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we expend far more than that on metal production already.
    To be more precise, it's about 60,000 megatons; and if need be, we can use hydrogen bombs. Put an H-bomb at the center of mass of an asteroid and light it off; I guarantee you that sucker will move. It's expensive, but not grossly so, assuming I have laser triggers for my H-bombs; only a few tons of hydrogen.
    I could also do it with fusion: at 10% efficiency I get 6.4 x 10 17 ergs per gram of hydrogen, and I need about 10 27 ergs total to move the rock; for an engine I use an ion engine, breaking up parts of the asteroid for reaction mass. What arrives is something less than I started with, but who cares? What I'll throw away as reaction mass is the slag from my refinery.
    (For those who haven't the foggiest notion of what I'm talking about: a rocket works by throwing something overboard. The reaction mass is what's thrown. Although the big space program rockets use gaseous exhaust as reaction mass, there's no reason you couldn't use dust, ground up rock, or slag from a metals refinery. It's all a question of whether you can throw it sternwards fast.)
    But that leads to another possibility: why not set up the refinery out at the Belt? Put up vast mirror systems and do the refining on the way in; use the slag as reaction mass to move the whole works, rock, refinery, and all. I can power that with solar mirrors. Or I can do all at once: use bombs for initial impetus, set up mirrors when I'm closer in, and while I'm at it run a hydrogen fusion plant aboard the moving strip-mine/refinery/spaceship I have created.
    At worst I have to carry about one Saturn rocket's worth of hydrogen, plus several shiploads of crew and other gear; and for that I get an entire year's worth of metals for the world. The value of my rock is somewhere near a trillion dollars once it's in Earth orbit; more than enough to pay for the space program and pay off the National Debt at the same time.
    So. For the price of some hydrogen and a rather complex ship system I've brought home enough metal to give everyone on Earth access to riches. If we do nothing else in space; if we come up with no new and startling processes such as I've described in other columns (and in other chapters of this book)—we'll have licked pollution and dwindling resources, thereby letting the developing countries industrialize, and thereby whipping the food production crisis for a while.
    We have avoided the fourth doom. And that's all of them.
    * * *
    Sure: there's a limit to growth. But with all of space to play with I'll be happy to leave the problem for my descendants of 10,000 years hence to worry about.
    I can hear the critics spluttering now. "But-but-but—what does this madman think he's doing? Flinging numbers like that around! Bringing in asteroids. It's absurd!"
    Really? Remember, we haul quantities like that around here on Earth even now; in trains, and on boats, and it takes far more energy to process them here than it would in space. To get that energy here we must burn fossil fuels—which are really far too valuable as chemicals to set a match to—and put up with the resulting pollution. And, after all, I've assumed that we're going to supply the whole world with metals at the rate that we produce them from all sources—including recycling—here at ground level US of A. What's so absurd about it?
    No, we won't be operating in space on this scale for a few years; but then we weren't producing all those tons of steel in 1930 either. Even the worst crunch models will not kill us off before 2020—a year in which we might very well be able to move asteroids around, boil them up for processing, and bring the resulting metals down for use here on Earth. It's a year in which we certainly will have Solar Power Satellites, always assuming that we want SPS. And there's approximately as much time between now and 1930 as now and 2020.
    Yes. We live on a finite Earth. But there's a whole solar system out there. If we