vaporize, or ablate. The meteoroid's particles and the particles in the atmosphere collide violently and are stripped of electrons, creating the meteor's intense luminosity, which can be seen more than 90 miles (150 km) away.
Tiny meteoroids last less than a second before burning up completely. Gravel-size meteoroids produce brighter and longer displays. Meteors as bright as the planet Venus are known as fireballs. These can cast shadows and last several seconds, leaving curling trails of glowing gas in their wake. Long-duration fireballs that explode or appear to fragment are called bolides.
The majority of meteoroids range in size from dust grains to small pebbles. Occasionally, larger meteoroids survive their fiery entry into Earth's atmosphere and reach the ground intact. When they do, they are called meteorites.