Comets were once described by Fred Whipple as nothing more than a dirty snowball that travels through space. Fred Whipple has a point as comets consist mainly of ice and dust. The make-up of a comet is theorised as being dust and blocks of ice held together by the cold.
Given a comet's size and the great distances they travel, it is not easy to work out their journey path, some comets such as Halley, the path is known, it travels out past Uranus before returning to the Sun. They have been seen throughout history and probably the most famous appearance of a comet that has been recorded is Halley Comet which appeared at the time of the Battle of Hastings (1066). Halley's Comet appears on the Bayeaux Tapestry which chronicles the battle.
A theory has been put forward that life on Earth hitch-hiked on the back of a comet due to its make-up. This has been neither proved nor disproved as far as I know. Recently due to the outbreak of the SARS virus, scientists are thinking about whether the so-called Emerging Diseases may have come from Comets as they pass-by. It is an interesting theory which could be worth looking into.
In March 2004, the European Space Agency (E.S.A.) launched the Rosetta space probe on a ten year mission to rendezvous with P67/Churyumov-Gerasimenko and land a space probe called Philae on the surface. On the 10th December, it was announced that results back from the mission and the experiments that it had carried out, water on Earth had not originated from comets. However it is only one comet that they've been looking at and more research is needed. Some scientists are now turning their attention to asteroids which they believe could contain water. Ref: B.B.C.
Although P67 was a disappointment in that it didn't find conclusive proof that water came from comets. According to Herschel space telescope, Comet Hartley 2 has the right kind of water that can be found on our planet. The reference for that is the same as the link at the end of the last paragraph. Hartley-2 and P67 are both comets that originate from the Kuiper Belt.
As mentioned before, the centre of a comet which is found at the head of a comet is a object made from ice and water, methane and other gasses. If you had been hoping for something a bit more exciting then you're a bit disappointed. The first picture below shows how a comet with a tail looks like as it approaches the Sun and the tail appears. The second picture is of Comet P67/Churyumov which when Rosetta examined it, it hadn't just started to burn and create a tail yet. Over time, the comet will eventually boil away into nothing or just a rock. There could be a rock centre but it has not yet been proved or disproved. Space
Comets take a highly elliptical path whereas planets are more circular. The picture below shows the path of Halleys Comet around the Sun. The pictures not drawn to scale so please don't complain, I already know. Also I've missed out the inner planets for picture clarity.
Below is the orbital path of Encke's Comet, which takes 3.3 years to orbit the Sun. It was drawn using N.A.S.A. Orbital Diagrammer which is a free on-line java program that will allow you to create maps of asteroid and some comet orbits. You'll need an old browser as a lot of browsers are dropping support for Java, the version I used is Internet Explorer 11, everything else had disabled it including Edge. The Java app will draw orbits to size which is why for Halley, I had drew it to fit everything in.
A Comet's orbital shape round a star is know as Eccentricity, a comet with an Eccentricity of 0 will have a near circular shape path. A higher number will be more elliptical, more oval shaped.
There is a theory that there is an encompassing cloud of comets that is in orbit round our star, the Sun called the oort cloud. The cloud is estimated to be at a distance of 1 light year. Something causes these comets to head towards the Earth. There is a theory around a Rogue Star called nemesis that when it comes close to us, it pushes comets to head towards us. Nemesis is believed to be a brown dwarf star which is why its not been detected as they are hard to spot.
There are two main times of comet term, short term and long term. If we take short term first, they can be broken again into two groups, Jupiter Comets where the term is less than 20 years, for example, Encke's Comet. The second short term comets are called Halleys after Halley Comet where their term can be from 20 years up to 200 years. Short term comets are believed to have originated in the Kuiper Belt.
Long term comets are anything where their orbit is greater than 200 years so that includes comets that are thought to originate from the Oort Cloud or beyond. Comet Hyakutake is a long term comet which will never see in our lifetimes unless of course this page is being read in 41,999 A.D. or there about.
Both long and short term comets are thought to have the same chemical composition. It is believed that the Oort Cloud comets were created within our solar system, at a distance closer to the Sun than the Kuiper Belt. The Oort Cloud comets are believed to have been ejected from our solar system, eventually forming a cloud which we know as the Oort Cloud. Ref: Consiglio Nazionale delle Ricerche
A Comet will only have a tail when it approaches the Sun. The tail is caused because of the heat of the Sun boiling the comet away. A comet will eventually disappear as it has been boiled away. Halley's Comet has quite a few orbits to go before it boils away.
A Comet's tail will leave behind dust particles which hit our atmosphere and burn up as meteors. Comet dust is what our Earth is hit by when a Meteor Shower occurs. For example, the Orionids is associated with Halley's Comet tail dust. Orionids are the result of the Comet approaching the Sun and the Eta Aquariids of when the comet finished going round the Sun and is on its way out.
The main convention is that the comet is named after the first person to spot it or make a detailed study of it. Halley's Comet, probably the most famous comet there is was named after Sir Edmund Halley who in 1682 predicted its return, 76 years later. Since then, the convention has roughly struck. The Comet of 1066 was not given a name until of course, Halley predicted its return.
Other older comets were just known as the Great Comet such as the Great Comet of 1680. Later comets were also given Great Comet names such as the comet of 1882 and 1680, presumably because the comets return was not predicted and proven to be right. If more than one comet appeared in a year, the month would be added to the name such as the 'Great January Comet of 1910'. In addition to a name, it is given a number and a letter. For example, Halley is 1P/Halley. The number signifies its order of being registered, Halley has the title of number 1. The letter doesn't always have to be a P.
Comets around other stars also known as ExoComet have been known about since 1987 when the first was discovered using Spectroscopy, the studying of the light and radiation. The comet was discovered orbiting Beta Pictoris. Since then, other exocomets have been found round other stars.
The main difference is what makes up the two objects, comets are made up of dust and ice whereas asteroids are rocks and metals. Comets when they get close to the Sun will burn away and produce a tail. Comets can have very long journeys round our Sun, sometimes never to be seen again in our lifetime whereas asteroids are in the asteroid or kuiper belts.
A lot of the periodic meteor showers that we receive are debris from comets as we pass through the left overs. The Meteor Showers Orionids and Eta Aquariids are both associated with Halley's Comet for example. When Halleys Comet passed by, its debris the Earth passes through at different points in the year. The showers are brightest when the comet has just passed by and dimmest when the comet is approaching.
Not all meteor showers are associated with Comets, the Geminids are one example of where the source of meteorites is not a comet. The source of that meteor shower is believed to be the asteroid Phaethon.
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