Question of the Month Submitted by Bob and Pat Gaskill, Orange County, and answered by Dr. William Bottke, Texaco Prize Fellow, Division of Geological and Planetary Sciences, Caltech.
Meteors and meteorites are small rocky fragments of other planetary bodies that fall to Earth. When they do so, they often produce spectacular audible and visual effects that can be seen from the ground. Meteorites, objects that survive their fiery passage through Earth's atmosphere, are of particular interest to scientists, since they are pieces of planetary bodies (mostly asteroids) for which samples have not yet been obtained through either manned or unmanned space missions. The oldest meteorites are remnants of the very first processes to occur in our solar system 4.6 billion years ago, giving us a glimpse into what conditions were when Earth was formed.
One common class of meteor is called a "fireball," named for the bright, streaming orbs produced when the surface of a fist-sized or larger body is boiled away by friction as it enters Earth's atmosphere. Fireballs decelerate from speeds of about 60,000 m.p.h. to 200 m.p.h. during this passage, often slowing enough at the end so that they literally drop to the ground. Their flight path is similar to a golf ball thrown at an angle into a swimming pool; once the water stops the forward momentum of the ball, it sinks to the bottom of the pool. The meteor is often not strong enough to survive this passage intact, which can make recovery of the fragments difficult.
Fireballs are mostly seen crossing the sky at night, though some are so bright they can be seen during the day. When a fireball is seen, it is usually several miles high. If any surviving meteoritic pieces were to survive to reach the ground, they would probably be over 500 miles from the observer. If enough people see the fireball from separate locations, however, scientists may be able to calculate where the fragments should strike Earth.
Studies indicate that about 25 meteorites weighing more than a fifth of a pound fall on California (or an area of equal size) each year. Three or four of these samples weigh about two pounds and are the size of your fist. Using these values, we can estimate that between 300 and 400 of these larger meteorites have fallen on California since the turn of the century. Most of these rocks, though, have not been found, leaving open the possibility that you yourself may discover one someday.