At one time or another, almost everyone has glimpsed a swift little streak of light dashing across the night sky. These sudden celestial visitors are meteors, commonly called falling or shooting stars. Meteors are pieces of space debris that plow into the Earth's atmosphere. --. Because they arrive at very high speeds anywhere from 11 to 74 kilometers (7 to 46 miles) per second -- they vaporize by air friction in a white-hot streak. Most meteor parents (meteoroids) range in size from sand grains to pebbles. Occasionally a larger object will survive its descent and fall to Earth -- then it's called a meteorite. Many meteroids move around the sun in swarms; each time the earth passes through a swarm we see a shower of shooting stars. The swarms are remains of dead comets. The most spectacular annual shower is known as the Perseids ( 25 July to 18 August). In addition to meteor showers, there are also non-shower or sporadic meteors, which may come from any direction at any moments. Meteorites are fragments of meteoritic rock that succeed in reaching the ground. They are thought to be associated with fragmented asteroids. They are of two main types: irons (siderites) and stones (aerolites), although there is no hard-and-fast distinction, and many intermediate types are known. Meteorites are named for the post office that is nearest to the place they land. Meteorites are valuable. Until rocks were brought back from the moon, they were the only materials from space that scientists could examine closely. A meteorite that fell near Murchison, Australia, in 1969 contained simple amino acids, the building blocks of all living things on earth. Amino acids have been found in meteorites collected in recent years in Antarctica, where they were well preserved. These finds indicate that the ingredients for all living things exist in space and have led to speculation that life may too.
A meteor that appears brighter than any of the stars and planets is called a fireball. A spectacular fireball is often reported over a distance of 200 miles or more. The sudden appearance and fast motion of bright meteor produces an illusion of closeness that can fool even well trained professionals. Airline pilots have swerved to avoid meteors that were actually 100 miles away.
Most meteors are seen 50 to 75 miles above the ground. Occasionally someone will claim to see a fireball land just beyond a tree or a hilltop, but in fact a typical fireball first appears at a height of about 80 miles and loses its brightness while still at least 12 miles above the ground.
Much more abundant are smaller, everyday meteors. While most look white, some appear blue, green, yellow, orange, or red. One that explodes at the end of its visible flight is called a bolide. At certain times of the year we see more meteors than usual. This happens when Earth passes near a comet's orbit and sweeps through debris that the comet has shed. Such events are called meteor showers. For the major annual meteor showers, seeing one meteor every few minutes is typical, though there are often bursts and lulls.
Shower meteors can appear anywhere in the sky, but their direction of motion is away from the constellation whose name the shower bears. This apparent point of origin is known as the radiant. Some observers feel that the best place to watch is between a shower's radiant and the zenith (the point directly overhead).
Here are some of the best of the year's meteor showers.
Leonids (Nov. 17-18). The Leonid shower occurs when Earth passes near the orbit of Comet 55P/Tempel-Tuttle. When the comet visits the inner solar system, the Leonid shower sometimes produces a veritable storm of meteors. This last occurred in 1966, when up to 40 meteors per second were seen for about an hour! Both 1998 and 1999 are good years to watch for a possible repeat of this performance. Best chances: the early morning hours of November 17 or 18, 1998, and November 18, 1999.
Geminids (Dec. 12-14). This is the only major meteor shower known to be associated with an asteroid (3200 Phaethon). It should peak on the night of December 13-14, 1998, and the nights of December 13-14 and 14-15, 1999. An observer may see 80 meteors in an hour under moonless, rural skies from late evening through dawn. A suburban observer may see only 30 meteors or fewer per hour. The primes observing hours are essentially Moon-free in both 1998 and 1999.
Perseids (Aug. 8-15). This best-known meteor shower has a broad peak, and it occurs at a pleasant time of the year for Northern Hemisphere observers. With the Moon near new and out of the sky all night, 1999 should be a great year for the Perseids. In recent years there have been two peaks about a half-day apart. The morning of August 13, 1999, should be the best time to watch, but rates should remain high for a day or two on either side. A single observer may see 50 to 100 meteors per hour under dark, rural skies in the hours before dawn. Rates in the evening are much lower. Suburbanites may expect closer to 25 to 40 meteors per hour even during the peak predawn hours. Brief outbursts sometimes enhance these rates greatly.
An Accident ! ! ! !
A mysterious explosion occurred over Siberia in the Soviet Union in 1908. Its effects were felt all around the world. In the immediate area, trees were blown down and charred. No craters have been found there, but the Russian scientists figure that 4,000 tons of material fell to the ground either from a huge fireball or a comet. They found diamond-like grains and ashes in the area that looked like the remains of a meteorite fall. The 66-ton Hoba West meteorite is the largest meteorite ever found. It is still where it fell in Southwestern Africa.
The meteoroid that caused the bright meteor was usually 1 centimeter in diameter. A meteoroid that weights only 0.00006 gram and is only 0.5mm in diameter causes a Leonid meteor of magnitude +5, which is barely visible with the naked eye in a dark sky. Most visible Leonids are between 1 mm and 1 cm in diameter.
That tiny particle can cause a light so bright that it can be seen over distances of hundreds of kilometers. The reason is the astronomical speed of the meteoroids. Just before they enter the Earth's atmosphere, Leonid meteoroids travel at 71 kilometers per second, or some 2,663 times as fast as a fast pitch in baseball, or, if you want, around the Earth in 3.8 minutes!
When meteoroids enter the Earth's atmosphere, they collide with numerous air molecules. Those collisions sputter away the outer layers of the particle. In this process, electrons are stripped off the individual atoms. When the atoms recapture these electrons, light is emitted. This is the same process as in gas discharge lamps.
The color of many Leonids is like the color of our sodium discharge lamps. For the same reason: meteoroids contain traces of sodium. The color of a meteor is an indication of its composition and the excitation temperature: sodium atoms give an orange-yellow light, iron atoms a yellow light, magnesium a blue-green light, calcium atoms may add a violet hue and silicon atoms give a red light.
Meteors do not normally cause audible sounds. Hence, they will pass by unnoticed if not seen. But watch out for hissing sounds that have been reported for very bright meteors. These sounds are thought to be due to low frequency radio waves interacting with the local environment. A sonic boom is sometimes heard for very bright meteors, the fireballs. If the particle is larger than the mean free path of the air.
This is the serious impact between a meteorite and a planet. This is a large depression is a meteor crater and can be found in Arizona, USA. It is 183m deep and 66m wide.
A Perseid meteor (left) streaks across the sky in this time-exposure photograph .The trail ends with several flare-ups in brightness as intense heating makes the body disintegrate.
A meteor trail (right) stands out against the starry background of the Milky Way. Meteor images are quite often caught by chance when some other object is being photographed.