Comets

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= = =What is a Comet?= Comets are small celestial bodies that revolve around the Sun. They consist of a nucleus, an atmosphere-like cloud called a coma, and a gas and a dust tail. All of a comet's activity comes from its small, solid nucleus which is made up of ices and dust. Most of the nucleus is surrounded by a hard crust. While far away from the Sun, a comet is just an icy, inactive body, meaning it has no coma or tails. When it nears the sun, however, the extreme heat vaporizes a lot of the ice, surrounding the comet in a large cloud of gas and dust ~ the coma. As a comet gets even closer to the sun, large amounts of pressure and high solar winds force some of the coma away, forming two tails: a curved, yellowish dust tail, and a straight, bluish gas or ion tail. Since the forces acting upon the coma push the tails away from the Sun, the tails stay behind a comet as it moves towards the Sun, and lead as the comet passes by the Sun.

There are two types of comets: short period comets, and long period comets. Short peiod comets travel around the Sun in under 200 years, and their appearences can be predicted. One example of a short period comet is Halley's comet, which can be seen from Earth approximately every 76 years. Long period comets take over 200 years to completely orbit the Sun. While these comets are brighter than short period comets, their appearences are not predictable. = = There are two main theories on where comets come from. The most widely accepted one is the Oort cloud theory. Suggested by Dutch astronomer Jan Hendrik Oort in 1950, the theory says that there are trillions of inactive comets that rest on the very edges of our solar system in a giant cloud called an Oort cloud. A comet in this cloud is activated when a gas cloud or star passes by and jolts it into orbit. Today, the Oort cloud is thought to lie between 50,000 and 150,00 astronomical units away from the Sun. Another theory about the origion of comets is the Kuiper belt theory. In 1951, the astronomer Gerard Peter Kuiper said that there is another collection of inactive comets is located just outside the solar system, about 35 to 1,000 astronomical units from the Sun. This Kuiper cloud contains only an estimated 10 million to 1 billion comets, much fewer than the Oort cloud.

There are also many theories on what happens to a comet at the end of its "life." One possibility is that the nucleus breaks or explodes. Another proposal is that comets become inactive, and turns into an asteroid. Yet another suggestion is that some force or disturbance causes comets to leave the solar system and fly out into space. = = =Comets and Earth=

Comets have affected people on Earth greatly over the years.Ancient civilizations thought that comets were warnings or omens of great news, due to their strange shape and out-of-the-blue appearences. The famous Greek philosopher Aristotle believed this, and also suggested that comets were related to weather. this idea kept the fear of comets alive for hundreds of years. Once printing was common, any sighting of a comet would be followed by papers with titles like "The Terrible and Fearsome Comet." On May 18, 1910, Earth traveled through the tail of Halley's comet. When news got out about this, people panicked. Many thought it was the end of the world, that the gasses in the comet's tail would surely kill off everything. Several people purchased gas masks and anti-comet pills to protect themselves. However, the comet passed harmlessly.

Many comets pass through the orbit of Earth, and it it speculated that a collision is likely to occur in the very distant future. A comet has crashed into Earth before. At the edge of Mexico's Yucatan peninsula is a crater 100 miles wide. The crash happened about 65 million years ago, and is associated with causing the extinction of many prehistoric creatures, including the dinosaurs, as the impact shot enough material into the atmosphere to cause the amount of sunlight reaching Earth to be reduced greatly. The ensuing temperature drop is what is believed to be the cause of the mass extinction.

It has been suggested that comets could be the source of space mining operations to gain raw materials in the future. However, this is not currently affordable or practical and there is no real plan going forward.

