Meteoroids to Meteorites: Lessons in Survival

Meteorites are rocks from other worlds. To reach Earth intact, all meteoroids must pass a rigorous test before they can become meteorites. They must survive passage through Earth’s dense atmosphere. Earth’s atmosphere provides an effective shield against most incoming meteoroids. With this protective shield in place, they stand little chance of reaching Earth’s surface without considerable damage. Particles from dust-sized to two or three millimeters diameter normally don’t make it. They are totally consumed, ablated away by frictional heating in the atmosphere. Larger bodies fortunately can survive their passage relatively unscathed, though reduced in mass and size but still intact. Every year, worldwide, about 40,000 tons of meteoritic debris make it through the atmosphere. Among them are the meteorites we see preserved in museums and in private collections. They are the ones that have passed the fiery test.

One should not confuse the occasional bright meteor with the most spectacular of meteors called fireballs or bolides. They are produced by relatively large meteoroids ranging from walnutsized to several feet across. By long established convention, a fireball is any meteoroid that has attained an apparent magnitude of −5 or brighter with no real upper limit (the Sun is −26.5). The brightest fireballs often have magnitudes exceeding the brightness of the full moon (−12.5). These chunks of rock are often large enough to survive atmospheric passage. They are pieces of asteroids that have been involved in impacts with other asteroids in space numerous times in their history. Often the most energetic impacts result in fractures that structurally weaken them. If they happen to encounter Earth’s atmosphere in their weakened condition they stand a good chance of fragmenting into several pieces.