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Comets have been known to mankind for centuries. They appeared suddenly on the night sky, along with a long diffuse tail. In many cultures their presence on the night sky was thought to bring bad luck. They scared people. The fear remains today, but for another reason: Earth has been hit by comets in the past, and there is a fear that it might get hit again. Though, the fear is in most cases very exaggerated and mostly due to ignorance.

Some famous comets are Halley's comet (which appears every 76 years, it was last seen in 1986), Hyakutake, Hale-Bopp and Shoemaker-Levy 9, which smashed into Jupiter in 1994.
Comets are giants balls of ice (water ice) and dust, when their orbit takes them close to the sun the water ice starts to evaporate, forming a characteristic tail pointing away from the sun. They are classified into two cathegories: Short period (orbit is less than 200 years) and Long period comets. The short period comets originate from the Kuiper belt, which is a belt of small icy bodies extending from past the orbit of Neptune (at 30 AU. 1 AU = 150 million km.) to 50 AU.

But where do the long period comets come from?

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The Oort Cloud

Did The Comets Bring Water To Earth?
Exploring Comets
Meteor Showers
Physical Properties

The Oort Cloud

After studying the orbits of several comets, the dutch astronomer Jan Oort (who also calculated that the centre of our galaxy is in the constellation Sagittarius) noticed some characteristics for long period comets in 1950:

* No found comet had an orbit which suggested that it had came from interstellar space.
* The correlation of long period comets most distant point from the sun (aphelium) correlated strongly around 50 000 AU.
* No point from which the comets seemed to come from existed, rather they appeared to come from all directions.

This made Jan Oort believe that there is a giant cloud of comets laying in orbit 50 000 AU, approximately 0.8 light years, (ranging to 3 light years) from the sun. Till this date, the existance of this cloud has not been proven, nor do astronomers know the exact size of this cloud. Though most astronomers believe in its existence. Some believe it might contains as many as 6 trillion comets. Being so far away from the sun the comets would be very scattered and far away from each other, probably tens of millions of kilometres apart. The could still hold a significant amount of the mass in our solar system. The cloud is thought to be a remnant from the nebula that created the sun and the other planets.
At 50 000 AU and further away from the sun, the solar gravity is very low (the sun would appear as any other star). Other stars that happen to come nearby could easily alter the orbit of a comet's stabile and maybe circular orbit, ripping it from the solar system, or make it move inwards to the center in a highly eccentric orbit.
Other stars likely possess similar clouds which could interact when two stars come close. The star with greatest possibility of bringing disorder in the solar oort cloud in the next 10 million years is Gliese 710 as it approaches the sun.

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Physical properties of comets

As mentioned earlier, comets are dirty ice balls, consisting of watery ice and dust (ammonia, carbon dioxide, carbon monoxide, frozen water and methane). They have a mass of only 10^14 - 10^15 kilograms. The Earth's mass is 6*10^24 Kg.
When they get close to the sun (around 5 AU) in their orbit, they start to heat up and the gas slowly evaporates from the surface, which leaves a several million kilometres long characteristic tail. When a comet is brightest several features are visible:

* Nucleus - the dirty iceball. 0.5-20 km in size.
* Coma - a fuzzy haze that surrounds the nucleus. Coma + Nucleus = Head.
* Hydrogen cloud - a huge, but sparse cloud of neutral hydrogen.
* Dust tail - this is the long white tail, which can be millions of km. long.
* Ion tail - a tail of ionized gas (by the UV radiation from the solar wind), pointing at the direct opposite direction of the sun. The famous comet Hale-Bopp had a beautiful blue ion tail.

During the comet's orbit the dust tail always points away from the sun, but it is the Ion tail that points at the exact opposite direction. The tail grows in size as the comet approaches the sun and it decreases in size when the comet gets further away.

