Comets by Ariella Fuzaylov 5/24/13

Halley's Comet as taken by NASA in 1986, last time it was visible from Earth.

http://www.space.com/19878-halleys-comet.html

Imagine it is 1986 and you have stayed up way past your bed time to watch Halley's comet come around for the first time in 75 years. Finally darkness descends and you see it, a streak of white light cutting through the sky. Leaving a blazing trail, but not appearing to move. You wonder what is this thing lighting up the night sky? What is it made of and where does it come from?

Comet diagram
http://www.enchantedlearning.com/subjects/astronomy/comet/

Another comet diagram
http://www.enchantedlearning.com/subjects/astronomy/comet/

This streak of light is a comet. A comet is an icy object that orbits around the sun. Each comet has a nucleus, a coma, an iron tail, and a dust tail. A comet is only visible when it is near the sun, even then the big slash in the otherwise navy sky is made up of the comet's two tails, not really the comet making them. A tail forms when the comet is near the sun, this is because as it nears the sun it starts to melt as it is mostly ice and rock, and it leaves small particles in its wake. The dust tail it leaves behind consists of microscopic dust particles, and it can be 1 to 10 million km long. These particles are bombarded with the sun's photons causing it to light up in a similar way to how dust is visible in the bright sunlight streaming into a dim room through a window. As the comet is leaving the sun the dust tail fades from view. The iron tail consists of completely different substances than the dust tail. It is made of charged gases, and can be as long as 150 million km long. Because of its composition the iron tail is always facing away from the sun because of the solar wind that pushes it into that configuration. As the comet nears the sun the iron tail trails behind it, as it leaves the sun the iron tail leads the way. Now you are probably wondering, what makes up the actual comet? Well the actual comet is made of a frozen nucleus at the head of it surrounded by the gaseous coma. The nucleus is made of ice, dust, and gas. Most of the comet's mass is contained in the nucleus even though it is usually very small, about 1 to 10 km across or more. The coma is an almost spherical blob surrounding the nucleus of the comet, basically the comet's atmosphere. It consists of various gases such as water vapor, carbon dioxide, ammonia, dust, and natural gases sublimed from the solid nucleus, the coma is about a million km across.

The orbit of a comet, including change in tail length as it orbits the sun.
http://cse.ssl.berkeley.edu/segwayed/lessons/cometstale/frame_orbits.html

Comets travel in regular paths around the sun called orbits. Most comets revolve around the sun in a more ovular pattern than planets do, or a highly elliptical orbit, with the sun near one end of the oval, but far from the other. This creates a point in the orbit of a comet where it is closer to the sun than it is in the rest of it's orbit. The comet's speed increases as it nears the sun and slows as it moves away. Though out most of a comet's orbit it has little to no tail, this is because the solar wind that radiates off of the sun melts the comet and creates its tails out of dust and gases from the comet. The farther away from the sun a comet is the less of an affect the wind has, causing the tails to disappear. From the simulation  you may also notice that the tail of a comet is always pointing away from the sun. This is also because of the solar winds, they blow the tail so that it always points away from the sun. So as a comet nears the sun its tail is behind it, and as it leaves the sun its tail is in front.

Oort Cloud and the Kuiper Belt

http://www.youtube.com/watch?v=n-rb6c4pNHM

The Kuiper belt crosses Pluto's orbit.
http://www.redorbit.com/education/reference_library/space_1/solar_system/2574576/kuiper_belt/

The Kuiper belt encircles our solar system
http://www.weirdwarp.com/2010/05/kuiper-belt-discovery-classification-shape-and-size/

The Kuiper belt is an area of the solar system full of Pluto size and smaller celestial bodies. It occupies the "space" from Neptune's orbit at 30 astronomical units or AU to 50 AU. An AU equals the distance between the Earth and the sun. Most of the object located in the Kuiper belt consist of ice and organic or carbon-containing material. They consist of the same things as most comets, and are believed by astronomers to be comets. Most short period comets which have an orbit of 200 years or less are thought to come from the Kuiper belt. Most of the objects in the Kuiper belt are  lumpy and not circular because they are not large enough for their gravity to pull them into a spherical shape. These objects mostly fall into the category of comet-like objects of dwarf planets.

The Oort cloud surrounding our solar system.
http://spaceplace.nasa.gov/tails-of-wonder/

The Oort cloud that surounds our entire solar system.
http://commons.wikimedia.org/wiki/File:Oort_cloud_lrg.en.png
Oort

The Oort cloud is a massive spherical cloud, whose size is unknown to scientists even today. Scientists believe that the Oort cloud starts at 2000 or 5000 AU and ends around 50,000 AU--almost a light-year. Some believe its edge extends past the edge of our solar system. The Oort cloud is believed to be filled with icy celestial bodies made of ammonia, water, and methane. Most comets in our solar system originate from the Oort cloud, the Oort cloud is made of the same materials as comets are. Long period comets which have an orbit of thousands of years are thought to originate from the Oort cloud. There are some exceptions where short period comets, that usually originate from the Kuiper belt originate in the Oort cloud, such as Halley's comet. The existence of this vast cloud has not been proven through direct observation, but its existence is widely exempted by scientists and astronomers alike.

Pluto and its tiny moon Charon to the right.
Photo courtesy of NASA
http://news.nationalgeographic.com/news/2001/02/0216_Pluto.html

Pluto's orbit in teal vs. Halley's comet's orbit in dark blue.
http://astronomy.swin.edu.au/s2plot/index.php?title=S2PLOT:Gallery

Pluto was once considered a planet, but it shares many characteristics of a comet. For one its volume is half ice, just like the nucleus of a comet is mostly ice. It is also very small compared to the rest of planets in our solar system, its size compares to that of a rather large comet. Another comet-like characteristic is Pluto has a very elliptical orbit, more elliptical than any other planet in our solar system. In the orbit diagram above you can compare Halley's comet's orbit with Pluto's. You may notice that Pluto's and Halley's orbits are both not on the same plane as the planets in our solar system. They both veer off on their own planes. You can also see that both orbits are not completely centered around the sun. While Halley's is less centered than Pluto's they are both very off center compared to the eight planets in our solar system. Also like a comet Pluto crosses the orbital path of another planet. It crosses paths with Neptune just as Halley does with Earth.