Meteoroids and Meteorites

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Meteoroids and meteorites

The word meteor is derived from the Greek meteron, meaning something high up. Today, however, the term is used to describe the light phenomena produced by the entry of a meteoroid into Earth's atmosphere. A meteoroid is defined to be any solid object moving in interplanetary space that is much larger than an atom or a molecule, but smaller than a few meters in diameter. A visual meteor, or shooting star, is produced whenever a meteoroid is vaporized in Earth's upper atmosphere. If a meteoroid survives its passage through the atmosphere without being fully vaporized and falls to the ground, it is a called a meteorite.

Upon entering Earth's upper atmosphere, a meteoroid begins to collide with an ever-increasing number of air molecules. These collisions will both slow the meteoroid down and heat its surface layers. At the same time the meteoroid is being decelerated, that energy is transferred from the meteoroid to the surrounding air. Some of the meteoroid's lost energy is transformed into light; it is this light that we observe as a meteor. As the meteoroid continues its journey through the atmosphere, its surface layers become so hot that vaporization begins. Continued heating causes more and more surface mass to be lost in a process known as ablation, and ultimately the meteoroid is completely vaporized.

The amount of surface heating that a meteoroid experiences is proportional to its surface area , and consequently very small meteoroids are not fully vaporized in the atmosphere. The size limit below which vaporization is no longer important is about 0.0004 in (0.01 mm). The smallest of meteoroids can safely pass through Earth's atmosphere without much physical alteration, and they may be collected as micrometeorites at Earth's surface. It is estimated that 22,000 tons (20,000 metric tons) of micrometeoritic material fall to Earth every year.

Visual meteors (shooting stars) are produced through the vaporization of millimeter-sized meteoroids. The speed with which meteoroids enter Earth's atmosphere varies from a minimum of 7 mi/sec (11 km/sec) to a maximum of 45 mi/sec (72 km/sec). The meteoroid ablation process typically begins at heights ranging 6271 mi (100115 km) above the earth's surface, and the whole meteoroid is usually vaporized by the time it has descended to a height of 43.5 mi (70 km).

Astronomers have found that the visually observed meteors are derived from two meteoroid populations: a continuously active, but sporadic, background and a number of specific sources called meteoroid streams.

On any clear night of the year, an observer can expect to see 1012 sporadic meteors per hour. Sporadic meteors can appear from any part of the sky, and about 500,000 sporadic meteoroids enter Earth's atmosphere every day.

Meteor activity is often described in terms of the number of meteors observed per hour. The observed hourly rate of meteors will be dependent upon the prevalent visual conditions. Factors such as the presence of a full moon , local light pollution, and clouds will reduce the meteor count and, hence, lower the observed hourly rate. Astronomers often quote a corrected hourly rate which describes the number of meteors that an observer would see, each hour, if the observing conditions were perfect.

Observations have shown that the corrected hourly rate of sporadic meteors varies in a periodic fashion during the course of a day. On a typical clear night, the hourly rate of sporadic meteors is at a minimum of about six meteors per hour at 6 p.m.. The hourly rate climbs steadily during the night until it reaches a maximum of about 16 meteors per hour around 4 a.m.

This daily variation in the hourly rate of sporadic meteors is due to Earth's rotation in its orbit about the Sun . In the evening, a sporadic meteoroid has to catch up with Earth if it is to enter the atmosphere and be seen. This is because at about 6 p.m. local time an observer will be on that part of Earth's surface which is trailing in the direction of Earth's motion. In the early morning, however, the observer will be on the leading portion of Earth's surface, and consequently Earth will tend to "sweep up" all the meteoroids in its path. An observer will typically see two to three times more sporadic meteors per hour in the early morning than in the early evening; and will see them at higher speeds relative to Earth.

Meteor showers occur when Earth passes through the tube-like structure of meteoroids left in the wake of a comet. Such meteoroid tubesor as they are more commonly called, meteoroid streamsare formed after a comet has made many repeated passages by the Sun. Meteoroid streams are composed of silicate (i.e., rocky) grains that were once embedded in the surface ices of a parent comet. Grains are released from a comet's nucleus whenever solar heating causes the surface ices to sublimate. New grains are injected into the meteoroid stream each time the comet passes close by the Sun.

The individual dust grains (technically meteoroids once they have left the comet) move along orbits that are similar to that of the parent comet. Gradually, over the course of several hundreds of years, the meteoroids form a diffuse shell of material around the whole orbit of the parent comet. Provided that the stream meteoroids are distributed in a reasonably uniform manner, a meteor shower will be seen each year when the earth passes through the stream. The shower occurs at the same time each year because the position at which the meteoroid stream intersects Earth's orbit does not vary much from one year to the next. There are long-term variations, however, and the days during which a shower is active will eventually change.

When Earth passes through a meteoroid stream, the meteoroids are moving through space along nearly parallel paths. Upon entering Earth's atmosphere, however, a perspective effect causes the shower meteors to apparently originate from a small region of the sky; this region is called the radiant.

The radiant is typically just a few degrees across when projected onto the night sky. A meteor shower is usually, but not always, named after the constellation in which the radiant falls on the night of the shower maximum. The Orionid meteor shower, for example, is so named because on the night of the shower maximum (October 21st) the stream radiant is located in the constellation of Orion. Some meteor showers are named after bright stars. The Eta Aquarid meteor shower, for example, is so named because on the night of the shower maximum (May 3rd) the radiant is close to the seventh brightest star in the constellation of Aquarius (by convention the brightest stars in a constellation are labeled after the Greek alphabet, and accordingly, the seventh letter in the Greek alphabet is eta).