Visuals Make sure to include the location of your image; add a caption with this information


 * [[image:http://galenet.galegroup.com/images/itkids/pct/SIBIO0119IMG002-t.jpg width="213" height="161" caption="Halley's Comet" link="http://galenet.galegroup.com/servlet/KidsInfoBits?subTopic=Space%2B%2526%2BAstronomy&locID=s0002&failover=0&srchtp=topic&topic=Science%2B%2526%2BMath&c=5&searchTerm=Comets&ste=6&tab=1&tbst=tsrch&relDocDisplay=SIBIO0119IMG002.jpg&docNum=BX3200510119&bConts=39"]] || [[image:http://galenet.galegroup.com/images/itkids/pct/00230184-t.jpg width="215" height="157" caption="Comet Hyakutake" link="http://galenet.galegroup.com/servlet/KidsInfoBits?subTopic=Space%2B%2526%2BAstronomy&locID=s0002&failover=0&srchtp=topic&topic=Science%2B%2526%2BMath&c=3&searchTerm=Comets&ste=6&tab=1&tbst=tsrch&relDocDisplay=00230184.jpg&docNum=BX3200950711&bConts=39"]] || [[image:http://galenet.galegroup.com/images/itkids/pct/00230185-t.jpg width="233" height="160" caption="The comet Hale-Bopp over a town in Bavaria."]] ||
 * [[image:http://callisto.ggsrv.com/imgsrv/FastFetch/UBER1/00002246-T2 caption="Comet West"]] || [[image:http://callisto.ggsrv.com/imgsrv/FastFetch/UBER1/00017835 width="208" height="200" caption="Halley's Comet"]] || [[image:http://callisto.ggsrv.com/imgsrv/FastFetch/UBER1/00044449 width="272" height="206" caption="Comet Hale-Bopp"]] ||
 * [[image:http://www.sciencenewsforkids.org/articles/20070725/a1522_1932.3a.RC.fob.jpg width="273" height="189" caption="The comet 73P/Schwassmann-Wachmann 3 breaking apart."]] || [[image:http://imgsrc.hubblesite.org/hu/db/images/hs-2010-35-a-small_web.jpg width="208" height="198" caption="Comet Hartley 2"]] || [[image:http://imgsrc.hubblesite.org/hu/db/images/hs-1994-21-b-small_web.jpg width="249" height="220" caption="Comet P/Shoemaker-Levy 9"]] ||
 * [[image:http://imgsrc.hubblesite.org/hu/db/images/hs-2007-40-c-small_web.jpg width="267" height="228" caption="The nucleus of Comet Holmes."]] || [[image:http://imgsrc.hubblesite.org/hu/db/images/hs-2004-52-a-small_web.jpg caption="Close-up of Comet NEAT taken from Kitt Peak Observatory"]] || [[image:http://imgsrc.hubblesite.org/hu/db/images/hs-1998-13-c-small_web.jpg width="248" height="218" caption="Radiation from the inner coma of Comet Hyakutake."]] ||

**Works Cited** **Sources** : Include the source information for all of the magazine articles, reference sources (encyclopedias) and web site pages that were used to complete your project. The source information for encyclopedias may be found at the end or beginning of each entry in iCONN. When using periodicals, the publication information will be at the beginning or end of the article. This needs to be formatted for MLA standards. If it is not labeled 'Source Citation' it can be formatted appropriately by using EasyBib.com. You should use EasyBib for the web sites. The final Works Cited should be listed in alphabetical order by the first word of the source citation. "Milky Way." //Kids InfoBits Presents: Astronomy//. Gale, 2008. Reproduced in Kids InfoBits. Detroit: Gale, 2012. "The Milky Way." //WMAP's Universe//. NASA, 28 June 2010. Web. 06 Mar. 2012. . Vergano, Dan. "Galaxy Bracketed by Big Bubbles." //USA Today// 10 Nov. 2010: 05A. Web. 6 Mar. 2012.
 * Sample:**


 * Your Source List:**

"Comets, Meteors, and Asteroids." //World of Scientific Discovery//. Gale, 2010. //Gale Science In Context//. Web. 7 Mar. 2012.

"Comet Hale-Bopp." //World of Scientific Discovery//. Gale, 2010. //Gale Science In Context//. Web. 7 Mar. 2012.

