Together, all of the asteroids mass only about one hundredth of the Earth. Yet the availability of those asteroidal resources is such that they could support a population thousands (perhaps millions) of times greater than our home planet. On Earth, we can only mine the outer few miles of crust, and drill only a little deeper than that. But asteroids, with nearly no gravity and no internal magma, are entirely accessible to our technologies.
Asteroids mostly reside in a wide range of orbits between Mars and Jupiter. There are an estimated 1,000,000 asteroids in the main belt that are at least one kilometer (0.6 miles) in diameter. Most of these are rich, carbonaceous chondrites, full of the stuff of life. Perhaps 5% are nickel-iron. But not all of the asteroids are there. Millions more reside in the Lagrange points in Jupiter's orbit. Thousands more orbit inside of Mars' orbit, some inside Earth's orbit. Many of these have highly inclined or eccentric orbits that would be difficult to reach. But not all.
Rather, our first choices should be Earth-crossing asteroids, the same asteroids that NASA is cataloging because they pose a potential threat to Earth. Given proper timing, some of these are even easier to reach than the moon. And these Earth-orbit-crossing potentially hazardous asteroids (PHAs) are, perhaps unfortunately, plentiful. Over a thousand are known whose diameter exceeds a kilometer (six tenths of a mile). There are likely 10,000 or more PHAs that are 300 meters (1,000 feet) in diameter; these are large enough to destroy entire states or countries. We believe there are 100,000 or more PHAs that exceed 100 meters (330 feet) in diameter, large enough to destroy a city. And there are millions in the 30+ meter (100+ foot) diameter range. A typical 50-meter diameter asteroid is a solid body massing over 200,000 tons, of which as much as 40,000 tons is iron-nickel alloy. Anything smaller poses no danger to the Earth, as they can't penetrate the atmosphere intact. Yet they can still contain valuable resources. A tiny, ten-meter (33 foot) diameter asteroid, (which may number in the tens of millions) could have a 1,600 ton mass, including 160 tons of water, 300 tons of iron-nickel metal grains, even 80 kilograms of platinum group metals worth $4M or more.
A particularly exciting option is the possibility of capturing an asteroid into Earth orbit. A captured asteroid puts vast resources into easy reach, protecting the earth from a future impact at the same time. Even a small Earth-crossing asteroid such as Apophis (only 270 meters wide) contains twenty-seven million tons of material, potentially including millions of tons of water, iron, carbon, nitrogen and other materials valuable to life in outer space. In addition to raw materials, an asteroid provides protection. Ten or fifteen feet of asteroidal rock will provide the same degree of protection from meteors, solar flares, and cosmic rays as our atmosphere on Earth.
Instead of worrying about how to deflect asteroids away from the Earth, we should think about how to place them into nice, safe, useful Earth orbits from which they could never again impact our planet.
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