The word comet is derived from the Greek word for 'long-haired star.' They are often called 'dirty snowballs' as they are known to be made up of ices of ammonia, hydrocarbons, water and cabon dioxide, that bind together pieces of meteoritic stone. They are not particularly large in size, averaging approximately a few tens of kilometres in diameter. The comet is only seen to be increasing in activity when it comes near to the sun. This causes a tail and head to appear- the tail always points away from the sun, and the head is the nucleus plus a tight ball of dust and gas that is left as some of the comet vapourises as it is on the part of it's elliptical orbit that draws it towards the Sun. Another contributor to the direction of the tail is the pressure from solar winds emitted from the Sun. The nucleus does not give out light, it is only seen by reflected sunlight. As the coma develops, dust reflects even more sunlight, and gas absorbs the ultra violet radiation from the sun and flouresces. A greater amount of UV radiation causes a greater flourescing, until at a distance of around 5AU (5 times the distance between the Earth and the Sun) from the Sun, this flourescing is more easily seen that the actual reflection from the other particles.
Comets often are shown to display two types of luminous tails. A straight one is made of gas, which has a force hundreds of times stronger on these ionised molecules than that from the sun's gravity on them, showing that it is the collisions caused by the solar wind that creates this tail.The wind is actually streams of sub-atomic particles projected radially at all directions from the Sun. The curved tail is made up of tiny particles of dust, which have been forced away from the coma by a force only just greater than that attracting the dust to the sun. This force is caused by collisions with the Sun's light.
Comets have been known to display enough light to actually cast shadows, especially in the case of the Great Comet of 1861 observed by Schiaparelli. He was also the first person to make the link between comets and meteors. Comets release particles along a cometary orbit via the tails. If the Earth passes through this tail the particles burn up as they enter our atmosphere. These are seen to be shooting stars but are actually called meteors. It only take a piece of matter as large as a grain of sand to be a clearly seen meteor, yet a piece that is the size of a grape creates a huge amount of light called a fireball, which can cast shadows. If a piece of this rock actually manages to survive to hit the ground, then it is called a meteorite. The chemical composition of the meteorite gives astronomers clues to it's origin. It was a meteorite from the planet Mars that has given rise to the plausible debate on basic life forms having existed on the planet.
As comets travel trough the gravitational fields of, for instance the planets, they are subject to the attractive forces. This may alter the shape and length of their orbits and even cause the comet itself to disintergrate. An example of this was the comet Shoemaker-Levy 9 that was broken down into smaller pieces that formed a line due to the influence of Jupiter's gravity. Eventually the pull was so strong that the pieces of material collided with the planet.
Comets are thought to have been some of the original creations of the big bang, were thought to fill the skies of the early solar system, and because of this are deemed to be important. They are speculated to, by collision with the planets, have provided ground water for them, enabling life on Earth to form. Now we only expect to see a comet with the naked eye about once every decade. They are theorised to exist in areas known as the Kuiper belt, and the Oort cloud. The latter was first theorised in 1950, and is said to be far outside of our solar system, up to 8 trilloin kilometers away from our sun. The cloud remains only a theory though, as it has never been detected. The Kuiper belt however was first theorised about in 1951, and accounts for the comets with short orbital periods of less that 200 years (such as Halley's comet). Scientists agree that in the main that the belt is in an area past the orbit of Neptune at around 30 to 50 AU from the sun. Well over 30 comets have been found in the region of the Kuiper belt. If they all remained in these areas, many people would never be able to experience their brilliance on a cloudless night, but due to the collisions that occur within this region and the stress from other objects such as gravity from nearby stars, occaisionally they are forced out of this region into an ellipitical orbit around the Sun. This orbit is in some cases perpendicular to the plain of the solar system. Only when the Earth is at the correct relative part of it's orbit compared to the comet's are we able to see the phenomena that has been the fascination of many scientists of our time.
Comets all through history have been seen as 'signs' showing different things to those that believed in them. They were feared because they were thought to be unpredictable, and were not understood until Issac Newton finally calculated their motion. Even then, they were taken as objects of great mysticism. For instance etchings have been found on rock from ancient times, the best examples coming from the Easter Islands (see digram), and China. This picture is notable due to the fact that the tail is pointed away from the sun.
It was not only the primitive civilisations that presumed comets to be of great consequence. Some of the great leaders of past times relied on them as guiders for the good and bad occurences of the times. Many comets coincidently have acted as landmarks for notable events. For instance the Great Comet of 1665 was seen by the people of London as the black death struck down 90 thousand people. A comet in 44BC was present soon after Julius Ceaser died, and was seen by all of Europe.
