On any clear night of the year a sometimes brilliant, rapidly moving streak of light may be seen against the background of stars. This streak of light is known as a meteor (or shooting star) and usually persists for less than a second. Meteors are caused by the collision with air molecules of tiny particles of interplanetary dust which enter the atmosphere at speeds of up to around 45 miles (70km) per second. Countless numbers of these tiny particles are orbiting the Sun, rather like miniature planets. When in space they are much too small and faint to be seen, revealing themselves only when they succumb to the Earth’s gravitational pull and are drawn in to the upper atmosphere. Here they are heated violently and burn themselves out, quickly destroyed in the brief blaze of incandescence we see as a meteor.
Meteors that appear at any time and from any direction in the sky are known as sporadic meteors, and can be observed on any clear night. However, during so-called meteor showers there is a sometimes dramatic rise in the number of observed meteors. Meteor showers occur as a result of the actions of comets which, as they travel along their orbits, are constantly shedding particles into space. These particles of cometary dust eventually become spread out all along the cometary orbit. At certain times of the year the Earth passes through the orbital paths of various comets, the upshot of which is that comparatively large numbers of particles can enter the atmosphere, whereupon meteor activity is seen to increase.
Particles of dust shed by a comet travel along the cometary orbit in parallel paths (see A in the diagram). As a result, when particles from the same comet enter the atmosphere, they all seem to radiate from a particular point in the sky, this point being known as the ‘radiant’ (B). The concept of a meteor radiant can be clearly illustrated by looking along a straight road. The road edges, telegraph wires and so on, all of which are laid in parallel paths, seem to meet at a point on the horizon (C). This point is analogous with the radiant of a meteor shower.
There are a large number of meteor showers per year. Some of these are quite feeble, although around a dozen or so are responsible for between ten and a hundred meteors per hour at times of maximum activity, which occurs when the Earth passes through the densest region of a swarm of cometary particles. Each meteor shower is named after the point on the celestial sphere containing the radiant. For example, the Lyrids radiate from a point in the constellation Lyra, while the radiant of the Geminids lies near Castor in the constellation Gemini. The most active shower of all is the Perseids (above right), which radiate from a point near the star Eta Persei (Miram) in the northern reaches of Perseus. The meteor captured in this image (left) is a Perseid, seen here streaking past the Milky Way.
Most particles entering the atmosphere are so small that they never reach the surface, although occasionally much larger objects succumb to the Earth’s gravitational pull and at least partially survive the journey through the atmosphere. These objects are called meteorites and appear much less frequently than meteors.
Many examples have been found, and the study of meteorites has shown them to be roughly as old as the Solar System itself. Unlike the tiny particles which produce meteors, many of which have their origins in comets, objects known as meteoroids travel independently through space, some of these being in orbits that carry them across the Earth’s path. Plotting their orbits from positional measurements taken during meteorite falls show that many of these objects originate in the asteroid belt, arising from collisions between larger bodies.
Very large meteorite falls are rare, yet when they do occur they can cause a great deal of damage, as borne out by the Arizona Meteorite Crater, located near Winslow in Arizona. This huge impact crater (right) is the result of a meteorite fall which took place thousands of years ago. Measuring over a kilometre in diameter, the floor of the Arizona Meteorite Crater lies nearly 200 metres below the surrounding terrain. Luckily no large meteorites have fallen on densely populated areas, although fatalities resulting from smaller meteorite falls have been recorded, albeit only of animals. A meteorite fall in 1860 killed a horse in Ohio and one in 1911 finished off a dog in Egypt. Injuries to humans have been reported, such as in November 1954 when a housewife in Sylacauga, Alabama was hit on the arm by a meteorite which came down through the roof of her house. Another near-miss account is that of a Ugandan boy who, in 1992, was hit by a meteorite fragment, although he was not harmed due to the fact that the speed of the fragment in question had been slowed by passing through the branches of a tree!