The Earth sweeps up debris from space

Press, 26 February 1987, Page 17

The Earth sweeps up debris from space

Several readers have written describing what they call “flashes of light moving across the sky.” They ask whether these objects are falling stars or satellites orbiting the Earth.

They describe the objects as lasting for a second or two, moving very rapidly across the sky and generally of a whitish colour. This leaves little doubt that these were meteors, commonly called falling or shooting stars. Actually they have nothing to do with stars. A meteor becomes visible when a solid particle from space, known as a meteoroid, enters the Earth’s atmosphere. Friction with the air causes it to burn up, reducing it to fine dust This dust slowly falls to the surface, usually during rain. We see a short flash of light as the meteor streaks across the sky.

Usually they enter the atmosphere at speeds of from 30 to 60 kilometres a second. The slower speeds are for those meteors that are overtaking the Earth, whilst the faster ones are meeting the Earth head on. Therefore, the latter are more common in the morning hours when we are facing the direction of the Earth’s motion. Normal meteors enter the atmosphere at heights of about 95 kilometres and burn up far above the surface.

Meteors often appear to move outwards from a point in the sky, so that if their tracks are traced backwards they appear to intersect at a radiant point These are called shower meteors. They come from a meteoroid stream, in which the in-

dividual particles are moving around the Sun in parallel orbits. Such showers are named from the constellation in which their radiant point lies, or by the name of a star close to that point

A meteor shower is seen when the Earth crosses the path of a meteoroid stream. Many showers are associated with comets. Typical examples are the showers seen each year in May and October. These occur when the Earth crosses the path of Comet Halley. This comet has made many returns to the vicinity of the Sun. At each return the Sun has melted the surface of the comet and driven off particles. These then lag behind the comet and, over many centuries, have become scattered right around its orbit The Earth crosses that orbit twice a year and hence there are two annual meteor showers from Comet Halley.

Comets which have made few returns to the Sun have not scattered particles right around their orbits. Instead the particles that they have lost tend to be bunched up into dense swarms. The result of the Earth encountering such a swarm is a meteor storm. These last only for a few hours at the most, resulting in meteors appearing at rates of up to 1000 a minute.

The best known example of meteor storms are the Leonid showers that were seen in 1799, 1833, 1866, and 1966. Then meteors appeared in such numbers that observers could not count them and likened the storm to the sky’s being

covered with darting snowflakes that almost hid the stars. After the brilliant display of previous years it was thought that the Earth crossed the path of this swarm every 33 years so a great display was expected in 1899 or 1900. Very few meteors from this shower were seen in those years and again in 1933 no abnormal display was seen. However, they appeared again in 1966. The Leonid meteors are associated with Comet Temple. It is apparent that its meteor swarm is very thickly bunched and that the Earth did not intersect this swarm in 1900 and 1933. Perhaps in 1999 a celestial fireworks display will herald the coming of the next century.

Sometimes a particularly bright meteor is seen that may even light up the whole sky, or be visible in broad daylight These last for a few seconds, or even a minute or two. They are called fireballs, or bolides. These are usually larger pieces of rock that penetrate lower in the atmosphere, and, because of greater air resistance at lower altitudes, move fairly slowly. Some come from showers, a few of which seem to be rich in fireballs, but most are sporadics unconnected with any shower. Astronomers are always interested in reports of these brilliant fireballs. Reports from separate observers a few kilometres apart enable the paths of fireballs to be established if the observers have been able to report accurate bearings on the beginning and ends of the path.

This then enables the heights of the beginning and ending to be calculated.

Most sporadic meteors are comparatively faint They can be seen at rates of from 6 to 14 on any clear, dark night with the greatest frequency just before dawn. They are simply some of the dust particles scattered in the solar system and are gradually spiralling into the Sun.

It is the atmosphere that protects us from being continually bombarded by meteors. However, there are larger fragments of rock or metal which survive thenpassage through the air and land on the Farih These are called meteorites. Accounts of such falls are found in the records of many early civilizations. There is no doubt that in the early history of the Earth it sustained a heavy bombardment from meteorites. Erosion has effaced most of the resultant scars. Some of the craters that resulted have survived erosion, especially in dry, arid regions. The best known example is the famous meteor crater in Arizona.

Examination of the airless worlds of Mercury, Mars and the Moon, in particular, show that they were all subjected to heavy cratering from bodies impacting on their surfaces. It is therefore safe to conclude that in the early stages of the solar system there was a lot more debris about The study of meteorites tells us something about the composition of that debris and hence the early history of the solar

system. Apart from the dust and rocks brought back from the Moon, meteorites form the only material from space that can be analysed and studied in laboratories. Next month’s articles will deal in more details with this subject Daylight Time will end on March 1 and the longer evenings will be welcomed by astronomers. On March 27 Mercury reaches its greatest elongation west of the Sun. This is the most favourable apparition of this planet in the morning sky for our latitudes. It will rapidly brighten from a magnitude of 2.6 to 0.2, making it a conspicuous object in the early-morn-ing sky during the second hSIf of the month. It can be seen in the east rising close to 4.30 a.m. during this period. Venus is also in the morning sky, rising at 2.45 a.m. on March 1 and at 3.30 a.m. on March 31. Its magnitude of -4.1 will make it a very brilliant object

Mars can be seen in Aries, low in the northwestern evening sky, where it will set at 9.45 p.m. on March 1 and at 8.20 p.m. on March 31. It has faded to magnitude 1.3 and as it has receded far from us its apparent disk is very small. Saturn, in Ophiuchus, is well placed for evening viewing, rising just before midnight on March 1 but twp hours earlier on March 31. Jupiter may be glimpsed very low in the north-western evening sky during the first few days of the month, after which it will become lost in the glare of the Sun.