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Diagram of composition of a comet
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Short-period Long-period Disintegrated
short-period comets: Halley's Comet, Encke's Comet, Tempel 1, Comas-Solá
long-period comets: Ikeya-Seki, Hale-Bopp, Kohoutek, Hyakutake
Shoemaker-Levy 9, Biela's Comet, Schwassmann-Wachmann

In ancient times comets symbolized all sorts of disasters such as wars, diseases, or punishments from God. It's no wonder that they cause such alarm when they appear unexpectedly with such fierce intensity.
Scientists originally thought they were a phenomena in the Earth's atmosphere, yet when Tycho Brahe calculated the distance in the 17th century, it became clear that they are very far away and not in our atmosphere. Orbits of comets look like long stretched-out ovals, or ellipses. Since the Sun is located near one end of this long ellipse, a comet spends most of the time moving slowly in the outer solar system before whizzing past the Sun. These cold and icy objects become very active when they approach the Sun and their remarkable tails form.

Astronomers often compare a comet nucleus with a dirty snowball. The small rocky core of a comet is covered with an icy layer mixed with dust. Scientists believe that comet nuclei have the same composition as the primordial material that formed the Solar System. It was quite surprising to find out that in addition to rocks and ice, comets contain some complex organic molecules that could be building blocks for life. Generally, a comet only becomes visible when it approaches the Sun and begins to melt. The frozen ice and dirt burn into a cloud of gas and dust which surrounds the nucleus. The dust and gas cloud are called the comet's coma. The coma is so thick near the center that it is impossible to actually see the nucleus without a spacecraft.
As a comet moves closer to the Sun, the solar wind blows the coma out behind the comet into a tail. The tail always points away from the sun. It has two parts: a yellowish dust tail and a bluish ion tail. The dust tail is the part we can see in the sky. Dust particles that escape from the nucleus can trail after the comet for up to 10 million kilometers. The ion tail is formed from the solar wind and is slightly curved following the lines of Sun's magnetic field. Even though comets shine brightly in the sky, the density of the molecules of a typical tail is a thousand times less than Earth's atmosphere.
The Oort Cloud is a big sphere surrounding the outer part of the Solar System. It begins at 50,000 AU and spreads up to one light year from the Sun. The Oort cloud contains a bunch of cold icy bodies which were left over from when the Solar System formed. Sometimes these objects get sent into the inner Solar System and become comets due to gravitational forces from the planets, nearby stars, or due to collisions. Some of the comets pass around the Sun only once and then leave the Solar System, others simply burn up when they get close to the Sun. If the comet becomes gravitationally trapped it will start to orbit in highly elongated pattern. Such comets are called long periodic comets and have orbital period longer than 200 years.
With telescopic observation astronomers have successfully determined the orbit and chemical composition of comet tails. However, the only way to learn more about the nuclei of comets is to use spacecraft. The spacecraft Giotto explored Halley's Comet in 1986. The Stardust mission was the first project that succeeded in returning samples to Earth.
Pluto is no longer classified as a planet. Ever since Pluto was discovered by Clyde Tombaugh in 1930, people debated about whether it was a planet or not.
What makes an object a planet? They are large, composed of rock or gas, and orbit the Sun. The inner four planets Mercury, Venus, Earth, and Mars, are all pretty small and rocky. The outer four planets Jupiter, Saturn, Uranus, and Neptune are all much bigger and mostly made of gas. Pluto is a ball of rock and ice, but is very small, even smaller than our Moon. Pluto's orbit is very stretched out and tipped when compared to the rest of the solar system. In fact, Pluto's orbit is so stretched out that sometimes it is closer to the sun than Neptune! Even though Pluto has its own moon, Charon, it is more like the Kuiper Belt objects than any of the planets in the Solar System. That's way the members of the International Astronomical Union (IAU) decided in 2006 to remove Pluto from the list of the major planets in the Solar System and make it member of the small planets family, where comets and asteroids belong. In order give Pluto recognition, people suggested calling Pluto the "King of the Kuiper Belt."
astronomer Edmund Halley

Edmund Halley determined that the comets from 1531, 1604, and 1682 are actually the same comet, and accurately predicted it would appear again in 1759. Even thought the same comet has been observed about 30 times since 240 BC, the comet was name after Halley. In 1986, Halley's comet was visible with the naked eye, but fainter than in the past.

meteor shower

Meteor showers are caused when the Earth moves through the stream of dust that trails behind a comet.

Kuiper Belt icon

The Kuiper Belt is like a ring, located beyond Neptune's orbit. So far, astronomers have found about 84 objects similar to comets. The former planet Pluto is probably the biggest of these. When a member of this belt is pulled to the Sun, it becomes a short periodic comet.

Tycho Brahe

Born in 1601, Tycho Brahe was a Danish astronomer known for his detailed observations of the sky.

Elliptical Orbit

An orbit that's elliptical, deviating from circular.
The orange planet has an elliptical, or eccentric orbit, while the blue planet's is circular.

Nucleus (comet)

Coma

Ion tail

Oort cloud

A cloud of primitve material surrounding the Solar System at great distances.
Tho Oort cloud also extends above and below the Solar System's plane, so long-period comets have large inclinations, and dip above and below the plane of the Solar System on their trip around the Sun.

Gravity

One of the fundamental forces of physics; gravitational attraction is responsible for the attraction of bodies on the surface of a planet to the mass of the planet.

Giotto (spacecraft)

Launced in 1986 by ESA, Giotto was tasked with approaching and studying Halley's Comet.

Halley's Comet

A comet visible on Earth every 75 years and last seen in 1986.
It's been observed for at least 2,000 years, but it was British astronomer Edmund Halley who realized that it was the same object that made periodic passes, and not individual objects as first thought.
Halley's comet won't be seen again until 2061.

Stardust (spacecraft)

A probe launched in 1999 by NASA which collected samples from the coma of the comet Wild 2.

Clyde Tombaugh

American astronomer born in 1906 who, while working as a researcher at the Lowell Observatory in Arizona, discovered the dwarf planet Pluto. He is also known for having discovered nearly 800 asteroids, several star and galaxy clusters and is partially responsible for the discovery of the Pisces-Perseus supercluster.

Orbit of Pluto

Charon

The largest satellite of the dwarf planet Pluto, with a radius of about 600 kilometers. It was discovered in 1978 by James Christy.

Photo of Pluto and Charon courtesy of NASA/JPL-Caltech.

Kuiper Belt

A region in the Solar System that extends beyond the orbit of Neptune and is the home of remnants from the formation of the Solar System, as well as the dwarf planet Pluto.