"Comets." //Kids InfoBits Presents: Astronomy//. Gale, 2008. Reproduced in Kids InfoBits. Detroit: Gale, 2012.

Comets." //Earth Sciences for Students//. Detroit: Macmillan Reference USA, 2010. //Gale Science In Context//. Web. 12 Mar. 2012.

http://scienceworld.wolfram.com/astronomy/KuiperBelt.html: Eric Weisstein’s World of Astronomy

H. Weaver/JHUAPL, M. Mutchler and Z. Levay/STScI, NASA, ESA

http://hubblesite.org/gallery/album/query/comets/

**Topic: Research Focus**
 * What is your topic? Comets**
 * State the focus of your research: Their composition, life cycle, and their impact on people/Earth.**

**Notes** Include notes, statistics and facts that you will use to write your final paper. You may want to label sections of your notes to help you be more organized as you write. As you take notes from a source, you should list the source citation in the Works Cited section above

Comets are small bodies that orbit the sun. For at least part of their orbit, they have an atmosphere, a tail, or both. The nucleus of a comet is made up of rock, ice, and gas; the comet's tail and atmosphere are created by the sun's radiation acting on its nucleus.

comets' nuclei are believed to be formed from dust and rocky material mixed with frozen methane, ammonia, and water.

The word //comet// comes from the Latin word for "hair." A comet looks like a star with long hair streaming out behind it. The "hair" is actually a gaseous tail.

Comets are sometimes called "dirty snowballs." They are made up of chunks of rock and iron surrounded by gas. The gravity from the planets and stars keeps the comets in motion. (Gravity is the pull between objects.) The melted ice becomes a gaseous tail that extends away from the source of the heat. As a comet gets closer to the sun, it moves faster and faster. The comet's tail will grow in length as more of the ice evaporates.

On May 18, 1910, Earth actually passed through the tail of Halley's comet. Scientists had predicted the event, and when newspapers reported the prediction, many people panicked. They were certain that the end of the world was near. The public feared that poisonous gas in the comet's tail would kill human life on Earth. People bought gas masks and anti-comet pills. However, the comet passed without incident. Halley's comet comes close to Earth every 76 years. It last appeared in our solar system in 1986.

Because of their unusual shape and sudden appearances, ancient civilizations often interpreted comets as warnings or signs of great news. Aristotle was among those who believed comets were atmospheric phenomena, and he also suggested that comets were signs related to weather, a view that kept fear of comets alive for centuries. As soon as printing was widespread, every appearance of a comet was followed by pamphlets with titles such as "News of the Terrible and Fearsome Comet." In 1456, the comet that would later be known as Halley's Comet appeared just three years after the Turks had conquered Constantinople, lending credence to this view.

The most commonly accepted theory about where comets originate was suggested by dutch astronomer Jan Hendrik Oort (1900-1992) in 1950. Oort's theory states that trillions of inactive comets lie on the outskirts of the solar system, about one light-year from the Sun. They remain there in what is now called an Oort cloud, until a passing gas cloud or star jolts a comet into orbit around the Sun. The Oort cloud, today, is known to lie somewhere between 50,000 and 150,000 astronomical units (AU) from the Sun.

" Comets are best described as "dirty snowballs." Clumps of rocky material, dust, and frozen methane, ammonia, and water, they streak across the sky on long, elliptical (oval-shaped) orbits around the Sun. A comet is star-like in appearance and consists of a nucleus, a head, and a gaseous tail. The tail (which always points away from the Sun) is formed when some of the comet melts as it nears the Sun and is swept back by the solar wind (electrically charged subatomic particles that flow out from the Sun).

In 1951, Dutch-American astronomer Gerard Peter Kuiper (1905-1973) suggested that there is a second cometary reservoir located just beyond the edge of Earth's solar system, around one thousand times closer to the Sun than the Oort cloud. His Kuiper belt is located somewhere between 35 and 1,000 AU from the Sun. It contains an estimated ten-million to one-billion comets, far fewer than the Oort cloud.

