Harbingers of Doom? Transcript Date: Thursday, 19 February 2009 - 12:00AM HARBINGERS OF DOOM? Professor Ian Morison Harbingers of Doom The comet Hale-Bopp seen close in the sky to the Andromeda Galaxy. Aristotle proposed the idea that comets were a gaseous phenomena in the upper atmosphere that occasionally burst into flames. He depicted comets as 'stars with hair' and used the Greek word 'kometes' to refer to them from the root 'kome' meaning 'head of hair'. They were regarded as bad omens foretelling catastrophe or the deaths of kings. (Comet Halley is seen in the Bayeaux Tapestry in the sky above King Harold, who was killed by an arrow in 1066!) Tycho Brahe made careful observations of the comet of 1577 and, by measuring its position from well separated locations, was able to show that it lay at least four times further away than the Moon. In 1687, Isaac Newton was able to show that the path of a bright comet observed through the winter of 1680/1681 could be fitted to a parabolic orbit with the Sun at one focus. He had thus shown that comets were Solar System bodies orbiting the Sun. 3.7.1 Halley's Comet Giotto de Bondone's "Adoration of the Magi". Giotto had seen Halley's Comet in 1301 and depicted it as the "Star of Bethleham" in his fresco painted in 1305. Edmund Halley calculated the orbits of twenty-four comets that had been observed between 1337 and 1698. He found that the comets that had been seen in 1531, 1607 and 1682 had very similar orbital elements and so believed that these were three 'apparitions' of the same comet. Halley could even account for the slight differences by taking into account the gravitational effects of Jupiter and Saturn. Its period was ~76 years so he predicted that it would return around the end of 1757. More accurate calculations by three French mathematicians indicated that it would, in fact, pass closest to the Sun in March 1759. It was first spotted by Johann Georg Palitzsch, a German farmer and amateur astronomer on Christmas day 1758. Halley, who had died in 1742, did not live to see the comet's return but it was named after him with the designation 1P/Halley - the "1P" indicating that it was the first known periodic comet. Halley's Comet in the Bayeux Tapestry as it had appeared in the sky in 1066 - certainly a bad omen for King Harold! Dependent on the Earth's orbital position as Halley nears the Sun, it can either appear bright and spectacular as it did in 1066 and 1910 or, as in its last apparition in 1985/86 when far from the Earth, barely visible to the unaided eye. The comet was first photographed in 1910 when it made a very close approach to the Earth - which even passed through its tail causing some alarm! It is thought that the artist Giotto di Bondone observed the comet in 1301, and used it to depict the Star of Bethlehem in his 1305 fresco, 'The Adoration of the Magi'. As a result, the spacecraft sent to fly-by Comet Halley in 1985 was called 'Giotto'. Halley's Comet will next appear in our skies in 2061. A comet is now classified as a Small Solar System Body that orbits the Sun and which, when close to the Sun, exhibits a visible coma (an extended atmosphere) and sometimes a tail. The 'nucleus' of a comet is typically of order 10 km in size and is composed of rock and dust bound together by ice. The term 'dirty snowball' that is sometimes used is thus quite apt. Those that are termed 'long-period' comets are debris left over from the condensation of the solar nebula and come from the outermost regions of the Solar System, up to a light year distant from the Sun, in what is usually termed the Oort Cloud. The cloud is thought to contain of order a trillion comets and is named after the Dutch Astronomer, Jan Hendrik Oort who popularised the idea in 1952. However the concept was first proposed by an Estonian Astronomer, Ernst Opik, in 1932 so it is alternatively, and more correctly, called the Opik-Oort cloud. Such comets will normally only ever been seen once, but occasionally their orbit will be sufficiently perturbed by Jupiter or Saturn to be 'captured' within the inner Solar System with a relatively short period. It will then become known as a 'short-period' comet which have, by definition, a period of less than 200 years. However, the majority of short-period comets are thought to originate in the Kuiper Belt, which lies beyond the orbit of Neptune. Of the ~3000 comets known by the end of 2007, several hundred have short periods. On average about one comet per year will reach unaided-eye visibility, but only about