Napolean Bonaparte thought that comets were important to him individually, and took them as guides to his battle conquests. Many of the battles he won, that totalled 60 in all, were won under the shadow of a comet, and Charles Messier (nick-named the 'Ferret of Comets'), a world recognised early comet hunter, became one of his closest associates. When a comet was seen in 1811 in the sky, Bonaparte made the decision to attack Russia. This unfortunately for him was not the good omen he took it to be, as this ended up to be the worst loss at battle of his career.
The birth of Mark Twain occured as Halley's comet moved through the sky. He took this to be a sign of his life. He also died as the same comet returned in 1910. When his life was drawing to a close, he said that he came in on the comet, and similarly he would go out on it. A sound observation.
During the nineteenth century, information was gathered together though the use of telescopes and photography. This allowed the calculations of orbital information to be completed extremely accurately, predicting the return of various comets correct to a couple of days. However this knowledge and understanding meant very little to some people, such as when Halley's comet returned in 1910. Astronomers were able to say that the Earth, while on it's orbit, would pass through the tail of the comet during it's visit. Some people took this to be a danger due to the fact that poisonous gases such as cyanogen had been discover to be in the tail of a comet that had passed a few years previous. Some people therefore presumed that going through the tail would no doubt mean instant death, even though they were instructed by the astronomers that they wouldn't even notice the effects. Business men began to catch onto the fears of the people, and sold what they claimed to be an antidote to the chemical. These so called "comet pills" were great sellers to the worried population, and thankfully kept the users away from the dangers of the tail. Fortunately for the rest of the world though, the astronomers were right in the first place, and no-one else felt the effects either.
This outburst of public fears has not been repeated until recently, when it was publisied that comet Hyakutake would hit the Earth in 1996. Luckily the early calculations that suggested this fact were incorrect, and no adverse effects were felt on it's passage past our planet. But 1997 brought the worst consequence for many, many years. Comet Hale-Bopp came into our skies, and due to the fact that the millenium was fast approaching, some people took it to be a sign from aliens. Some people thought that the astronomers studying the comet had not told the public the truth, that the comet would hit our planet. Others believed that a UFO was in the tail of the comet, and upon their death on Earth they would be transported to their true destination. Fake photographs were manufactured to support this claim. All of the knowledge we have of comets was shrugged aside by the Heaven's Gate cult, when they commited a mass suicide of 39 people in order to fulfill their predicted future calling on March 26, 1997.
This has been the most prominant comet of late, the picture shown having come from the Hubble Space Telescope. The comet is named after the two amateur astromers that independantly happened to notice it's existence within the group M70. They saw it on 22 July 1995, when it was 560 million miles from the Earth, just outside the orbit of Jupiter. It was unusually bright for the distance it was discovered at, and was of great excitment to astronomers worldwide. The last time it was visible from Earth was 4200 years ago, but this orbital period is shrinking somewhat to 2380 years due to the effect of Jupiter's gravity. The plain of it's orbit is perpendicular to that of the solar system, and it's orbit is of a fairly predictable elliptical shape. It was seen to be at it's most active in the beginning of April 1997, when it is less than one AU to the Sun. It demonstrates a remarkable pin-wheel shape, the top bright part of the picture is thought to be a disintigrated part of the icy crust. The shape comes from the spinning of the nucleus of the comet- and does so approximately once per week.
The first person to recognise that this was a periodic comet was Edmund Halley. He had already been able to compute the parabolic orbits of 24 other comets. Although he actually never saw the comet, he calculated that the comets orbital period was around 75 years, as he read about the sitings of it in earlier times, and saw the resemblance of the orbital paths. The most well known appearance it has made was woven into the Bayeux tapestry, when the Norman's won the battle of Hastings in 1066. He predicted it's next appearance in 1758, but unfortunately died in 1742. His prediction was correct, as it reappeared on December 25, 1758.
After the comet had returned, astronomers realised that they could link it to sitings from even earlier than Halley had done. They found an additional 23 records pre1531, to as early as 240BC. This was recorded by the ancient Chinese, who described it as a "broom star" that "appeared in the east and then was seen in the north". They also specified that it was observed in a specific lunar month that began on May 24 and finished on June 23. This viewing has been computed to be extremely accurate considering the time that it was made.
The closest that planet Earth has ever been to the comet is still 3.2 million miles (the equivilant 0.0342 Astronomical Units). It was at this distance in in 837 and it's tail ranged a large 60 degrees over the night skies.
On it's viewing in the mid 1980s, it was the most observed returning comet of all time.
By Anne-Marie Cumberlidge, Keele University- 1997.
"Who Threw That Dirty Snowball?" by Gregory Beekman.
"Comet Hale-Bopp" by Andrew Beardmore.
Comet hysteria informaton.
General comet information.