|An artist's impression of meteoroids (potential meteorites) about to enter the earth's atmosphere. |
Most meteorites which land on our planet are believed to have originated within the Asteroid Belt.
A small rocky or metallic object in orbit around the Sun (or another star). A meteoroid which strikes the Earth (or other large body) is called a meteorite. As a meteoroid encounters the Earth's atmosphere frictional heating begins at an altitude of 100 to 120 km. What happens next depends on the speed, mass, and friability (tendency to break up) of the meteoroid. Micrometeoroids radiate heat so effectively that they survive unchanged to reach the surface as micrometeorites. Objects about the size of sugar grains burn up as meteors or "shooting stars". Friable meteoroids break up and are destroyed at altitudes of 80 to 90 km. Those which are tougher survive longer and produce fireballs as their surface is melted and eaten away at temperatures of several thousand degrees. If they avoid destruction high up, they enter the lower, denser part of the atmosphere where they are rapidly decelerated. Finally, at subsonic speeds the fireball is extinguished and the residue falls to the ground as a meteorite. The last melted material on the surface of the object solidifies to form a thin, usually black, rind known as a fusion crust.
Meteorites are among the rarest materials found on earth and are also the oldest things any human has ever touched. Chondrules—small, colorful, grain-like spheres about the size of a pin head—are found in the most common type of stone meteorite, and give that class its name: the chondrites. Chondrules are believed to have formed in the solar nebula disk, even before the planets which now inhabit our solar system. Our own planet was probably once made up of chondritic material, but geologic processes have obliterated all traces of the ancient chondrules. The only way we can study these 4.6 billion year old mementoes from the early days of the Solar System is by looking at meteorites. And so meteorites become valuable to scientists as they are nothing less than history, chemistry, and geology lessons from space.
There are a number of periodic meteor showers visible each year in the night sky: the Perseids in August, and the Leonids in November usually being the most interesting to observe. The annual meteor showers are the result of our planet passing through debris trails left by comets. The meteors we see during those annual displays are typically small pieces of ice which rapidly burn up in the atmosphere and never make it to the surface of our planet.
|Perseid meteors appear to stream from a point — |
called the radiant — in the constellation Perseus.
The shower is best after midnight local time, when the radiant
rises in the northeast.
Perseids. The best known of all meteor showers, the Perseids never fail to put on a good show and — thanks to the shower's late-summer peak — are usually widely observed. The earliest record of this event comes from China in a.d. 36. Generally visible from July 17 to August 24, meteor speed (37 miles [60 km] per second), brightness, and a high proportion of trains (45 percent) distinguish the Perseids from other showers active at this time. It became the first meteor shower linked to a comet (109P/Swift-Tuttle) in 1865. Models of the Perseids predict a gradual decline in activity from a peak in 2004.
Leonids. Leonid meteors generally arrive between November 14 and 21, with a peak hourly rate on November 17 of between 10 and 15 meteors per hour; about half of these meteors leave trains that can persist for several minutes. Because Earth runs into the orbiting particles almost directly head-on, Leonid meteors travel faster than those of any other shower — 45 miles (71 km) per second. The shower's most notable feature is its habit of producing periodic, dramatic meteor storms as Earth intercepts streams of dense material ejected at previous returns of Comet Tempel-Tuttle. Our planet passed through such streams annually from 1998 to 2003. Computer models show that Jupiter's tug on the dense Leonid streams causes them to miss Earth until at least 2098. Because the stream responsible for the predicted outbursts was ejected in 1933, only its smallest particles have been able to drift into a path that Earth will intersect. This means any outburst, if one occurs at all, will be rich in faint meteors.