Astronomers think there are about half a million asteroids the size this one near Earth, NASA said, but less than one percent have been detected.

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Between 500, 000 to 1 million large asteroids near are near Earth and Mars.

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In 2014 it intends to launch, at a cost of a few million dollars, a set of small space telescopes whose purpose will be to seek out asteroids which are easy to get to and whose orbits return them to the vicinity of Earth often enough for the accumulated spoils of a mining operation to be downloaded at frequent intervals.

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