Formation
It is believed that asteroids formed in the same manner as the rest of the solid bodies in our solar system - during the collapse of the solar nebula - with the majority arising in the area between Mars and Jupiter.
Here, it is theorized, another planet was forming in orbit around our Sun. However, as Jupiter grew in mass, the gravitational pull on the forming world caused it break apart.
Many of the asteroids, specifically the larger ones, were ejected from the orbit. Computer models suggest that as little as 1% of the original mass remained, with the two largest chunks - Ceres and Vesta - absorbing some of the remaining material and cooling into nearly spherical orbs.
Classification
Asteroids are usually classified by one of two methods: either their orbital mechanics or their composition (determined from their reflectance spectrum).
The problem has generally been that the spectral classification has proven unreliable. This has led, over time, to several different spectral rubrics. Hence there doesn't seem to be general agreement on this issue.
Therefore it is more common to refer to asteroids by their orbital characteristics, specifically where they orbit in our solar system. The major groups of asteroids include those orbiting in the asteroid belt, the Trojan asteroids and the Apollo asteroids.
The Asteroid belt
Perhaps the origin of virtually all the asteroids in our solar system, the asteroid belt contains millions individual objects, though the vast majority of these range in size from small rocks to the size of cars. However, there are estimated to be up to 2 million "large" asteroids - those with diameter greater than a kilometer (0.62 miles).
Trojan Asteroids
Trojan asteroids are those characterized by orbiting the Sun along the same path as one of the planets.
The most famous grouping of Trojan asteroids are those that lead and follow the planet Jupiter.
Located at the lagrange points,60 degrees ahead and behind the body in question, the orbits of the asteroids remain in relative position to the planet; orbiting at the same speed around the Sun.
While it could be possible for Trojan asteroids to orbit around the inner planets, only Mars' orbit has been found to contain them. This perhaps isn't surprising as the population probably arose from gravitationally captured asteroids emerging from the belt.
It is also possible that Trojans exist in the orbits of the outer gas giants, though this has not been confirmed.
Apollo Asteroids and Near-Earth Asteroids
It seems like there is a near-constant barrage of news reports dealing with asteroids or comets that are heading towards Earth. The media seems intent on striking fear into the hearts of average citizens by playing off the hollywood doomsday scenarios.
The reality is much less exciting. There are actually thousands of asteroids that find their way near Earth on a near constant basis. The ones that actually cross Earth's orbit are known as Apollo asteroids.
The rest simply have orbits around the Sun that occasionally will bring them near to the Earth. These asteroids are monitored, quite closely, by NASA researchers. And while occasionally they find that one of them could potentially impact Earth (usually decades in the future), it usually turns out that it won't happen.
But even if one of these objects were to impact the Earth, it wouldn't be for some time in the future. And most of them are two small to actually cause much damage at all - unless of course one actually falls on your house. Which isn't likely.
Asteroid Discoveries
The first asteroid to be discovered was Ceres; a large example of the species found orbiting in the asteroid belt.
Astronomer Giuseppe Piazzi found the roughly 600 mile diameter asteroid in 1801. Ceres is the largest asteroid discovered to date and is the only object to be classified as a dwarf planet in the inner solar system. (The next closest dwarf planet to Earth is Pluto.)
Since asteroids are rather small objects, observing them was difficult. So discoveries were rare until the late 20th century.
At that time computers were being increasingly used to analyze images taken of the night sky. This allowed researchers to find faint objects moving against the deep space background.
Now advanced satellites like the WISE infrared observatory allow scientists to find asteroids more readily while they are still far away from Earth.
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