Half a million asteroids

Look at them, up there, dancing around and taunting us beyond our steep little gravity well...

Video link, courtesy of Bad Astronomy.

Yeah, I saw that video, linked from Freedom's Phoenix a couple of days ago. Pretty spiffy.

What's wild is, despite there being so many rocks out there, when you're in the Belt, you hardly see any of them. It's not the crowded field of hurtling boulders they show in the movies.

Because Space is big. Really, really big.

Yah.  It's maybe a tenth of Earth's mass, spread in a really wide orbital torus.  But it's almost all accessible, unlike well over 99% of Earth's mass.  (Ask them guys down in Chile how accessible resources are on Earth).

Much less than a tenth - more like 1/2000 (total mass of the main belt is about 3 x10²¹ kg, of which nearly 1/3 is Ceres, whereas the Earth masses 6 x 10²⁴ kg). 

However you're right about the accessibility, particularly of metals; most of the metal in the Earth has long ago sunk to the core, and we only have access to the upper bits of the crust.  And most of the deposits of many metals there came from asteroids, according to some geologists.

If you have not done so, I strongly recommend that this be viewed in full-screen.