Comets

In 1986, the Giotto spacecraft flew within 400 miles of the nucleus of Halley's Comet. This close encounter provided valuable data and images of the comet's nucleus and helped scientists learn more about these cosmic objects.

The first "certain" appearance of Halley's Comet is from 240 BC, in the Chinese chronicle Records of the Grand Historian or Shiji, which describes a comet that appeared "in the east and moved north".

Comets are classified based on the length of their orbital period and their origin. Short-period comets have orbital periods less than 200 years, while long-period comets have longer orbital periods. Comets can also be classified based on their composition and physical characteristics, such as size and brightness.

Comets have been compared to "dirty snowballs"; rocks and dust held together by various types of ice. In deep space, everything cools down because the Sun is so far away, and several elements that we think of as "gasses" are frozen solid in comets.

As the comet approaches the Sun, the Sun's heat begins to melt some of the surface layers into gas, and the sunlight hitting the gas is reflected back to Earth as the "tail" of the comet. Some comets have been known to explode, as the heat of the sun melts a large ice pocket into vapor and breaks off pieces of the comet's core.

Maria Mitchell, an American astronomer, discovered a new comet in 1847. This was the first comet discovered by an American and she received global recognition for her achievement.

Some scientists think that a gigantic asteroid may have hit the Earth where the Gulf of Mexico now is, leading to the extintion of the dinosaurs and many other forms of life because of the global cooling created by the dirt and dust thrown into the atmosphere. Other scientists do not agree with this hypothesis and believe the Gulf of Mexico was formed by the drifting apart of the Caribbean and North American tectonic plates.

The period of naked eye visibility for comet Kohoutek spanned the end of Nov. 1973 until late Jan. 1974. {That means if you knew where and when to look, you could see Kohoutek without a telescope.} Its orbit was found to be hyperbolic, meaning Kohoutek is on an escape course from the solar system, never to be sen again.

Comets follow eccentric orbits that bring them closer to the Sun, causing them to release gas and dust that forms their tail. Occasionally, Earth crosses paths with these comet orbits, bringing them relatively close to our planet, but they typically pose no threat to Earth due to their small size and distant trajectory.

The nucleus of a comet is the solid, icy core of the comet. It is composed of a mixture of ice, rock, dust, and organic compounds. As a comet gets closer to the Sun, the heat causes the nucleus to release gas and dust, forming the characteristic coma and tail of the comet.

• A comet's tail is only seen when the comet is relatively close to the Sun. The heat of the Sun evaporates particles from the comet and illuminates them.
• Far from the Sun there is neither evaporation nor illumination.
• If a comet tail is not visible, then probably it doesn't have a tail. The comet has a tail when it comes near the Sun, and gets heated up. It is interesting to note that the comet's tail will always point away from the Sun (it gets blown out by the solar wind), this may be "behind" or "in front of" the direction of movement of the comet.
• Usually, the center of a comet is composed of ices and meteoric materials. These ices are sublimated (transformation of solid into gas) under the action of sunlight when the comet goes to the sun. The dust and ice (that is transformed into gas) are expelled from the center of the comet. Ultraviolet radiation from the sun breaks the molecules of gas and dust. These broken molecules becomes bright because of exposition to ultraviolet. That's why we can see a comet's tail.

Comets can be seen with the naked eye when they are close to Earth and reflecting enough sunlight to be visible. They are more commonly seen during certain times of the year when a comet's orbit brings it closer to the Sun and Earth. Many comets are also visible with the help of binoculars or telescopes.

Exobiology speculates that microscopic life originated elsewhere and was brought here by a meteorite sent off by some violent distaster, which while it has not been proven is very scientifically plausible, as germs and viruses have proven to be able to survive in outer space and there is a gigantic gash on mars (a planet with water likely to have once supported life) which could have sent such a meteorite into outer space. Also, since comets are thought to be made substantially of water ice, it is theorized that they are the primary source of earth's water. It may seem unlikely, but in the early stages of earth's development there were probably many, many more comets than there are today, and it is not impossible that enough comets could have collided with earth over millions of years to supply us with our water.