one in ten of these will become easily visible. Cometary Nuclei The nuclei of comets range from about one half to fifty kilometres in size and have a very rich composition; primarily made of rocks, dust and water ice along with frozen gases such as carbon dioxide, carbon monoxide, methane and ammonia. This 'Dirty Snowball' idea was proposed by Fred L. Whipple in 1950 who had discovered 6 comets himself. It is more formally known as the 'icy conglomerate' theory. They also contain many organic compounds such as methanol, formaldehyde, ethanol and ethane and possibly even more complex molecules such as amino acids. They are far too small to become spherical through the force of gravity and are thus irregularly shaped. Due to their small size and low albedo, comets cannot normally be seen in the outer Solar System. As a comet approaches the inner solar system, solar radiation causes water, frozen gases and other volatile materials within the comet to vaporize and stream out of the nucleus. This releases the dust that is bound up within the ice and together the dust and gas form a huge, extremely tenuous atmosphere around the comet called the coma. The forces exerted on the coma by the Sun's radiation pressure and outflowing solar wind causes tails to form which naturally point away from the Sun - as first shown by the German astronomer Peter Apian in 1531. The comet of 1532 observed by Peter Apian showing that the tail pointed away from the Sun. Both the coma and tail may become visible from Earth as the comet passes through the inner solar system. The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The dust tail is yellowish in colour and tends to lie along the orbit of comet and so often appears curved. Sometimes, due to that angle that we observe the comet from, part of the dust tail can actually appear to point from the nucleus of the comet towards the Sun - an antitail. This rare sight typically occurs when Earth crosses the plane of a comet's orbit and the cometary dust, which lies in a thin sheet, may be seen edge-on. One of the most prominent antitails that we have seen was that of comet Arend-Roland as it rounded the Sun in 1957. The Antitail seen in the comet Arend-Roland. The gasses, in the form of ions, are most strongly affected by the solar wind and always appear to point directly away from the Sun.The ion tail often appears bluish in colour (as you may be able to see in the picture of Hale-Bopp at the head of this transcript) due to emission from (CN)2, cyanogen. The majority of comets are too faint to be visible without the aid of binoculars or a telescope, but perhaps once each decade a comet become bright enough to be visible with the naked eye. These are called Great Comets. Occasionally a comet may experience a huge and sudden outburst of gas and dust and thus the size and brightness of the coma temporarily greatly increases in size. This happened in late 2007 to Comet Holmes which was visible to the unaided eye for some time. The coma may exceed the Sun in size, and ion tails have been known to extend over one AU in length, and so are the largest objects in the Solar System. The size of the coma and tail increases as the comet nears the Sun and the comet will tend to be most apparent immediately after it has passed the Sun. We thus tend to observe them in the period before dawn or after dusk. With each passage around the Sun, the comet loses material and will eventually disintegrate into a trail of dust or become an inert, asteroid-like, body of fractured rock. The fact that comets may contain significant amounts of organic compounds is indicated by their very low albdo. The Giotto space probe showed that Comet Halley's nucleus reflected only ~4% of the incident sunlight whilst Deep Space 1 showed that Comet Borrelly only reflected ~2.6%. The image of the nucleus of Halley's Comet observed by the Giotto spacecraft on the 13th March 1985. The Giotto image showed that the nucleus was not spherical and the outflows were only coming from a number of vents on the sunward side. The fact that the side away from the Sun shows no activity implies that there is little heat conduction through the interior of the comet. The density of the nucleus was just 0.5 gm/cc - a half that of water - so as ice has a density of 1 gm/cc and dirt ~ 2.5 gm/cc this implies that there must be quite a lot of empty space within the nucleus! Meteor Showers Each time a comet swings by the Sun in its orbit, some of its ice vapourises and fragments of cometary material, are shed.