Even smaller space rocks can have mineral prizes worth tens of trillions of dollars. The smallest known metallic asteroid that is an accessible near-Earth object has 40 times as much metal as all the metal in Earth's history, Lewis pointed out. He has joined Planetary Resources as perhaps the most recognized expert on asteroid wealth.
Knowing what asteroid wealth consists of depends on incomplete but enticing scientific surveys. Scientists sitting on Earth can detect chemical signatures of asteroids based on reflected light, or directly sample space rocks fallen to Earth as meteoroids. Japan has carried out the only successful space mission to retrieve asteroid samples in space, but the U.S. is planning its own asteroid sample and retrieval missions.
An M-class asteroid about 79-feet (24-meter) long could have as much as 33,000 tons of extractable metal and possibly one ton of platinum group metals. The platinum alone could be easily worth about $50 million dollars in Earth's commodity markets, according to studies cited by the paper "Assessment on the feasibility of future shepherding of asteroid resources" in the April-May issue of the journal Acta Astronautica.