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The asteroid belt lies between the orbits of Mars and Jupiter.
They orbit the Sun in periods of between 3 and 6 years.
Another term for asteroids is "minor planets", this is because when the first asteroid was discovered it was thought to be a planet.
The asteroid belt was once thought to be the debris from a planet that had broken up, but it now seems more likely that the minor planets are cosmic rubble of the kind from which planets formed, so that the asteroid belt represents a failed planet rather than an exploded planet.
Some minor planets (about 5%) have quite different orbits, and some of these are in elliptical orbits which take them closer to the Sun than the Earth's orbit, crossing the Earth's orbit as they do so. Such objects have collided with the Earth in the past, and may do so in the future.
The moon has hundreds of thousands of impact craters while the Earth only possesses 120 or so.
Many of the craters that have survived on Earth are deformed by erosion or buried beneath layers of sediment.
In 23 years time, a small asteroid, measuring 390 metres in diameter, named "2004 MN4" will pass closer to Earth than any other in recorded history. It will pass by about 5.7 Earth radii. It shouldn't have any "pull" upon the Earth's tidal forces, like that of the moon.
There is only one asteroid that is ever visible to the naked eye, that is "Vesta", it measures 576km in diameter. Its not the largest member of the asteroid belt, that is "Ceres", which was the first asteroid discovered, by G. Piazzi, january 1st 1801.
Ceres measures 940km in diameter, Vesta, the next largest, 576km, followed by Pallas, 525km, Hygeia, 430km and Davida, 384km diameter.
Asteroids are divided into various types, according to their surface characteristics, as derived from their spectral affinities. The main classes are:
C (Carbonaceous) most numerous,darker than coal and featureless.
S (Siliaceous) most numerous in inner part of the main zone, generally reddish.
M (Metallic) metal-rich cores of former larger bodies which have been broken up by a collision.
E (Enstatite) relatively rare.
D -reddish, surface is 90% clay with carbon-rich substances, mostly remote from the Sun.
A -almost pure olivine.
P -spectra similar to class M.
Q -close-approach asteroids.
V -very rare,igneous rock surfaces.
U -unclassifiable.
The "NEAR" (Near-Earth Asteroid Rendezvous) spacecraft, which was designed to study the Near-Earth asteroid, Eros was launched february 14th 2000. It was renamed "NEAR Shoemaker" after its launch, in honour of the planetary scientist Eugene Shoemaker.
The main mission objectives of the probe were to return bulk properties, find out the composition and minerology, study its internal mass, distribution and magnetic field and fundametally determine to understand the characteristics of asteroids in general and their relationship to meteorites and comets and the conditions of the early solar system.
NEAR Shoemaker was equipped with an X-ray/gamma ray spectrometer, a near-infrared imaging spectrograph, a multi-spectral camera fitted with a CCD imaging detector, a laser rangefinder, and a magnetometer.
NEAR Shoemaker was launched february 17th 1996.
Its studied the S-type asteroid, Eros. (see image at top of this page.) Eros measures 13x13x33km in size, its the second largest Near-Earth Asteroid (NEA).
When NEAR Shoemaker began to exhaust its supply of nuclear fuel, the scientists at mission control decided to try and crashland the craft to get a closer view of the surface.
So, on february 12th 2001, it touched down, undamaged and operational after landing, transmitting 69 close-up images of the surface.
The final unsuccessful attempt to communicate with the craft was made on december 10th 2002.

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