The reason for the renewed interest is the steady rise in the price of gold and other base metals during the past decade. Back in 1994, William Hartmann at the Planetary Science Institute in Tucson, Arizona, estimated that a 2-kilometre-wide asteroid would be worth $25 trillion in metal and mineral resources (see diagram). That's enough to pay off the US's $15 trillion national debt, use the loose change to settle up for Greece and still make the investors very rich indeed. "I don't see how you can look at any economic study of Earth and not think about the potential resources of the inner solar system," says Hartmann.