Probably the best-known meteor shower is the one known as the Perseid shower. This shower reaches its peak on the night of August 12th each year, but meteors can be observed from the stream for several weeks on either side of the maximum. The shower's radiant first appears in the constellation of Andromedia in mid-July, and by late August it has moved into the constellation of Camelopardalis. The radiant is in the constellation of Perseus on the night of the shower maximum.

The steady eastward drift of the radiant across the night sky is due to the motion of Earth through the Perseid meteoroid stream. The nearly constant year-to-year activity associated with the Perseid meteor shower indicates that the stream is very old. Essentially, the earth encounters about the same number of Perseid meteoroids each year even though it is sampling different segments of the stream. Since 1988, however, higher than normal meteor rates have been observed about 12 hours before the time of the traditional shower maximum (August 12th). This short-lived period (approximately half an hour) of high activity is caused by new meteoroids that were ejected from the stream's parent comet, Comet Swift-Tuttle, in 1862. Comet Swift-Tuttle last rounded the Sun in late 1992, and it is expected that higher than normal meteor rates will be visible half-a-day before the time of the "traditional" Perseid maximum until around 2015.

Another meteor shower known as the Leonid occurs every year in November, caused by the tail of comet Tempel-Tuttle, which passes through the inner solar system every 3233 years. Such a year was 1998; on November 17 and 18, 1998, observers on Earth saw as many as 200 meteors an hour. The shower was so intense that it generated widespread concern about the disruption of global telecommunications and the possible damage or destruction of space telescopes. Partly as a result of careful preparation by satellite and telescope engineers, however, concerns appeared to be minimal.

If a meteoroid is to survive its passage through Earth's atmosphere to become a meteorite, it must be both large and dense. If these physical conditions are not met, it is more than likely that the meteoroid, as it ploughs through Earth's atmosphere, will either crumble into many small fragments, or it will be completely vaporized before it hits Earth's surface. Most of the meteoroids that produce meteorites are believed to be asteroidal in origin. In essence, they are the small fragmentary chips thrown off when two minor planets (asteroids ) collide. Meteorites are very valuable then, for bringing samples of asteroidal material to Earth. A few very rare meteorite samples are believed to have come from the planet Mars and the Moon. It is believed that these rare meteorite specimens characterize material that was ejected from the surfaces of Mars and the Moon during the formation of large impact craters.

Accurate orbits are presently known for just four recovered meteorites (the Pibram meteorite, which fell in the Czech Republic in 1959; the Lost City meteorite, which fell in Oklahoma in 1970; the Innisfree meteorite, which fell in Alberta, Canada, in 1977; and the Peekskill meteorite, which fell in New York State in 1992). All four of these meteorites have orbits that extend to the main asteroid belt between the planets Mars and Jupiter.

Meteorites are superficially described as being either falls or finds. A meteorite fall is scientifically more useful than a find because the exact time that it hit Earth's surface is known. Finds, on the other hand, are simply thatmeteorites that have been found by chance. The largest meteorite find to date is that of the 66 ton (60 metric ton) Hoba meteorite in South Africa . Meteorites are either named after the specific geographic location in which they fall, or after the postal station nearest to the site of the fall.

An analysis of meteorite fall statistics suggests that about 30,000 meteorites of mass greater than 3.5 oz (100 g) fall to Earth each year. Of these meteorites, the majority weigh just a few hundred grams, only a few (about 5,000) weigh more than 2.2 lb (1 kg), and fewer still (about 700) weigh more than 22 lb (10 kg). In general, the number of meteoroids hitting the earth's atmosphere increases with decreasing meteoroid mass: milligram meteoroids, for example, are about a million times more common than meteoroids weighing a kilogram.

Meteorites are classified according to the amount of silicate and metallic nickel-iron that they contain. Three main meteorite types are recognized; these are the irons, the stones, and the stony-irons. The iron meteorites consist almost entirely of nickel-iron, while the stone meteorites are mostly silicates. The stony-iron meteorites contain both nickel-iron and silicates. The stony meteorites are further divided into chondrites and achondrites. The term chondrite is applied if the meteorite is composed of many small, rounded fragments (called chondrules) bound together in a silicate matrix. If no chondrules are present then the meteorite is an achondrite. Most (about 85%) of the stony meteorites are chondrites. Meteorite fall statistics indicates that about 96% of meteorites are stony, 3% are irons and 1% are stony-irons.

Even though many thousands of meteorites fall to Earth each year it is rare for one to hit a human being. The chances of a human fatality resulting from the fall of a meteorite have been calculated as one death, somewhere in the world, every 52 years. Thankfully, no human deaths from falling meteorites have been reported this century. A woman in Sylacauga, Alabama, was injured, however, by a 8.6 lb (3.9 kg) meteorite that crashed through the roof of her house in 1954. Another close call occurred in August of 1991, when a small meteorite plunged to the ground just a few meters away from two boys in Noblesville, Indiana.

In contrast to the situation with human beings, meteorite damage to buildings is much more commonthe larger an object is the more likely it will be hit by a meteorite. A farm building, for example, was struck by a meteorite fragment in St. Robert, Quebec in June of 1994. Likewise, in August 1992, a small village in Uganda was showered by at least 50 meteorite fragments. Two of the meteorites smashed through the roof of the local railway station, one meteorite pierced the roof of a cotton factory, and another fragment hit an oil storage facility. One of the more spectacular incidents of meteorite-sustained damage in recent times is that of the Peekskill meteorite which fell in October of 1992 and hit a parked car.

See also Astronomy; Barringer meteor crater; Impact crater; Murchison meteorite; Space and planetary geology