Comets are not physically strong objects in the traditional sense. They are composed mostly of ice, dust, and gas, and can break apart easily if they come too close to the Sun or other celestial bodies due to gravitational forces. However, they can be powerful in terms of the impact they have on our understanding of the solar system and the valuable scientific information they provide.

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The densest part of a comet is typically its nucleus, which is a solid, icy body made up of dust, rock, and frozen gases such as water, ammonia, methane, and carbon dioxide. The nucleus is surrounded by a coma, a cloud of gas and dust that forms as the comet gets closer to the sun and begins to heat up.

Haley's comet is a comet that passes around every 76 years, it was last seen in 1986 and will next be seen in 2061. It is the only short-period comet that is visible to the naked eye. It's orbit is highly elliptical and focuses on the sun.
Correctly named Halley's comet, this comet was named after Edmond Halley in 1705 and is visible to the naked eye about twice in a person's lifetime.

The Bayeux Tapestry, not Halleys comet, is a famous 11th-century embroidered cloth that depicts the Norman conquest of England in 1066. Halley's Comet did make an appearance in 1066, which is believed to be depicted in the tapestry as a bad omen for the English.

Comets do not fall. They orbit around the Sun in elongated orbits, with their paths often bringing them close to the Sun before they swing back out into the farther reaches of the solar system. This motion is due to the gravitational forces acting on the comet from the Sun.

halleys comet will return in 2061.

Not much different. Stony metorites contain the same minerals as continental crust.

Iron-nickel meteorites, such as the one that created Meteor Crater, near Flagstaff, Arizona, more closely resemble the composition of the Earth's solid core.

Comets are just very icy asteroids, they become comets when a large enough of a body of mass, i.e. jupiter or saturn. When this happens, the asteroid gets thrown into an extremely high elliptical orbit. They ideally have an orbit perpindicular to the ecliptic.

Halley's comet has a period of 75.3 years. The last time it passed near the Earth was 1986. Before that was 1910, 1835 and so forth. The next visit will be on July 28th 2061 so start making plans.

The main difference is that asteroids tend to be dense chunks of solid rock, iron and organic material, whereas comets are less dense and more loose lumps of ice, dirt, rock and organic matter - "dirty snowballs." Asteroids seem to be dead, inert chunks of solid rock, but the ice in comets means that when they get close to the Sun, some of that ice starts to melt and evaporate into space, making the comet grow a fuzzy, thin atmosphere called a coma, which ultimately streams out behind the comet (pushed out by the Solar Wind, a stream of charged particles flowing from the Sun into space) and grows into a tail.

Most asteroids are held in a belt between Mars and Jupiter, where they follow roughly circular, planet-like orbits. Comets, on the other hand, tend to come from much further out in the Solar System - either from the Kuiper Belt beyond Neptune, or the Oort Cloud much further out - and have very long, thin, highly eccentric and elliptical orbits.

Recently though scientists have found the distinction between asteroids and comets isn't all that clear cut. Objects called centaurs, which travel between the orbits of Saturn and Neptune, look like asteroids and have asteroid-like orbits, yet are made of significant amounts of ice and often get fuzzy when they get relatively close to the Sun. Many asteroids in the outer part of the main asteroid belt contain a lot of ice. And many asteroids we can point to seem to be ancient ex-comets whose ice has all melted and evaporated, leaving behind the dust and rock that has compacted into a single mass - an asteroid.

They are from extreme outer parts of the solar system (known as "Kuiper belt") and from a huge cloud of comets lying far beyond the orbit of Pluto, completely surrounding the Sun (known as "Oort cloud").

Comets spend most of their time far from the Sun in the outer solar system and are only visible when they come close enough to the Sun to develop a visible coma and tail. Additionally, their orbits may not bring them close to Earth very often, making them appear infrequently in the night sky.