The asteroid belt lies between the orbits of Mars and Jupiter; it is here most asteroids are located. The closest approach between Earth and Mars is about 35 million miles, which is more than 1,500 times the distance between Earth and Moon. So, traveling beyond the orbit of Mars for a mining expedition would seem unfeasible—at least at first. Asteroids nearest the Earth (and in the metal-rich inner belt) would seem the ideal candidates for Man’s asteroid-mining initiative.
Technically, closeby asteroids (those that come within 1.3 astronomical units of the Sun) are categorized as Near Earth Objects (NEOs). NASA has been cataloguing asteroids that fall into this category since 1980. The objective of the search is to identify asteroids that pose the greatest risk to Earth. Of the 6,500 that have been identified so far, about 2,300 of them have a diameter greater than 300 meters. 800 have a diameter of one kilometer or greater.
On July 3, 2006 asteroid 2004 XP14, measuring 500 meters, came within 400,000 kilometers of our planet. Asteroids 2009DD45 and 2008TN166 approached Earth in 2009. The 30 meters wide 2009DD45 came within 70,000 kilometers of Earth. Asteroid Apophis, measuring 350 meters, is to approach Earth at a distance of about 36,000 kilometers in 2029.