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An asteroid is any of numerous small planetary bodies that revolve around the sun. Asteroids are also
called minor planets or planetoids. Most of them are located in the asteroid belt between the orbits of Mars and
Jupiter. The belt contains more than 1,150 asteroids with diameters greater than 18 miles (30 kilometers). Astronomers
estimate that more than 30,000 asteroids exist, and they have accurately determined the orbits of about 6,000 of
them. The average temperature of the surface of a typical asteroid is -100 degrees F (-73 degrees C).
There are two main theories for the origin of asteroids. One theory is that a planet located between the orbits
of Mars and Jupiter broke up, forming the asteroids. According to the other theory, there was enough material for
a planet between the two orbits, but no planet formed. Instead, the material simply became a belt of rocky objects.
Asteroids vary greatly in size. The largest and first known asteroid, Ceres, was discovered in 1801. It is about
600 miles (970 kilometers) in diameter. Ceres is believed to contain about 1/3 the total mass of all the asteroids.
One of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across.
Astronomers determine the size of an asteroid by comparing its distance from the sun with the amount and the wavelength
of light it reflects and the amount of heat it gives off. Astronomers may also measure an asteroid during an occultation,
when the asteroid passes in front of a star and is silhouetted against it. In addition, astronomers use radio telescopes
to produce images of asteroids.
In 1991, the United States space probe Galileo took the first detailed photograph of an asteroid. This was Gaspra, an irregularly shaped object measuring about 12 by 7 1/2 by 7 miles (19 by 12 by 11 kilometers).
Composition
Studies of an asteroid's reflected light as well as analyses of meteorites have provided information about the
composition of asteroids. Astronomers classify asteroids into two broad groups based on their composition. One
group of asteroids dominates the outer part of the belt. These asteroids are rich in carbon. Their composition
has not changed much since the solar system formed. Asteroids in the second group, which are located in the inner
part of the belt, are rich in minerals. These asteroids formed from melted materials.
A meteor is a bright streak of light that appears briefly in the sky. Observers often call meteors shooting
stars or falling stars because they look like stars falling from the sky. People sometimes call the brightest meteors
fireballs. A meteor appears when a particle or chunk of metallic or stony matter called a meteoroid enters Earth's
atmosphere from outer space. Air friction heats the meteoroid so that it glows and creates a shining trail of gases
and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat
up when the meteoroid passes through the atmosphere. Most meteors glow for about a second.
Most meteoroids disintegrate before reaching the earth. But some leave a trail that lasts several minutes. Meteoroids
that reach the earth are called meteorites.
Millions of meteors occur in Earth's atmosphere every day. Most meteoroids that cause meteors are about the size
of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the earth. They disintegrate
at altitudes of 30 to 60 miles (50 to 95 kilometers).
Meteoroids travel around the sun in a variety of orbits and at various velocities. The fastest ones move at about
26 miles per second (42 kilometers per second). Earth travels at about 18 miles per second (29 kilometers per second).
Thus, when meteoroids meet Earth's atmosphere head-on, the combined speed may reach about 44 miles per second (71
kilometers per second).
It is a well established fact that the comets are very similar in composition to the large planets such as Jupiter,
Saturn... This fact indicates that the comets may have been formed at the same place as large planets. This place
is called the Kuiper’s Belt. However, one does not find any more comets around giant planets. So a second question
comes to mind: why would the fragments have left and where are they now?
They would have left because of gravitational instabilities created by the large planets. They would have been
ejected to 50 000 AU, in the Oort’s Cloud, place which is definitely more stable as there is no large celestial
body nearby. But then, how can they come close to the Sun?
It is assumed that they would undergo a change of orbit caused by the influence of a hypothetical star. When the
orbit is perturbed, the comet can be sent towards the Sun or be ejected from the solar system. Hale-Bopp would
have indeed undergone such a change and been sent into an orbit which passes close to the Sun.
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