Meteors Are Very Minute Bodies

Press, Volume CX, Issue 32210, 31 January 1970, Page 16

Meteors Are Very Minute Bodies

Even the most casual observer of the night sky has seen a star-like object appear suddenly, move rapidly across the sky and then disappear. Usually it is visible for only a few seconds. Such objects are popularly called “falling” or “shooting” stars. Scientifically they are termed meteors.

They are very small bodies following a path around the Sun. .If its path brings one close to the Earth it is pulled out of its orbit by the gravitational attraction M 1 the Earth. The meteor then enters the Earth’s atmosphere where the friction with the air causes its energy, derived from its rapid motion, to be transferred to heat and light. It then becomes visible as a momentary flash of light moving across the sky. In spite of their apparent brightness meteors are, on the whole, very minute bodies. Most are no larger than a grain of sand. Even a bright meteor is no larger than a grain of wheat. Occasionally a very bright meteor, rivalling the Moon in brilliance, is seen. This can light up the countryside for several hundreds of miles. These are called fireballs and have a mass of around 2001 b. Away from the city lights it is possible to see several meteors an hour on most clear, dark nights. The actual number varies both with the time of night and the season. It has been calculated that over 4000 m of these tiny particles enter the Earth’s atmosphere every day. Fortunately the atmosphere acts as a protective barrier so that most meteors burn out at heights of between 40 and 50 miles. Large Masses Occasionally a very much larger mass enters the atmosphere and the heat, generated by friction, is insufficient to pentrate its interior. Such bodies reach the Earth’s surface and are known as meteorites. A number have been recovered and it has been found that they can be divided into two main classes. The stony meteorites resemble terrestrial silicate rocks, whilst the iron meteorites consist mainly of metallic iron with about 30 per cent of nickel.

The largest single meteorite recovered is a mass of 50 tons from Hoba West, in South-West Africa. However, there have fallen on the Earth masses very much greater than this. It is estimated that the mass which formed the famous meteor crater in Arizona was something of the order of 2m tons. The crater itself has a diameter of 4000 ft with a depth, from floor to rim, of 570 ft. This meteorite arrived some 20,000 years ago with an energy that equalled that of a 30-megaton hydrogen bomb. When a meteor enters the atmosphere it ionises the air In a cylindrical volume. This conducting cylinder scatters electro-magnetic waves at right angles to its own direction. The application of radar techniques to the study of meteors takes advantage of this to enable the numbers, direction and velocities of the separate objects to be determined. Such methods have the great benefit that they can be used in daytime and when the sky is cloudy, whereas visual and photographic studies require clear night skies. Canterbury Studies Using radar survey

methods, C. S. L. Keay and C. D. Ellyett made extensive studies of meteor rates in the Southern Hemisphere from a field station near. Christchurch. These observations were made with equipment at the University of Canterbury under a grant from the National Science Foundation, Washington, whilst the analysis of the data has been done at the Universtiy of Newcastle, in Australia, under a grant from N.A.S.A Their results for the 31rnonth period between February, 1963, and August, 1965, have recently been published as an important memoir of the Royal Astronomical Society, London, These results extend an earlier survey they made during a period of 12 months in 1960. The importance of the data is enhanced because another meteor survey was made at the same time for the Northern Hemisphere from Canada using similar equipment. The New Zealand results produced records of 2,304,333 meteors yielding an average rate of 114 meteors an hour down to the brightness of eighth magnitude, which was the limit possible. Keay and Ellyett found that the over-all activitiy varies widely as from one year to another. This was particularly so in May, June and July of 1963 when the rate was over 100 per cent greater than in 1960. The reality of this effect was proved by confirming observations obtained in Canada and Sweden. The increase has been attributed to variations in atmospheric density inversely related to the solar cycle activity. From a map in this paper it can be seen that the greatest number of meteors recorded an hour was in late July between 2 a.m. and 4 a.in. when the average hourly rate exceeded 351. The minimum daily rate occurs around 6 p.m., although the exact time of minimum varies in a regular seasonal manner within a period of two hours during the year. The highest rates occur between 2 a.m. and 9 am.

It has been known for many years that there are more meteors visible after midnight than before and that those seen in the morning are generally swifter and bright-1 er. Before midnight we see; meteors that are overtaking 1 the earth, whilst after mid-1 night we are meeting meteors! head on. Meteor Showers Often several meteors follow parallel paths in space. If their apparent paths across the sky are plotted and traced backward" they are found to intersect at one point in the sky. Such a group forms a meteor shower and the point from which they appear to radiate is called the radiant. When we pass through a meteor shower there are usually several times the number of meteors visible each hour compared to other times. When a great meteor shower occurs this number increases several hundred times. Generally such showers are associated with a comet The comet has scattered particles right round its orbit If the earth crosses the comet’s orbit the result

dan be a meteor shower on approximately the same date each year. In our latitudes a good shower is visible, radiating from the constellation Capricornus from mid July to mid August. In 1970 the maximum of this shower is calculated to occur on July 25. This shower is associated with the comet known as 1948 n. In early May we see a shower radiating from close to the star Eta Aquarri. This shower, like the one from Orion in October, is associated with Halley’s Comet. Occasionally a meteor swarm Is very much denser along part of its orbit whilst elsewhere the swarm is comparatively thin. This means that in some years there are only a few meteors each hour from the radiant with other years bringing a very rich display. A radiant visible in November is one such example. Normally this yields a number of bright meteors with persistent trains, but, for a short interval on the morning of November 17, 1966 meteors fell from this shower so thickly that the sky appeared as if it was covered with meteors. For a period of twenty minutes they fell at the rate of 150,000 per hour, rivalling the historic Leonid displays of 1799 and 1833. The remains of meteors which have burnt out in the atmosphere gradually drift down to the surface as dust, as do the smaller particles known as micrometeorites. Whilst the individual masses of these are all very small, when the vast numbers entering the atmosphere daily arc taken into account the result is to add a considerable quantity of dust to the Earth each year. Whilst calculations differ widely, a reliable estimate places the amount of dust as 2000 tons each year. Of course this is spread over the entire surface including the oceans. It therefore sounds a lot more than is actually the case. If the rate was constant the falling dust would add a uniform layer to the Earth one fifth of an inch thick in 3000 m years.