In the late 1700's a German astronomer named Titius predicted that a planet should exist between the orbits of Mars and Jupiter. On January 1, 1801 (the very first day of the 19th century), Guiseppe Piazzi discovered a new object that moved in relation to the background stars. Further observations showed that this object was exactly where the missing planet should be. Piazzi's object was named Ceres. At this point the solar system had 8 planets (Neptune was not discovered until 1846). A little over a year later, a second object was discovered with an orbit very similar to Ceres'. This second object was named Pallas. Now the solar system had 9 planets. By 1851 there were 15 known asteroids (asteroid means "star-like;" because asteroids are so small, a small telescope cannot resolve them as disks, so they appear like faint stars that move slowly across the sky). These objects were unlike the other planets that had been observed because they were very small and they orbited relatively close to each other in space. Astronomers decided to ``demote'' these smaller objects from planet-status and call them ``minor planets'' (sound familiar?). For a more detailed history, go here.
Astronomers initially believed the asteroids were pieces of a planet that had somehow disintegrated, but this is no longer considered the best explanation for their existence.Since then we have discovered thousands
of asteroids, ranging in size from 100s of kilometers down to 10s of
meters.
Most asteroids in the solar system have orbits around the sun which
keep
them between Mars and Jupiter. The asteroid belt lies roughly 2-4 AU
from
the sun. The total mass of the asteroids is 2.3e21 kg, roughly 2500
times
less than the mass of the Earth (about 30 times less than the mass of
the
Moon).
Planetary scientists now believe that the asteroids could have formed
another
(very small) planet
in the solar system, but gravitational perturbations from Jupiter
prevented
the asteroids from getting together. Thus, asteroids are leftover
planetesimals and represent the same
material that went into building the planets.
In contrast, iron and stony-iron
meteorites probably come from an asteroid that looks like this:
To see how congested the solar system
in
the area around the Earth is, check out:
Animations
from the Minor Planet Center
Figures
from Armagh Observatory
Different problems arise, depending on whether the asteroid crashes into land or water. So what can be done? This is still an area of research and debate. Blowing up a potential impactor as illustrated in Armageddon (bad, bad, bad movie) probably is not the best idea. We may be able to "nudge" it with different types of bombs, to push it into a non-threatening orbit may or maybe nothing will really work. The feasibility of the different strategies that have been proposed depends strongly on how early we detect the impactor.