Meteoroids are small chunks of matter left in space. They are up to tens of meters in diameter. When the Earth encounters a meteoroid (Fig. 26.9), the meteoroid may fall through the Earth's atmosphere. It is then heated by the friction between the air and the meteoroid. It glows brightly as it streaks across the sky, as shown in Fig. 26.10. At this point, we refer to it as a meteor. Most meteors burn up as they pass through the atmosphere. However, some do reach the ground. The ones that reach the ground are called meteorites. The largest meteorites produce craters, including the large one shown in Fig. 26.11.
Some very small meteoroids, much less than 1 mm across, may settle into the upper atmosphere, to be collected by balloons. These are of interest because there are many more small mete-oroids than large ones.
Most meteoroids that produce meteors are probably the debris of comet tails. They are therefore left behind in the orbit of the comet, as shown in Fig. 26.12. When the Earth crosses the comet's orbit, we see a large number of meteors -a meteor shower. These showers occur at the same time each year, since they represent the passage of the Earth through the orbit of the comet. This scenario explains why we see most meteors after midnight. After midnight, an observer is on the side of the Earth facing in the direction of the orbital motion of the Earth.
, Earth's Orbit
Earth moving through meteoroids. Note that the activity is greatest when the observer is on the leading edge of the Earth, which occurs after midnight.The maximum effect is actually at dawn, so the best time to see meteors is just before it begins to get light in the morning.
Since we think that comets are left over from the formation of the Solar System, meteorites give us a chance to examine that material directly. Two different compositions have been identified. One group, called irons, are mostly iron and nickel. The other type, called stones, have an appearance similar to ordinary rocks and are hard to find on the ground unless the fall has been witnessed. There is another type, stony irons, which is a combination of two types of material. Most meteorites are stones.
Most of the stones contain small rounded glasslike particles, called chondrules. Meteorites containing chondrules are called chondrites. The others are called achondrites. Chondrites with large amounts of carbon are called carbonaceous chondrites. It is believed that these are the oldest meteorites. An example of a famous carbonaceous chondrite is the Murchison meteorite, which fell in Australia in 1969. This meteorite contains amino acids of a type (left-handed vs. right-handed) not found on Earth. The largest carbonaceous chon-drite is the Allende meteorite, which fell on Mexico in 1969. We think that the carbonaceous chondrites were never strongly heated after formation. They therefore preserve the original material out of which the Solar System formed. The Allende meteorite has centimeter-sized inclusions of minerals rich in calcium and aluminum.
It has been suggested that there is some relationship between asteroid types and meteorite
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