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Incorrectly called "shooting stars," these brief dashes of light are actually small rocky debris passing through Earth's atmosphere some 50 to 70 miles above the ground. They are called meteors. Ranging in size from a pea to a grain of sand, these rocks travel at speeds ranging from 30 to 50 thousand miles per hour! Moving so fast through the air, meteors experience air friction, which causes them to heat up, glow, and either shatter or disintegrate entirely. (To understand a little about friction, rub your hands together very quickly; what happens?)
Simultaneously, the rapidly moving stone also heats up air molecules, causing them to glow as well. As a result, what you see is a quick streak of light in the night sky as the meteor passes through our atmosphere. The smallest of these meteors, called micrometeorites (see Activity 36), drift slowly toward Earth, whereas meteors about the size of your fist and larger may hit Earth after a colorful passage through the air. Sometimes called "nature's fireworks," such meteors are most commonly known as fireballs. Should the meteor "pop" and crack into visibly colorful pieces during its atmospheric entry, it is called a bolide.
The major meteor showers, shown in Table 35-1, surprisingly come primarily from comets. Upon nearing the sun, a comet, also known as a dirty snowball, is heated and slowly vaporizes, creating a gaseous tail and a stream of small chunks of rock (see Activity 37, especially Figure 37-3). This region of cometary debris then remains in its own orbit; individual particles are called meteoroids. As Earth passes through this stream of debris, the particles, called meteors, enter Earth's atmosphere. The ones that do not disintegrate but survive to impact Earth are called meteorites (see Figure 35-3). Meteor showers and sporadic meteors (about 7 per hour per night are visible) together contribute some 70,000 tons of disintegrated rock to Earth's atmosphere each year! Meteorites are generally indistinguishable from Earth's rocks, although most Earth stones, unlike meteorites, show signs of weathering and erosion, often giving them rounded shapes. Thus, it often requires careful scientific studies to reveal if a rock is meteoric or not.
During its history, Earth has experienced a barrage of meteor impacts that have left countless pockmarks, called CRATERS. After 4.5 billion years of resurfacing, weathering, and erosion, many craters have filled in, making them virtually undetectable. Some, though, remain nearly pristine, such as Meteor Crater near Flagstaff, Arizona (Figure 35-4). Nearly 600 feet deep and 1 mile wide, this impressive sight was created about 49,000 years ago by a meteor weighing several thousand tons.
Though most meteors in these showers usually burn up in our atmosphere, occasionally a meteor will impact Earth's surface. Such larger meteors usually don't originate from any particular comet, but may instead be remnants of asteroid collisions in the Asteroid Belt (between Mars and Jupiter). In fact, some meteorites found on Earth are believed to be ejected moon rocks kicked up during a meteor impact on the moon, some 240,000 miles distant. Some are even from Mars, some 40 million miles from Earth at its closest.
The main classes of meteorites -- stones, stony-irons, and irons -- are determined by the constituents of the meteorite. Stony meteorites, the most abundant seen to fall, are similar to Earth rocks and therefore difficult to find. Some contain a great deal of carbon, an element that's present in living things. In fact, some contain amino acids, the building blocks of life. Having remained unchanged since their formation 4.5 billion years ago, stony meteorites are the oldest material ever to be held by humans. Irons, on the other hand, are very dense mixtures of metals, mostly iron and nickel, that have experienced significant heating and melting since their formation. Stony-irons are a mix of the two. Iron-rich meteorites attract compass needles or cause watch windings to spin.
An odd meteor impact in 1992 crumpled the trunk of a parked car (see Figure 35-5) belonging to Michelle Knapp of Peekskill, New York. The damage was initially considered to be the work of a vandal, but the perpetrator was soon discovered to be a twenty-seven-pound stony meteorite whose path was witnessed by many along the eastern seaboard on the evening of October 9. Ironically, Ms. Knapp quickly received handsome offers for both the meteorite and the car from scientists, museums, collectors, and even London's Sotheby's auctioneers!
