Space exploration is back. After decades of disappointment, a combination of better technology, falling costs and a rush of competitive energy from the private sector has put space travel front and center. Many analysts (even some with their feet on the ground) believe that commercial developments in the space industry may be on the cusp of starting the largest resource rush in history: mining on the Moon, Mars and asteroids. Whether this proves to be the dawn of a gigantic adjunct to mining on earth — and more immediately, a key to unlocking cost-effective space travel — will turn on the answers to a host of questions ranging from what resources can be efficiently. As every fan of science fiction knows, the resources of the solar system appear virtually unlimited compared to those on Earth. Billionaires including Jeff Bezos and Elon Musk imagine heavy industry moving to space and Earth becoming a residential area.