From Planetary Resources:
There are near-limitless numbers of asteroids and more being discovered every year. More than 1,500 are as easy to reach as the Moon and are in similar orbits as Earth. Asteroids are filled with precious resources, everything from water to platinum. Harnessing valuable minerals from a practically infinite source will provide stability on Earth, increase humanity’s prosperity, and help establish and maintain human presence in space.
Asteroids are the low-hanging fruit of the Solar System. There are close to 9,000 near-Earth asteroids, and nearly 1,000 more are discovered every year.