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Asteroid & Comet: Friend of Foe?Rod Jenkins, Project Manager of Giotto Space Mission, on 3 May 2002The speaker first described the formation of the planets after the Sun was formed 4½ billion years ago. They were produced by agglomeration of the dust and gases surrounding the Sun; asteroids and comets formed from the material left over. During this process the material from which they were formed was in violent motion so that there were many impacts between planets, moons, asteroids and comets. Some of the latter were thrown out of the solar system. It is thought that some of these comets form the Oort Cloud around the solar system at a distance of one light-year from the Sun, but others travelled into distant space. The craters visible on the Moon, Mars, Mercury and the moons of Saturn and Jupiter, as well as on asteroids, are evidence of these impacts. There are also many craters on Earth, although these are not as clearly obvious as those on the Moon, having been worn down by weathering and earth movements. All the planets together amount to only 1/4000th of the mass of the solar system. Many asteroids are located in a belt between Mars and Jupiter, where their gravitational pulls prevents them agglomerating into another planet, and can changes their orbits. They are made of the solid material of the inner planets like the Earth. There are between 1 and 2 million with a diameter of less than 1 km, which each weigh about 20 billion tonnes; about 200 with a diameter greater than 100 km. They travel through space at around 45,000 mph. Together they have a mass similar to that of the Moon. Comets are made from the material left over from the formation of the large gaseous planets and their moons. In 1986 the Giotto space craft got within 600 km of the nucleus of Halley's comet and it was seen that it was very black, reflecting only 3 % of the light falling on it - it was not a `dirty snowball'. It could only be photographed because the Sun back-lit the `tail' to allow a silhouette to be taken.
Asteroids and comets are also formed in gas clouds at the time new stars are forming, and some of these are ejected into space and may enter our solar system. It is suggested that the Earth got its atmosphere, its water and the start of life forms from these comets. It is difficult to account otherwise for the very quick appearance of these features on Earth only 600 million years after it was formed. Table 1 compares the composition of the nucleus of Halley's comet with that of a human being. Perhaps we owe our existence to comets. On the other hand, there have been five major events that have almost destroyed the Earth due to the impact of asteroids; the most famous 65 million years ago when the dinosaurs were
Other impacts have been discovered in Australia, Brazil (1930), Arizona and Siberia (1947). In the 90s 30 impacts of bodies of 2 - 30 metres diameter were recorded. The impact that caused action by Governments was that of the Shoemaker-Levy 9 comet on to Jupiter in 1994, where the `crater' in the gaseous planet was the size of the Earth. In 1996 Spaceguard was started to watch for NEOs (near earth objects) that might be dangerous to Earth. In 2000, the UK Government issued a report on Potentially Hazardous NEOs and the National Asteroid and Comet Information Centre was formed in Leicester.(1). The report estimated there was a 1 in 5000 chance per 100 years of an individual being killed by a NEO. The possible fatalities resulting from various events were tabulated but the example of Tunguska shows how these numbers are meaningless - they quote an average value for such an event as 5000 casualties but the range is clearly nil to 10 million depending on where it occurs, so the average is useless. Action to mitigate the risk is also proposed and equally pointless; who can see a world-wide effort being instituted to: · identify and track all potentially dangerous objects [how many thousand?] · establish disaster management plans · ensure [!] that misidentification can't trigger nuclear war · move people away from estimated impact point and areas that could be affected by tsunami [how many millions? how quickly?] · store a year's food supply · develop defence mechanisms, e.g. means to destroy or deflect incoming objects. So, `Asteroid and Comet: Friend or Foe?' - comets could have brought life to Earth and allowed mammals to flourish; asteroids and comets could cause extensive damage and loss of life or even extinction of Man. The Government report can only say: "One the one hand we can rejoice in them; on the other we can fear for our future". I suggest: "Que sera, sera - whatever will be, will be"; you have 1 chance in 5000. Discussion The answers to questions provided extra information: Halley's comet visits every 76 years and was first reported in 240 BC. As it loses very little matter each time it approaches the Sun it will probably make another 10,000 visits. A comet that loses all its volatile matter may become an asteroid. The risk of death from volcanoes, tsunami and similar events is probably larger than from an asteroid impact, but the risk is easier to mitigate because the location is more predictable. The origin of the Star of Bethlehem and the Magi story may be a deputation of Arab diplomats to Rome in 66 AD and a visit from Halley's comet in 64 AD. Matthew wrote his Gospel in about 85 AD and wished to demonstrate that prophecies were being fulfilled. If the theory that comets from molecular clouds deposited the elements for Earth's water, atmosphere and life is true then life could be expected throughout the universe. Many stars may have planetary systems, but half the stars are double stars so planetary orbits could be complex. Donald Lovell 1.(www.spacefrontier.org/PROJECTS/THE_WATCH/index.html)
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