There are many theories as to what happens at the end of a comet's life. The most common theory is that the comet's nucleus either splits or explodes, which may produce a meteor shower. It has also been proposed that comets eventually become inactive and end up as asteroids. Yet another theory is that gravity or some other disturbance causes a comet to exit the solar system and travel out into interstellar space.

Comets are small bodies of ice and dust that travel around the sun. When they approach close enough to the sun, they develop trailing tails of light. If close enough to Earth, these may pass slowly across the night sky for weeks or even months and then disappear until their orbits again carry them close to the inner planets. Many civilizations have thought that comets were warnings of coming disaster or death. Later, these spectacular sky travelers were recognized as cosmic fossils that have changed little since the earliest days of the solar system, about 4.55 billion years ago.

How Comets Look from Earth
Comets trail clouds of glowing gas and dust; their name comes from a Greek word meaning "the long-haired one." When a comet is far from the sun, it is simply an inactive icy body. As it approaches the sun, however, heat causes the ice to vaporize, surrounding the comet with a cloud of gas and dust called a coma.

Pressure from sunlight and the solar wind pushes the coma away from the sun, forming two tails. The dust tail is usually curved and appears yellowish because the dust particles scatter sunlight. The gas, or ion, tail is straight and may appear blue because it contains carbon monoxide ions (CO+). As the comet nears the sun, these tails point behind the comet. When the comet moves away from the sun, however, its tails lead. The tails always point away from the sun.

Types of Comets
British astronomerEdmund Halley, in the late 1600s and early 1700s, was the first to use Newton's new theory of gravitation to calculate the orbits of comets. Noting that the comets observed in 1531, 1607, and 1682 had very similar orbits, he decided that all three had been the same comet, and he predicted that it would return in 1758 or 1759. It did, and was named Halley 's Comet in his honor.

Since Halley 's time, astronomers have divided comets into two groups. Short-period comets orbit the sun in less than 200 years, and their appearances are predictable. Halley, which appears every 76 years or so, is one of the brightest of the short-period comets. Long-period comets take longer than 200 years to orbit the sun. They are often brighter and more active, but their appearances are not predictable.

When CometHalley swung by Earth in 1986, several spacecraft approached it for close study. One of them, Giotto, approached CometHalley and later came close to Comet Grigg-Skjellerup. These missions, together with ground-based observations, have yielded much information about Halley and about comets in general.

Structure of Comets
The Halley 's Comet probes confirmed an earlier theory about the makeup of comets. All of the comet 's activity comes from a small, solid body called a nucleus. Although the internal structure is uncertain, Halley 's nucleus is made up of ices and dust. Of the ices, about 80 percent are water. A hard crust covers most of the surface of Halley 's nucleus; the rest is exposed ice. If the crust were to cover all of a comet 's nucleus, that comet may stop being active and would look like an asteroid. Comet Wilson-Harrington, discovered in 1949, became inactive and was lost until it was rediscovered as minor planet 1979 VA.

A number of studies have focused on the chemical composition of comets. Comet observers have detected water, carbon monoxide, carbon dioxide, methane, formaldehyde, and other chemical molecules in comets. Similar proportions of molecules exist in dense molecular cloud cores--the places where stars form. This likeness leads scientists to believe that comets contain the same raw material from which the sun and planets were formed. Cometary material is of particular interest to scientists because it is the most primitive material in the solar system. Comet dust from comet Wild 2, returned to Earth by the Stardust space probe in 2006, shows minute amounts of high-temperature minerals. In March 2010, scientists announced the tentative discovery of two grains of interstellar dust in the Stardust Interstellar Dust Collector (SIDC) used during the mission. The interstellar dust grains contained heavy elements formed during nucleosynthesis. Elements initially identified included aluminum, chromium, copper, gallium, manganese, magnesium, iron, manganese, and nickel. Interstellar dust is ejected from exploding supernova and then condenses to form new generations of solar systems, planets, and other astronomical bodies.

