Comets do not always have large visable detail or tails! When a comet gets closer to a star ice particles are blown off which creates the comet's tail. Usually tails extend behind the comet in the opposite direction from a star. The image of a Star with long hair is appropriate as the name derives from the Greek "Icometes" meaning a "long haired star". Some Comets can be gigantic - some are thought to be more than 100,000 miles across, with tails extending up to 75 million miles.
Halley's Comet..
Edmond Halley was a good friend of Isaac Newton. In 1705 he used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He was the first to calculate its path. Having seen it in 1682 he correctly predicted its return in 1759. The Royal Astronomical Society honoured him, naming the comet after him, but he did not live to see it happen, he is however widely regarded as the farther of geophysics.
Observations of the comets return appear through out our history, dating
back over 3,000 years. It was noted in the Bayeux Tapestry, but was first
seen and recorded by the Chinese in 1058/1059BC. It is the brightest of
the comets whose paths astronomers can accuratley predict. Halley's comets
recent ruturn in 1985/86 was seen at a distance of 39 million miles (63
million km). In AD837 it passed only 3,700,000 miles (6 million km) from
the Earth, this must have been a breath-taking sight, as bright as Venus
with a tail spanning 90 degrees across the sky.
The central part of the comet is just nine miles long and five miles wide. The nucleus consists of filthy ice, this is protected from the intense heat of the sun by a thick covering of the blackest dust. Nothing much is darker in the whole solar system than the dust coating of a comet. This thick covering cracks with the violent heat of the Sun and leaves the ice exposed. This in turn causes jets of gas and dust to shoot out leading to increased observable activity in it's tail. Estimated mass loss of ice per visit to the Sun is 250 million tonnes. The ice mass of Halley's comet will be melted at it's present rate in 170,000 years.
Comet Hale-Bopp.
Hale-Bopp was discovered independently in 1995 by Alan Hale and Tom Bopp. It is the farthest comet ever discovered by amateurs. Hale-Bopp appeared 1000 times brighter than Comet Halley did at the same distance from the Earth. It is the brightest comet to be seen since 1976. It is estimated that Hale-Bopp last visited the inner solar system, around the time Stonehenge was nearing completion. This is speculative as any number of cosmic events could have changed it's course. Gravitational tugs by the planets, particularly Jupiter, mean it will return in about 2380 years. Its closest approach to the earth this time around was 122 million miles. Hale-Bopp's mass would equal the size of a mountain. With a tail length of 50-60 million miles, it is and was certainly an impressive sight.
COMET NEWS.
Rosetta Probe Set For Lift Off.
A £600m ambitious mission to land a probe on the nucleus of a comet, is scheduled to launch on the 26th February 2004. The Ariane-5 G+ rocket will launch a 10 year mission in an effort to answer questions about the origin of our Solar System. Rosetta has the most instruments of any spacecraft, which makes it challenging and one of the most exciting missions ever. Comets are the oldest material present during the formation of the solar system.
Rosetta will dispatch a lander named Philae, that will land on the comets surface to search it's icy preserves. Scientists studies show strong indications that complex organic molecules, such as hydrogen, carbon, oxygen and nitrogen, sit beneath the surface of comets. Could life on Earth have begun with a comet?
Rosetta will have to catch up with the comet, then guide in the lander named Philae.
Once Rosetta catches up with the comet named 67P/Churyumov-Gerasimenko, the space craft plans then to orbit the comet in order to map the surface. When a suitable landing site is found the deployment of the lander will begin. Philae will then approach from a distance of 1 km at a speed of 1 meter per second then fire harpoons at the comets surface. All this will be done after a ten year journey, at a distance of several hundreds of millions of miles away. Rosetta will have to orbit the Sun 4 times to pick up the speed it will need to reach the comet by 2014.
Once anchored to the surface Philae will begin drilling for answers...
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