Comets are also called planetesimals because they are the remains that did not become planets or moons. Comets are believed to orbit the sun in a region called the Oort Cloud.
The Oort cloud is believed to contain over a trillion comets and the Oort Cloud begins beyond the orbit of Pluto and Neptune and extends more than 20 to 100 thousand AU, almost half the distance to the star called Alpha Centauri, the closest star to earth.
Within the Oort cloud is a region called the Kuiper belt where comets with short orbital periods travel. The most famous comet that travels within the Oort cloud is Halley's comet which can be seen from earth every 75 years. Comets are believed to consist of ice, dust, and rock or as Fred Whipple, a famous astronomer calls them, "dirty snowballs."
When a comet comes close enough to the sun, the ice begin to melt at such a great pace that sublimation occurs. Sublimation is the change of phase from solid to gas. The sublimation of the ice that carries away dirt and rock causes the appearance of a large white fireball with a long tail. The layer that envelopes the comet is called the coma. A tail of dust particles, hydrogen, or ions extend more than a million miles away from the comet.
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These are NASA Hubble Space Telescope images of comet Hyakutake, taken at 8:30 P.M.. EST on Monday, March 25 when the comet passed at a distance of only 9.3 million miles from Earth. The sunward and tailward directions are at approximately the 4 o'clock and 11 o'clock positions, respectively. The image on the left is 2070 miles across (3340 km) and shows that most of the dust is being produced on the sunward-facing hemisphere of the comet. Also at upper left are three small pieces which have broken off the comet and are forming their own tails. Icy regions on the nucleus are activated as they rotate into sunlight, ejecting large amounts of dust in the jets that are faintly visible in this image. Sunlight striking this dust eventually turns it around and "blows" it into the tailward hemisphere. The image on the bottom right is an expanded view of the nucleus. The top right image shows pieces of the nucleus that apparently broke off and were first detected during ground-based observations on March 24. |
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These NASA Hubble Space Telescope pictures of comet Hale-Bopp show a remarkable "pinwheel" pattern and a blob of free-flying debris near the nucleus. The bright clump of light along the spiral (above the nucleus, which is near the center of the frame) may be a piece of the comet's icy crust that was ejected into space by a combination of ice evaporation and the comet's rotation, and which then disintegrated into a bright cloud of particles. The debris follows a spiral pattern outward because thesolid nucleus is rotating like a lawn sprinkler, completing a single rotation about once per week. The Comet Hale-Bopp was discovered on July 23, 1995 by amateur astronomers Alan Hale and Thomas Bopp. |
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Thi is a picture of the Comet Kohoutek as taken from Skylab |
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This is a picture of the Comet Shoemaker-Levy 9 taken from the Hubble Space Telescope |
EXPLORATION
Halley's comet is the only comet to date that has actually been visited by spacecraft. However, it is extremely difficult for spacecraft to study the comet because it has a retrograde orbit. The comet orbits in the opposite direction that the planets orbit the sun.
All spacecraft launched from earth travels in the direction of earth's orbit because of launch specifications. Most launches are assisted by the motion of earth's spin which allows the rocket to gain speed easier and accelerate faster to escape earth's atmosphere.
A total of five spacecraft have been sent to explore Halleys comet. Two from the Soviet Union, two from Japan, and one from Europe has been sent. Vega 1 and 2 from the Soviet Union flew within 5000 miles (8,400 km) of Halley's Comet and sent back images of its nucleus and tail.
The Japanese spacecraft Susei and Sakigake were sent to study Halley's corona as well as its nucleus. The spacecraft Susei was hit by dust particles from the comet at a distance of more than 93,800 miles (151,000km) but still managed to study the comet. It was discovered that the comet completed one rotation every 53 hours.
Giotto, the European spacecraft, holds the record for flying the closest to the comet. It flew within 376 miles (605km) and was hit by a barrage of high velocity particles that came from the comet. If it wasn't for the two layer shield that protected Giotto from the impacts, the high resolution pictures of Halley's nucleus and coma would have never been sent back to earth. What was revealed about Halley was incredible. Halley's comet has a potato shaped nucleus that was more darker than coal. There are also hills and impact craters on the comet. It was also discovered that more than 80% of the comet's coma is water vapor and a very small percentage of organic compounds.
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Comet Halley as taken with the Halley Multicolor Camera on the ESA mission Giotto. |
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Comet Halley as taken March 8, 1986 by W. Liller, Easter Island |
Click here to download a great animation of the Shoemaker-Levy 9 crashing into Jupiter (637K)
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