Comets contain a high percentage of both water and organicmaterial, in this case molecules made up mainly of carbon, hydrogen, oxygen, and nitrogen. This discovery may link comets to the history of life on Earth. Some scientists believe that at least part of Earth's water came from comets that crashed into the planet. Comets may also have brought the organic material that was the source of the first life-forms.

Sources of Comets
In 1950 a Dutch astronomer, Jan Oort, published his research showing that visible comets likely originate in a cloud or shell of material located between 20,000 and 100,000 astronomical unitsfrom the sun. This shell is called the Oort cloud. However, although the Oort cloud is a source of comets, many astronomers do not believe that the comets formed there.

Many scientists think that the material in the Oort cloud is left over from the formation of the giant planets Neptune and Uranus. Building blocks from these planets could have scattered across space to form the cloud. From time to time, a random combination of gravitational forces causes one of these chunks to fall into the inner solar system to appear as a comet.

Another possible source of comets is the Kuiper Belt, a ring of icy bodies that some astronomers think revolves around the sun beyond the orbit of Pluto. If the Kuiper Belt does exist, fragments from it may sometimes enter the solar system to join the Oort cloud or to travel in new cometary orbits.

Comet Shoemaker-Levy Strikes Jupiter
In the summer of 1994, people watched for the first time as a comet struck a planet. Telescopes on Earth, the Hubble space telescope in Earth's orbit, and cameras on the Galileo spacecraft (on its way to Jupiter) were trained on this event, which spanned several days as successive pieces of the comet struck Jupiter. This marked the first time humans had witnessed the effects of collision of comet material with another planet.

The comet in question was Shoemaker-Levy 9, which had split into 21 icy fragments as it approached Jupiter, the solar system's largest planet. Many people compared the comet fragments stretched across space to a string of pearls. Excitement was intense when astronomers who had studied the path of the fragments announced that they would strike Jupiter. Although the impacts took place on the side of the planet that was turned away from Earth, Jupiter's rapid rotation brought the impact sites into view within a few minutes. NASA's Galileo spacecraft had a direct view of the impacts.

Astronomers labeled the fragments and their impact sites A through V. Fragment A produced one of the largest impact sites, about twice the size of Earth. Most impacts were surprisingly dramatic, producing visible flares in Jupiter's atmosphere.

Scientists studying the observations of the impacts have learned much about the chemical makeup and physical structure of Jupiter's upper atmosphere. At the same time, the collision of Comet Shoemaker-Levy 9 with Jupiter also made many people wonder what would happen if a comet were to strike Earth. Eugene Shoemaker, one of the comet 's discoverers, estimated that the impact with fragment G, the largest, released several hundred times as much energy as all the nuclear weapons in the world.

Comets and Earth
The orbits of many comets cross that of Earth. A collision could occur. At the edge of Mexico's Yucatán peninsula is a crater about 100 miles (160 km) across, created about 65 million years ago when a large comet or asteroid crashed into Earth from space. In March 2010, a multidisciplinary panel of geologists, geophysicists, and paleontologists examining the Cretaceous-Tertiary (K-T) mass extinction published a report in the journal Science concluding that K-T mass extinction 65 million years ago--including the disappearance of the dinosaurs--was primarily caused by a large asteroid impact. The impact blasted enough material into the atmosphere to reduce the amount of sunlight reaching the surface of Earth. The ensuing global winter resulted in mass extinctions.

It has been speculated that comets may one day become sources of raw materials for space mining operations. It would be far less costly to extract large amounts of raw materials from comets than to haul them into space through Earth's gravitational field.However, the fact that it would be cheaper does not necessarily mean that it would be affordable or practical, and as of the early 2000s no specific plans to extract resources from comets or asteroids existed.