Macroscopic bodies
As themacroscopic bodies penetrate the Earth’s atmosphere, the air resists their motion and part of their energy is lost in the form of heat. The heat generated causes the ablated material and the atmospheric path to become ionized and, when the atoms recombine, light is emitted and the rapid progress of the body through the upper atmosphere is seen as a flash of light along a line in the sky. The flash might last for a few seconds. The event is known as a meteor (popularly known as a shooting star). The rate of burning of the meteor is not constant and fluctuations in brightness may be seen on its trail, usually with a brightening towards the end of the path. Positional measurements can be made of the meteor and the event can be timed. Simultaneous observations of a meteor at different sites allow determination of its trajectory within the Earth’s atmosphere.
On occasions, many meteors can be observed during a relatively short period of time and, by observing their apparent paths across the sky, it is noted that there is a point from which the shower of meteors seems to originate. This point in the sky is known as the radiant of the shower. Meteor showers are often annual events and can be seen in the same part of the sky at the same time of the year, although the numbers counted vary widely from year to year. The regular appearances of showers result from the crossing of the Earth’s orbit of a fairly tight band of orbits followed by a swarm of meteoritic material.
Meteors can also be detected during the day by radar. As a meteor passes through the upper atmosphere, as has already been mentioned, some of the gases there are ionized. The ionized trail which persists for a short time acts as a good reflector for a radar beam and the effect of any daytime meteor can be displayed on a cathode ray tube. Several daytime showers have been discovered by the use of this technique.
Some of the larger meteors have such large masses that they are incompletely ablated or destroyed in the atmosphere. In this case, the meteor suffers an impact on the Earth’s surface. The solid body, or meteorite, is frequently available either in the form of a large piece or as scattered fragments. The material can be exposed to the usual analyses in the laboratory.
The smaller meteors or micrometeorites can now also be collected above the Earth’s atmosphere by rocket and analysed on return to Earth. It also appears probable that some micrometeorites are continuously percolating through the atmosphere. Because of their size, they attain a low terminal velocity such that any local generated heat by air friction is radiated away at a rate which prevents melting of the particle. Previous micrometeorite sedimentation can be explored by obtaining cores from ancient ice-fields. It is now a difficult problem to separate any fresh contribution from the general dust which is constantly being stirred in the lower atmosphere of the Earth.