A rare motion picture made in August 1972 in the Grand Teton mountains of Wyoming captured a meteor as it passed through our atmosphere some 36 miles above the ground never striking Earth's surface! This incredible footage, taken by James Baker of Omaha, Nebraska, was invaluable to astronomers, who used it to track the meteor's path, size, and altitude. The 13-foot-long meteor left a clear smoke trail visible for nearly thirty minutes. The meteor itself, visible during daylight for no more than a minute, caused several sonic booms over Utah, Wyoming, and even Alberta, Canada, before leaving Earth's atmosphere.
Take careful notes on the time, direction, brightness, and colors of the meteors you see. Look for the radiant, the constellation from which all the meteors of a particular shower appear to emanate. This will give you a general idea of the location in space from which a particular meteor swarm will occur each year. The radiant also gives the name of the meteor shower. For example, the constellation of Orion gives its name to the Orionid meteor shower. To best determine the shower's hourly rate, observe with many people, each facing a different direction. Patience is important, so plan to observe for at least an hour and dress appropriately.
Speaking of hourly rates, one of the most amazing showers is the Leonid meteor shower, which has been known to flare up from a low 10 meteors per hour to an incredible 10,000 to 100,000 meteors per hour once every thirty-three years. The next anticipated Leonid burst of this magnitude is expected in mid-November of 1999. Don't miss it!
Micrometeors, unlike larger meteors, do not burn up in the atmosphere as they approach Earth because they are too lightweight to do so. In the same way that a piece of plastic foam floats in a pool of water, micrometeors float upon our atmosphere. They have such little weight that the air can support them.
The greatest concentration of micrometeors occurs around the appearance of a major meteor shower (see Table 35-1 in the previous activity for a list of shower dates). Thereafter, the particles can remain suspended in the air for weeks or months until they are washed toward Earth's surface by wind, rain, or snow. It is estimated that some 100 tons (about 200,000 pounds) of micrometeors enter Earth's atmosphere each day with more entering during large meteor showers. Their presence is not normally noticed because they leave no visible trail.
Start by lining the bottom of your 12-inch by 18-inch aluminum pan with aluminum foil. To serve as an isolated weight, place the brick in a plastic bag, tie the bag closed tightly, and place it in the pan. Now put the pan in as high and clear an outdoor location as possible (the best spot is on a rooftop in a clearing). Allow the pan to collect rainwater before, during, and after a meteor shower.
Several days after the peak of meteor activity, it's time to see if you've captured anything. Put the magnet in a clear plastic bag. Run the bagged magnet through the rain-filled pan to attract metallic micrometeorites. The bag makes it easier to remove the particles from the magnet. Line the small bowl with aluminum foil, fill it with distilled water, and put the bagged magnet and adhering particles into the bowl.
Carefully remove the magnet from its bag, allowing the particles to settle to the bowl's bottom. You can then collect the particles by letting the bowl stand covered with gauze for a few days and allowing the water to evaporate. (You can also collect the particles by pouring the water from the bowl into a foil-lined pot and then boiling off the water, but this is a less desirable method.) Similarly, evaporate or boil off the water from the large aluminum pan to collect nonmetallic micrometeorites. Allow the particles to dry, being sure to keep them from dust.
Once you have collected and dried the particles, sort them by size. A magnifying glass or microscope with a 100× eyepiece may be helpful in looking at the very small pieces. Though many of the particles you have collected will be from Earth (such as pollutants and dust), many will have originated from comets or interstellar space. If you collect over enough days, you should be able to notice that the amount of Earth debris is relatively constant, whereas the amount of space debris varies over the course of time.
The particles you collect may be from any of the three main classes of meteor material: stones, irons, and stony-irons. The metallic micrometeorites will most likely fall into the iron or stony-iron categories. Because of their magnetic properties and high iron-nickel content, these rocks are fairly easy to distinguish from terrestrial stones. The nonmetallic particles in your collection will most likely be of the stony classification. These are harder to distinguish from Earth stones. Geologists and planetary astronomers use sophisticated equipment to analyze the stones to determine their actual composition.