If you view the sky on a clear moonless night you may see streaks of light several times per hour. These luminous streaks are called shooting stars or METEORS . The Solar System contains a large number of small particles moving in orbits around the Sun. In the vicinity of the Earth these particles typically have a speed of 05 km s-1. The orbital speed of the Earth is 30km s-1, so that a relative speed anywhere between 35 and 95kms-1 is possible. If one of these particles intercepts the Earth and enters the atmosphere, the air resistance quickly heats up the particle to incandescence and it is visible as a streak of light. At the same time the air along the meteor trail is ionized. The light of a meteor can, with only very rare exceptions, be seen only at night but the trail of ionized gas reflects radio waves and can therefore be detected by radar both by day and by night. Typically, a meteor is completely vaporized in less than a second. The larger and brighter meteors may last longer and give a trail hundreds of kilometers long. The brightest meteors emit so much light that they are able to cast shadows. They are called FIREBALLS and may even be seen during daylight . They are occasionally accompanied by a sound like thunder which is presumably a sonic boom like that associated with super¬sonic aircraft. Fireballs sometimes explode in flight and are then called BOLIDES. Although most meteors are completely destroyed during their flight through the Earth’s atmosphere a few penetrate to the surface. They are then known as METEORITES. The word meteor is used somewhat loosely to describe either the visible trail, the associated ionized gas or the actual solid particle responsible for these. It is now customary to use the word METEOROID for the particle before it reaches the surface of the Earth, and METEOR for the observable trail which it produces in the atmosphere.

If a meteor trail is photographed from two places its path can b obtained by surveying techniques. The two observing stations need to be about 30km apart: this is far enough to provide a good base¬line but not so far that the meteors are not visible from both stations! Meteors typically become visible about 100km up since the Earth’s atmosphere is too tenuous at greater heights to raise the particle to incandescence. The brighter meteors remain visible down to a height of 55km or less while fainter (and smaller) ones fade at about 80km. The speed of a meteoroid may be obtained by the use of a shutter which periodically cuts off the light to the camera and breaks the photographed meteor trail up into segments. The orbit of the meteoroid around the Sun, before it entered the Earth’s atmosphere, can then be determined. Radar observations of meteor trails can also be used in a similar manner. All the orbits so far determined are elliptical, indicating that the meteoroids come from within the Solar System; no hyperbolic orbits, indicative of an interstellar origin, have been found. The spectra of the brightest meteors can be obtained by placing a prism over the objective of a meteor camera. The results indicate that meteoroids have a composition similar to meteorites.

Although a single, naked-eye, observer will see about six to eight meteors per hour he is only seeing the brightest meteors over a small part of the Earth’s surface. The total number of particles entering the atmosphere every day is enormous, but when allowance is made for the very small mass of each particle, a total mass of about 1000 tonnes falls on the Earth each day, although this figure is extremely uncertain. Moat of this mass is in the form of MICROMETEORITES. These are particles that are so small that the air resistance is very large compared to their masses. They are slowed down so much that they do not glow and vaporize, but instead drift undamaged to the ground. They are smaller than a few micrometers across and can be found in large quantities on the ground and in deep-sea sediments. Naturally it is difficult to distinguish them from ordinary terrestrial dust. A better way to collect micrometeorites is with rockets and spacecraft.

The rate at which meteors appear is not constant and varies both during the night and over a year. It is always greater after midnight than before, and in northern latitudes it is greater in the autumn and winter months. On some nights a much greater number of meteors than usual appears to give what is known as a METEOR SHOWER. Meteors not in a shower, that occur at all seasons, are called SPORADIC METEORS. The meteors in a shower seem to radiate from one point in the sky, called the RADIANT; hi a-few cases there are two radiants within one shower. The apparently divergent trails are an effect of perspective on the actual trails which are parallel. These meteor showers recur each year regularly at the same date, although the actual number of meteors may vary. A list of the main. Most of them are named after a star close to the radiant or the constellation containing the radiant. In general a shower lasts just a few days each year, although some, such as the best known, the PERSEIDS, are visible for a month or more. The number of meteors in a shower is often measured by the ZENITH HOURLY RATE (Z.H.R.) at maximum. The observed rate at which a naked-eye observer sees meteors depends strongly on the altitude of the radiant and is greatest when the radiant is at the zenith (i.e. directly overhead). At a radiant altitude of 27° the observed rate is reduced to a half of this and at 2.6° to only 10 per cent. The ZHR is the hourly rate for an observer watching under very good conditions with the radiant at the zenith. For an experienced observer who can see meteors as faint as magnitude 6.5. The actual rates vary somewhat from year to year; for some showers the variations are substantial. The most extreme example of these variations is shown by the Leonid shower. This shower gave spectacular displays in 1799, 1833 and 1866, suggesting a period of 33 or 34 years. Few meteors were seen in 1899 or 1933 and the general expectation was that this would continue in 1966. In fact the most spectacular display in meteor history occurred. After a steady climb over several hours, the rate suddenly peaked sharply over a few minutes at 150 000 meteors an hour.

