<b> I don't think these resources will be mined before the earth is depleted (and afterwards it will be too late)</b><br /><br />That's the statement I disagree with. What exactly do you mean by "earth is depleted"? Many people glibly talk about "earth resources running out," but except for fossil fuels all "earth resources" are <b>reusable</b>. Do you know that US does not mine iron any more? It is all recycled. Most of the iron in your car was originally mined for WWII. Likewise with copper, tin, silver, and pretty much any large-scale use metal. Modern civilization needs LESS of these metals, not MORE -- and what it does need has already been mined out, smelted, scrapped, and only needs to be re-melted. Platinum, gold, and other precious metals are being mined because demand for them remains high, but it will be VERY long time before they are really "depleted" -- and by that time extracting them from asteroids will become more attractive.<br /><br />The only things we truly <i>use up</i> are non-reusable energy sources -- fossil fuels and uranium. But those (the former, anyway) have many viable alternatives, and will soon have to be phased out anyway. And (solar) energy is exactly one thing which IS profitable to bring from space -- or soon will be.<br /><br />So before you toss out phrases like "earth resources will be depleted," think: Which resources? And are they really getting depleted, or just reused over and over?<br /><br />I suppose I was unclear in my previous post. The only "resource" truly beyond Earth capabilities which is necessary for interstellar travel is ENERGY. And while I do not ever expect anyone to export iron or magnesium from asteroids to Earth, ENERGY is one thing I do expect to be so exported. Think of a solar panel 1000 km on the side, orbiting Sun as close as Mercury. THAT could power a starship -- and long before such use is needed, could power the increasingly energy-hungry Earth.