Comets often regarded as harbingers of doom

Press, 4 December 1985, Page 38

Comets often regarded as harbingers of doom

By

JOHN HARFORD

Most races, throughout history, have viewed comets as harbingers of doom and destruction, wicked fairies, the sword of an angry god, omens of great change, and even the end of the world. This was hardly surprising since comets arrived either at times of great upheaval in the world or a nation’s leader has died during its visit In 1910 King Edward VII died while Halley’s Comet was showing in southern hemisphere skies. Maori mythology described comets as sons of the Sun, sent to Earth with the gift of fire for mortals. The comet Te Auahituroa, the Longstanding Smoke (probably Halley’s Comet), became the lover of the beautiful goddess Mahuika. Their offspring were fire children whom Maui tried to drown. To escape, the surviving fire children were hidden in trees. The ancient Greeks called comets astron kometes, or “haiiy stars,” an allusion to the long wispy tails they display while near the Sun. Some historians claim Pope Calixtus 111 regarded Halley’s Comet as an agent of the devil and in 1456 he excommunicated it. Until Tycho Brahe proved that the comet he discovered in 1577 was further from Earth than the Moon, the celestial wanderers were thought to be atmospheric phenomena. More than a century later, Sir Isaac Newton proved that Comet Kirch moved in an orbit congruent with his theory of gravity. Other astronomers, particularly Edmond Halley, further improved methods of determining cometary orbits, and in the mid 1700 s studies of the physical structure and behaviour of comets began. Often described as a “dirty showball,” a comet is a small body of icy substances which moves in a highly elliptical orbit round the Sun. Although most of their lives are spent in frozen reaches far from the Sun’s warmth, their orbits might periodically bring them close enough to be heated by it. The chemical composition of a comet nucleus has yet to be precisely determined but the widely accepted theory holds that it is formed of dust particles loosely compacted with water ice, frozen carbon monoxide and carbon dioxide. As the comet hurtles to perihelion, solar heat melts : the nucleus, releasing

enormous volumes of gas and dust. This creates a fuzzy head or coma, consisting of water vapour, carbon monoxide, carbon dioxide, hydroxl and small amounts of other molecules containing carbon, hydrogen and nitrogen. These molecules are dispersed into smaller fragments which form a plasma or ionised gas. Solar wind carries the gas, and the pressure of sunlight forces dust away from the coma to produce tails. Prominent tails develop only in large, active comets close to the Sun. The plasma streams from the coma at speeds up to 640 km a second when carried by solar wind, creating a straight-rayed, gaseous tail pinpointed directly away from the Sun. Dust particles are heavier than the ionised gases and move more slowly away from the head, producing a fan-shaped, curved tail. Occasionally dust particles might appear to stretch out before the coma, creating an anomalous spike or antitail. As comets move towards the Sun, the stars’ light excites their gases to fluorescence, causing a rapid brightening. The reflective properties of a comet’s dust amplifies this brightness, so dust-rich periodic comets, such as Halley’s are much brighter than “old” comets which have lost much of their dust to solar radiation. Solar activity can also produce unexpected variations in the brightness of comets. Comet nuclei may have diameters of up to 16km. Coma diameters range from 16,000 km to 200,000 km, and tails can reach as long as several hundred thousand kilometres. Although comets and their tails cover vast areas, their masses are so small they have never been accurately measured. A 2000-cubic-mile of comet tail contains less material than one cubic inch of air. A comet’s mass is reduced at each perihelion by 0.1 to 1 per cent. Halley’s nucleus is estimated to have a diameter of 6km and a mass of 100

billion tonnes. Although the total number of comets in our solar system is not known, it is estimated that at least 10 million have their aphelion (the distance furthest from the Sun) within the orbit of Neptune. Since most new comets move towards perihelion from a great distance, it has been suggested that they originate in a vast, spherical cloud formed at the edge of the solar system as the Sun and its planets were being born. Known as the Oort Cloud, after a Dutch astronomer Jah Oort, it is 20,000 to 60,000 astronomical units away and contains an estimated 100 billion . comets. (One astronomical unit 149 million kilometres — the average distance of the Earth from the Sun). Gravitational forces occasionally “kick” a complacent comet from the cloud, sending it on its journey toward the Suh. Another. theory suggests that the solar system, , as it moves through the galaxy, will occasionally “plough” through a cloud of. gas and dust, leaving a wake and creating swirls and eddies. Attracted by the gravitational pull of the Sun, these fall in towards its line of motion and coalesce to become comets. Periodic comets, trapped within the orbit of the outer

Slantes by gravitational )rces, generally have a lifespan that lasts about 1000 revolutions round the Sun. Halley’s Comet is thought to

have travelled at least 2200 revolutions and shows no significant signs of aging. Tomorrow: Viewing Halley’s Comet. \ .