When you think about it, it is remarkable that so many asteroids pass within 5 to 10 million miles of earth. The earth’s orbital radius is 93 million miles. The earth orbits the sun in a plane, so any asteroid coming close to earth has to pass through our orbital plane AND be at the location where the earth is in its orbit. Almost all the bodies in the solar system travel in roughly the same orbital plane. Those with highly elliptical orbits, hyperbolic orbits, or highly inclined orbits can intersect the earth’s path.
It's because over the eons, the Earth (with the moon) have plowed away most of the rubbish in its path. That effect is part of the definition of a planet; something Pluto may not have.
Yeah, it's almost like there were literally thousand or even millions of those things out there! But that could't be true, could it?
Regards,
P.S.
1. A hyperbolic orbit is no orbit at all; it's a one-time encounter. Same for a parabolic trajectory.
2. As far as how many asteroids there are… well, if you only consider those larger than 100 meters orbiting within the inner Solar System, there's over 150 million. Count smaller ones and you get even more. - Universe Today, Posted on September 25, 2012 by Jason Major