The LINEAR system alone had discovered 121,346 asteroids by March 2011. Among all the automated systems, 4711 near-Earth asteroids, (having orbits that pass close to Earth), had been discovered, including >600 being >1km wide. As of May 2010, 7,075 near-Earth asteroids were known, ~500-1,000 being >1km, [~ means approximately, > means greater than].
The main asteroid belt has 1.1-1.9 million asteroids >1km (>5 city blocks), & millions smaller, orbiting between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (not very elongated) orbits. "Trojan" clusters of asteroids orbit around the L3 & L4 Lagrangian stability points 60deg before and after a planet in its orbit.
Many asteroids are in the two Torjan Asteroids clusters spread from about 30-90deg before and after a planet in its orbit. The biggest population of Trojan asteroids are the Jupiter Trojans. Although fewer Jupiter Trojans had been discovered by 2010, it was thought that they are as numerous as the asteroids in the main asteroid belt in between Mars' & Jupiter's orbits.
>300,000 asteroids had by 2010 been identified and cataloged; >1,000,000 are expected to exist in the main asteroid belt between Mars and Jupiter. The Kuiper belt beyond Neptune has >100,000 asteroids >100km size. And there's the even much farther out Oort cloud, both also having many more smaller ones.
Most main belt asteroids have semi-major axes (mean distances to Sol) 2.2 - 3.2 astronomical units (1 AU = avg. distance from Earth to the Sun = 1.496×10^8 km, where e8 or *10^8 means times 10 raised to exponent power 8). >1,200 asteroids >~150m wide, bigger than a sports field park, have been identified that regularly pass near the orbit of Earth at distances of <=.5 AU.
Asteroids widths & #s: >100m ~ 25,000,000, (200m~city block), >300m ~ 4,000,000, >500m ~ 2,000,000, >1km ~ 750,000, >3km ~ 200,000, >5km ~ 90,000, >10km ~ 10,000, >30km ~ 1,100, >50km ~ 600, >100km ~ 200, >200km ~ 30, >300km = 5, >500km = 3, >900km = 1.
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