Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.

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3242-22med.jpg  Human kind has always looked to the heavens in awe and wonder, and sometimes in fear. Perhaps no other astronomical phenomenon except a total solar eclipse has historically evoked as much fear as comets. When the specter of fear is removed, however, they emerge as strikingly beautiful objects in the sky. It was once believed that if earth passed through a tail of a comet, its inhabitants would die; this theory has been discredited. Comets are messengers from a time long past. Most are chunks of dirty ice, locked away in the Oort cloud for billions of years.
Comets are familiar to nearly everyone as striking star like objects with long tails stretching across a wide band of the sky. The most famous comet, Halley’s comet makes its return to the skies every seventy-five years. The word "comet" is derived from a Greek word meaning "long haired" Comets were greatly feared before the twentieth century as bad omens. Since then, they have been identified and catalogued as objects that come from deep space. Most of them occupy orbits that carry them far outside the solar system. Many comets make only a single approach to the sun and never return again, while others exist in stable, but highly elliptical orbits that allow them to return after an extended period of time, such as the Halley’s comet.

In 1986, the European space probe Giotto passed about 600 kilometers from the Comet Halley as it made its close approach to the sun. The probe verified existing theories that comets are made up of ices covered by black dust or soil. The spacecraft confirmed a theory that had been advanced prior to the reconnaissance that described comets as "dirty snowballs." Using data taken by the spacecraft, scientists determined that the dust is composed of carbon, hydrogen, oxygen, and nitrogen. Other metals have also been discovered in comets, such as iron, calcium, nickel, potassium, copper and silicon. Halley’s comet was one of the darkest objects ever seen in the solar system; it was basically flat black.

Comets are composed of a mixture of ices and dust. As a comet approaches the sun, it absorbs the suns energy and warms up. The main body of the comet is called the nucleus. As the nucleus warms, the ice beneath the comets soil evaporates. Because the comet has no atmosphere, the evaporated substance (also called a volatile) escapes into the vacuum of space as a gaseous envelope that surrounds the comet called "coma." As the coma grows, it forms a plume of vapor that carries away some of the comets surface as well. This mixture of evaporated volatile particles and dust is carried away from the comet by solar wind, is ionized by high-energy particles, and creates the spectacular tail of the comet. The comet’s tail, glowing in the solar wind, can stream behind the comet for millions of kilometers. The nucleus of the comet consists of mostly volatile ices and dust. The ice is nearly all water ice, but there is also evidence of ices composed of carbon dioxide and methane. More elementary compounds of nitrogen, oxygen, and carbon monoxide may exist as volatile ices.

Comets are typically small bodies (comparatively!!). Halley’s comet is a potato shaped object, 14 by 17 kilometers. The largest known comet is Chiron, which is estimated to be approximately 200 kilometers in diameter. Comets are thought to have formed as the solar system evolved. The material of which comets are composed was constructed by accretion at the outer edge of the disk of material that ultimately became the sun and planets. Because the comet material was fashioned at the outer edge of the solar system, the sun did not evaporate the volatiles in the cometary material. At the same time, the giant planets of the solar system formed at what would become the outer orbits of the solar system. These massive planets encountered the newly formed comets, and the comets that were not engulfed by the giant planets were, over the first billion years, catapulted into interstellar space by the planets massive gravitational fields. Not all comets met that fate, however. Some were gently nudged into stable orbits closer to the sun. Others were flung into the inner solar system and impacted the inner planets.

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The study of comets involves detailed knowledge of its composition of the outer regions of the solar system and the space between the last planet and 100,000 astronomical units onwards. Cometary study also seeks to understand complex gravitational interactions between bodies separated by wide distances and even gravitational interactions between tiny comets, their behavior when approaching the sun, and something of the makeup and evolution of the early solar system.

New comets approaching the sun for the first time have been held in deep freeze in the Oort cloud and are thought to be composed of primordial material of the newly forming solar system. They have been tied up in the Oort cloud for billions of years at temperatures slightly above absolute zero. As they approach the sun, their internal gases begin to steam away. A detailed study of an approaching comet may tell cosmologists about the composition of the early solar systems. Comets and their approach have also hinted at the existence of the elusive brown dwarf, thought to be one of the most common bodies of interstellar space. Because they are so dim, they are all but invisible from Earth. On the other hand, because brown dwarfs are thought to be so plentiful, the study of comets and their orbits may give the first real clues to the former’s reality and abundance.

In the early 1980’s the existence of the galactic tidal action was merely speculation. Since then, careful study of cometary orbits and approaches has favorable supported the theory of galactic tides. In the close approach of Halley’s comet by an unmanned spacecraft in 1986, a wealth of information was recovered on the shape, behavior, and composition of comets. The existence of the Oort cloud and the concept gravitational interactions by passing objects in space have led to the theory of periodic comet showers. Such comet showers, separated by periods of tens of millions of years, may be responsible for the mass extinction on Earth. There is a wide speculation that Earth was struck by one or more comets 70 million years ago, which wiped out the dinosaurs. Some scientists have speculated that this extinction was the result of a shower of comets from the Oort cloud, sent on their close approach to the sun by a passing star or brown dwarf through the Oort cloud.

Comets have been used to judge vast distances, evaluate the composition of the solar system as it was being born, and even test the idea that the gravity of the entire galaxy can make a difference to the smallest objects in space. Comets have been used as yardsticks to evaluate what may be the most type of star in the galaxy-the brown dwarf-which ironically is one that may never be seen. Comets have also been called as dirty snowballs. Halley’s comet was so black that it was the darkest object ever seen in space. Yet, from these dirty specks of ice, cosmologists have witnessed some of the most spectacular light shows. Ultimately comets may also generate clues to some of the most fundamental secrets about the solar system and planets. From these tiny messengers, cosmologists may unlock and examine pristine elements from creation itself.

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