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Meteor Showers

When the comet orbits the sun, it leaves a dust trail behind it. When the earth passes through a comet's orbit, observers of the night sky can see a meteor shower, which is a concentrated burst of "falling stars". A meteor shower could last for days and when the intensity of the meteors is highest, one can sometimes see several hundred meteors per hour, depending on how much dust was left behind by the comet.
The meteors from the comet enter the atmosphere, sometimes at the speed of 70 km/s. This high velocity exposes them to very much friction as they enter the atmosphere and the vast majority of them burn up. Some fall down as small pebbles.
Many storms are annual, such as the Geminids, the Perseids and the Leonids. The name of the shower is given by the constellation from which they appear to radiate (in other words, from which they appear to be coming from). Below is a list of some annual meteor showers.

Quadrantids: - Jan 1 - 5, Maximum: Jan 3, 120 meteors.
April Lyrics: - Apr 16 - 25, Maximum: Apr 22, 15 meteors.
Eta Aquarids: - Apr 19 - May 12, Maximum: May 5, 60 meteors.
Arietids: - May 29 - June 19, Maximum: June 7, 60 meteors.
z-Perseids: - May 20 - July 7, Maximum: June 9, 40 meteors.
d-Aquarids: - July 12 - Aug 19, Maximum: July 28, 20 meteors.
Perseids: - July 17 - Aug 24, Maximum: Aug 12, >100 meteors.
Orionids: - Oct 2 - Nov 11, Maximum: Oct 21, 20 meteors.
Leonids: - Nov 14 - 21, Maximum: Nov 17, 20 meteors.
Geminids: - Dec 7 - 17, Maximum: Dec 14, 110 meteors.
Ursids: - Dec 17 - 26, Maximum: Dec 22, >12 meteors.

Data retrieved from http://www.meteorscatter.net/metshw.htm.

When the comet that left the trail behind visits the inner parts of the solar system, especially the parts which cross earth's orbit the debris in the track is filled up. When the earth later passes through this trail the meteor shower is especially intense.
The most spectacular meteor shower is provided by the comet Tempel-Tuttle, which is associated with the storm Leonids. Every 33 years the meteor storm is extremely intense. During the peak, which lasts for about an hour, the rate of meteors could get as high as or higher than 100 000 meteors/hour! There has been events recorded where the witnesses said that meteors were raining down like snowflakes. Notable storms were recorded in 1698, 1799, 1833, 1866, 1966. For some reason, the storm that was 1999 was not as intense as predicted. Maybe the peak was "only" 4 000 meteors/hour (which is still a high number).

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Did The Comets Bring Water To Earth?

Some scientists believe that all the water present on earth was originally locked into rocks, and slowly leaked out over millions of years. The Earth did form from molten rocks and asteroids, so this is a plausable theory.
However, another theory which is getting more and more attention says that the water we drink actually came from outer space, by impacting comets! If scientists can prove that the comet water is similar to that found in Earth’s oceans, then they have solved a puzzle. Else, if the comet water is really different from Earth's water, then it suggests that the comets may not be of this solar system! To study this, scientists have sent a probe named Rosetta to a comet called 67P/Churyumov-Gerasimenko. It will arrive in 2014, and a small lander called Ptolemy will be sent to the surface to measure the chemical signature of the comet's water, and see if that matches our water.

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Exploring Comets

Why send spacecract to study comets? Because they contain clues about the formation and evolution of the solar system. They are composed of gas, water and dust and are virtually unchanged since the formation. By sending probes there we can learn more than only viewing them from earthbound telescopes.

As mentioned above Rosetta is due to meet up with the comet 67P/Churyumov-Gerasimenko in 2017 to study the similarites with Earth's water and the water found on the comet. In 1986 the spacecract Giotto visited the Halley's comet and sent back pictures of the core (among others).
NASA has launched another probe, Deep Impact to meet up with the comet Tempel-1 on 4th of July, 2005. The spacecraft will deploy a 1 metre impactor which will then guide itself to collide with the comet at a speed of 10.2 km/s. The spacecraft itself will pass 500 km below the comet and gather as much information it can about the comet and the impact that it possibly can, before the spacecraft comes too close to the hail of particles from the nucleus (13 min).

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Space Art 1: Nucleus of Halley's comet. Approaching the sun and gradually gaining a larger tail.

Illustration 1: Illustration of the range of the Kuiper belt.


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