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In addition to the Sun and the planets, there are millions of smaller objects that are gravitationally bound to the Sun as part of our solar system. These objects are much smaller than the planets, have irregular orbits and can be broken up into three major categories: icy comets, rocky asteroids and meteoroids.
A comet can be a beautiful sight and is unlike any other object in the sky. The brightest comets are visible with the naked eye as extended fuzzy patches, although this is rare and most require a telescope to see. They progress across the sky very slowly, remaining in our sky for weeks before fading out of view. Comets are small bodies composed mainly of ice and dust whose orbits take them to remote regions outside the solar system. It is believed that comets originated during the formation of the solar system as condensed chunks of ice and rock, and continue to orbit the Sun in highly elliptical orbits inclined at various angles from the ecliptic.
It was not until 1986 that a cometary nucleus was observed, when spacecraft were sent to take high-resolution photographs of the nucleus of Halleys Comet. Astronomers now know that the nucleus of a comet is composed of ice and dust and typically measures a few kilometres across. As a comet approaches the Sun, solar heat begins to vaporize the ice in the nucleus and it becomes enveloped in a coma, a fuzzy ball of glowing gases which hides the nucleus from our view. The solar wind blows the vaporized gases from the coma, producing a long, luminous tail of gas and dust. A comet does not produce its own energy or light, the gases illuminate because they reflect sunlight. As the comet approaches the Sun, solar radiation becomes more intense, melting more ice and increasing the size and length of the comets tail, which can vary in appearance from fat and short to thin and extended. (Comet Ikeya-Seki in 1965, for instance, had a long, skinny tail which was 150 million kilometres long -- equal to the distance between the Earth and the Sun.) Solar radiation produces two comet tails, although it is not always possible to distinguish both individually. The ion tail is straight and is caused by the solar wind, pointing directly away from the Sun. The dust tail is typically curved slightly and is caused by radiation pressure exerted on the dust particles by photons from the Sun. The tail of a comet will always point away from the Sun, and is not necessarily an indication of the comets direction of travel.
Comets are generally classified in two ways. Some are known as short-period comets and have orbital periods of less than 200 years. They appear at regular and predictable intervals, allowing astronomers to prepare for their return. The most famous short-period comet is Halleys Comet, which has a period of 76 years and is set to return in 2061. The majority of comets, however, are long-period comets that can take up to a million years to orbit the Sun. Most comets discovered each year are of this type, and as such have never been scientifically documented. Because of their long orbital period, there may be millions of long-term comets which have yet to be discovered. It is believed that these comets spend most of their orbital period in the Oort cloud, a cloud of comets some 50,000 AUs from the Sun. This cloud contains millions of comets travelling extremely slowly in their orbits because the Suns gravitational pull is weak at such extreme distances. The best known long-period comets in recent years have been Hyakutake (1996), Hale-Bopp (1997), and Shoemaker-Levy 9, which crashed into Jupiter in 1994.
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There are approximately 4000 catalogued asteroids and countless smaller ones which have not been scientifically recorded and named. The vast majority of asteroids are contained in the asteroid belt located between the orbits of Mars and Jupiter, although they are scattered throughout the solar system. Asteroids are known as minor planets, and are visible as star-like points of light that move nightly with respect to the stars, but can only be seen through binoculars or a telescope. Asteroids are rocky objects composed of material similar to the crusts of the terrestrial planets, and are thought to be remnants from the formation of the solar system that did not condense into a planet. Most asteroids have diameters less than a kilometre, but range in size from 100 metres up to the largest known asteroid, Ceres, with a diameter of about 900 kilometres, nearly half the size of the smallest planet, Pluto. The best method to determine the size of an asteroid is to observe it during an occultation of a star. This is when an asteroid passes directly in front of a star, visible along a very narrow path along the surface of the Earth. The time period of the occultation gives valuable information about the approximate size of the asteroid.
Although planetary orbits are generally quite circular, asteroids have slightly elliptical orbits, albeit not as much so as that of comets, and do not normally cross the orbits of Mars or Jupiter. Asteroids with high eccentricities may come inside the orbit of the Earth, and are termed Earth-crossing asteroids. Because these asteroids cross the path of the Earth, there is the possibility that one of them will strike the Earth in the future. The chance of such an occurrence in our lifetime, however, is slim, as only a few asteroids strike the Earth every million years.