Many meteor showers have been associated with comets, particularly in the cases where there is little difference between the orbits of the comet and the meteoroid particles. For example the Leonids are associated with comet 1866 I (P/Tempel-Tuttle), the ? Aquarids and Orionids with P/Halley and the Taurids with P/Encke. Matter from comets is spread around the comet’s orbit, and when the orbit passes close to the Earth’s, the Earth will intercept the cometary material once (or occasionally twice) a year and produce a meteor shower. In some cases the matter is unevenly spread around the cometary orbit so that the shower strength varies from year to year; this is particularly so for the Leonids.

On 7 April 1959 a Czech fireball of magnitude – 19, which led to the Pribram meteorite fall, was successfully photographed by accident. This showed the possibility of predicting impact points of meteorites by the photography of fireballs. Three networks of automatic cameras were set up to cover large areas of the American and Canadian prairies and most of Czechoslovakia and West Germany, a total of three million square kilometers of the Earth’s surface. The results of the photography showed that fireballs are about ten times more frequent than was previously thought but that only about one per cent of them actually lead to a meteorite fall. Most of the meteorites that cause fireballs must be made of fragile material, possibly of cometary origin, which is destroyed in the Earth’s atmosphere. The few objects that survive to become meteorites are probably of a different nature and, quite possibly, are effectively very small asteroids. This is confirmed by the photo-graphs of the Pribram fall and of a fall on 8 January 1970 at Lost City, Oklahoma, which showed that the orbits of the meteorites around the Sun were elliptical with aphelia in the asteroid belt Detailed calculations have shown, however, that the supply of material from the asteroid belt is insufficient to provide the parent bodies of more than a small fraction of the bright meteors.

About 500 meteorites fall on the Earth each year. Since only 30 per cent of the Earth’s surface is land the number that is accessible for recovery should be about 150, but of these only about ten are actually recovered. A meteorite is generally named after the town or village nearest to which it was found or some readily identifiable topographical feature.

Meteorites are classified according to composition and structure. The basic classification is into IRONS, STONY -IRONS and STONES. Most of the stones contain small spheroidal aggregates known as CHONDRULES which are typically one millimeter across. The stones are therefore divided into CHONDRITES (those with chondrules) and ACHONDRITES (those without). About 60 minerals are known in meteorites but many of these are only minor constituents; the common minerals are listed . These differ in several ways from terrestrial minerals: for example nickel-iron is practically absent from terrestrial rocks. The common meteorite minerals are largely magnesium-iron silicates whereas quartz (silicon dioxide) and aluminosilicates are abundant on Earth. Meteorites are largely anhydrous (i.e. free from water of crystallization) whereas hydrated minerals are common and abundant on Earth. From the types of minerals found we can deduce much about the conditions under which meteorites formed; for example iron and nickel must have been present largely as the free metal. Meteorites do not contain minerals formed under high pressure, which would indicate formation as part of a large parent body. The only exception to this is the diamond found in iron fragments of the Canyon Diablo meteorite which formed the Arizona Meteor Crater and the small group of meteorites known as ureilites. This diamond appears to have been produced by the shock of impact in the first case and by extraterrestrial shock effects in the second.

Most meteorite finds are irons, which are resistant to weathering and are easily recognized as peculiar objects. An iron is therefore more likely to be found than a stone. This bias in the relative proportions of irons and stones can be largely overcome by considering only those meteorites which are seen to fall. ‘About 84 per cent of falls are achondrites with nine per cent achondrites, six per cent irons and one per cent stony-irons.

The chemical composition of meteoritic matter. The average compositions of iron meteorites and the metal in chondrites are almost identical, indicating a common origin. The differences between the other types, both in the proportions of the various elements and of the minerals that they make up, show that however the meteorites may have formed, they do not come from a single parent body.

There is another small class of meteorites which we have not yet mentioned, the CARBONACEOUS CHONDRITES, which differ fundamentally from the other classes and consist mainly of the mineral serpentine (Mg,Fe)6Si4O10(OH)8. They are remarkable for the considerable amounts of organic compounds of extraterrestrial origin that they contain and for the close correspondence between their chemical composition and that of the Sun. Chondrites, and especially carbonaceous chondrites, are now recognized to be relatively well-preserved samples of the non-volatile material of the nebula from which the Sun and planets formed. Most chondrites show signs of having been heated within their parent bodies to around 800°C or higher. If a fairly small proportion of these parent bodies reached 1400-1500°C they would have melted and separation of the various constituent elements could have occurred (as in the Earth) to give the achondrites, irons and stony-irons.

The cooling rate, and hence the size of the parent body, can be deduced from examination of a meteorite. Such studies have indicated that 90 per cent of iron meteorites have come from at least six and possibly as many as eleven parent bodies and that all but one of them were larger than 200km in diameter. Similarly, there were at least five,or as many as ten, parent bodies for the chondrites with diameters in the range 180 to 300 km.

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