�Asteroids: Our Minor Planets�

Term Paper Title	�Asteroids: Our Minor Planets�
# of Words	1409
# of Pages (250 words per page double spaced)	5.64

�Asteroids: Our Minor Planets�

Earth is one sole object in the universe, which everyone is familiar with.  Stretching beyond this planet are several other planets in the solar system and several thousand other celestial objects also known to revolve around the sun.  These minor objects are called asteroids.
Asteroids are metallic, rocky objects without atmospheres that orbit the Sun.  Even though they orbit the sun, they are too small to be classified as planets, so they are often referred to as minor planets or sometimes as planetoids.  Tens of thousands of these minor planets come together in a region of the asteroid belt.  The asteroid belt is a vast, doughnut-shaped ring located between the orbits of Mars and Jupiter from approximately 2 to 4 A.U. from the Sun.  They are given a number and sometimes then named when their orbits are well established.
European astronomers discovered the first asteroids early in the 19th century as they searched for an additional planet in the region between Mars and Jupiter, where the Titius-Bode �law� suggested one [planet] might be found.  Italian astronomer Giuseppe Piazzi was the first to discover an asteroid.  He detected Ceres in 1801 and measured its orbital semi-major axis to be 2.8 A.U. exactly as the �law� predicted.  (Chaisson and McMillan 298)
Asteroids are thought to be primitive material prevented by Jupiter�s strong gravity from becoming a planet-sized body when the solar system was born about four and a half billion years ago.  Known asteroids range in size from the largest � Ceres, the first asteroid discovered in 1801 � at about 600 miles in diameter down to the size of pebbles.  Sixteen asteroids have diameters of 150 miles or greater.  They [asteroids] have been found inside Earth�s orbit to beyond Saturn�s orbit.  Most however, are contained within a main belt that exists between the orbits of Mars and Jupiter.  Some have orbits that cross Earth�s path and some have even hit the Earth in times past.  One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona.  The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.
Asteroids are material left over from the formation of the solar system.  One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago.  More likely, asteroids are material that never coalesced into a planet.  In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would less than half the diameter of our Moon, about 932 miles across.
Our understanding of asteroids has been derived from three main sources: Earth-based remote sensing, data from the Galileo flybys, and laboratory analysis of meteorites.  Asteroids are classified into different types according to their sizes, shapes, and compositions.
Asteroids are classified in terms of their spectroscopic properties.  The darkest or at least reflective asteroids contain a large fraction of carbon in their makeup.  They are known as C-type (or carbonaceous) asteroids.  The more reflective S-type asteroids contain silicate, or rocky, material.  Generally speaking, S-type asteroids predominate in the inner portions of the asteroid belt, and the fraction of C-type bodies steadily increases as we move outward.  Overall, about 15 percent of all asteroids are S-type, 75 percent are C-type, and 10 percent are other types (such as those containing large fractions of iron [M-type, or metallic asteroids]).  (Chaisson and McMillan 303)
Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth.  Asteroids that are on a collision course with Earth are called meteoroids.  When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor.  If the meteoroid does not burn up completely, what�s left strikes Earth...

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