By observing their motions in our sky, astronomers can determine asteroids orbital periods, but their physical properties often remain unknown. There are two types of asteroids: carbon and silicate. Carbon asteroids comprise about three quarters of the asteroids and are dim with non-reflective surfaces. Silicate asteroids occupy the inner portions of the asteroid belt and make up the remaining 25% of the asteroids, having highly reflective surfaces. The characteristics of the asteroids are difficult to determine because telescopes reveal little information about these small objects. The spacecraft Giotto passed by two asteroids, Gaspra in 1991 and Ida in 1994, and gave the first close-up views of a minor planet. Both these asteroids were very irregularly shaped with numerous craters of various sizes pocketing their grey surfaces.
A meteoroid is much like an asteroid, its only distinction being size. Rocky objects smaller than about 100 metres are generally termed meteoroids, and are typically asteroid fragments that were broken off during collisions between asteroids. Meteoroids are classified into three categories: stones, irons and stony irons. Stony meteoroids are much more common than irons, but they are similar in their appearance and age. Whereas meteoroids are primitive rocks that are remnants from the formation of the solar system, giving valuable information about the state of the material in the early solar system, planets have evolved since their creation so give few clues to their initial composition and structure. Occasionally a meteoroid will enter the Earths atmosphere and burn up as a meteor, also known as a "shooting star". From a dark site away from city lights, a few sporadic meteors are generally visible every hour. The meteors that streak across the sky are usually dust particles the size of a grain of sand. These particles enter our atmosphere at speeds of hundreds of thousands of kilometres per hour, and the friction between the particle and our atmosphere releases energy in the form of heat and light. The particle burns up in a matter of seconds and streaks across the sky in a flash of light.
Meteors occur randomly, but also occur annually in predictable meteor showers. A meteor shower occurs when the Earth passes through a swarm of dust particles, most often the debris left by a passing comet. Because the Earth passes through the debris at the same point along its orbit, meteor showers will occur at the same time each year and will typically last a couple of nights. The meteors from a meteor shower appear to originate from a single point in the sky, and are named after the constellation from which they radiate. The Leonid meteor shower occurs around November 16 every year and radiates from the constellation Leo with a meteor visible every few minutes. Comet Temple-Tuttle produces the debris for the Leonids and leaves a fresh trail of dust particles with each passage near the Earth, creating a brilliant display of thousands of meteors, known as a meteor storm. The orbital period for this comet is 33 years, causing the meteor storms to occur with approximately the same time interval
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An extremely bright meteoroid larger than a few centimetres in diameter entering our atmosphere is rare and is called a fireball. A larger or denser meteoroid will endure its journey through the atmosphere and will not burn up completely. The surviving piece of rock will reach the Earth as a meteorite, and can give scientists valuable information about the rocks in space and their origin. A meteorite may be large enough to cause an impact crater when it strikes the Earth, although this is very rare. Because meteorites occur so infrequently, and because our surface is in constant change, covering and modifying craters from the past, there are few visible craters on the surface of the Earth today.
The comets, asteroids and meteoroids in our solar system are relatively unimportant objects which in total are only about 10% the mass of our Moon. Comets are beautiful objects, with a bright head and extended tail of glowing gases. The majority of comets are located in the Oort cloud, a distant cloud of slow moving comets. Most comets in our sky originate from this cloud, and have extremely long orbital periods. Because of the length of the orbits of most comets, new ones are discovered each year and after their passage will not be seen again for generations. Asteroids and meteoroids are small pieces of rock that are debris left over from the formation of the solar system. Asteroids are larger than meteoroids and typically orbit the Sun between Mars and Jupiter, while meteoroids are small and are randomly located in the solar system. Meteoroids often travel through the Earths atmosphere and cause meteors, bright flashes of light often known as shooting stars. The light is caused by the release of energy as the grain sized particle slows due to the friction between it and the upper atmosphere. Meteor showers occur in regular intervals and are caused when the Earth travels through small dust particles left by a passing comet, and can be amazing displays of hundreds of streaks of light crossing the sky in a single night. If a meteoroid is large enough or dense enough, it will survive its journey through the atmosphere and will reach the Earth as a meteorite. A meteorite is a significant object for scientists studying the early solar system, as its relatively unchanged composition can give valuable clues to the formation of our solar system.