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The Best Near-Term Future of Space Exploration?
An anonymous reader writes "Much fanfare has been made about manned missions to moons and planets, but little has been done about travel to the asteroids — until now. NASA is working on plans for a trip to the asteroids by 2025. This type of mission has great potential for positive economic return based on the fact that no effort has to be spent on getting in and out of a distant planet's gravity well. Yes, we should go to the planets, but we should master mining the asteroid belt for resources first because it is easiest. What do you think?"
In fact, everything that we currently mine (copper, iron, zinc, platinum, gold, etc.) came from asteroid impacts.
Only in the sense that Earth is basically built of asteroids in the first place. But in that limit, you're just advocating mining on Earth again, the nearest and most habitable such body.
all those elements moved to the core, leaving only things like calcium and silicon and carbon in the Earth's crust when it cooled. All the useful elements came from asteroid impacts after that.
Good lord, no. Certainly elements did tend to head to the core preferentially. Such siderophilic (iron-loving) elements are fairly rare in the Earth's upper layers. Others are still fairly common. Or at least common enough. Even iron, which lead the charge to the core during differentiation, is awfully common in the crust.
In fact, silicon (the second most abundant element in the crust) is only about ten times more common than iron, which is about as abundant as calcium (which you cite as being abundant). Aluminum is more abundant than calcium and is in fact only a few times less abundant than silicon. (Oxygen, incidentally, is the most common element in the crust, beating silicon out by a factor of a few.) In fact, most metals we're particularly attached to are about one-in-ten-thousandth as common as silicon. If you factor in the fact that they're usually found in clumps, that's a very cheerful thought.
(For the record.)
By the way, if your theory of asteroid delivery were true, I'm pretty sure we wouldn't have very much metals to work with. The Earth's crust is tectonically recycled every several hundred million years (any given chunk has been subducted and recycled several times, more or less; we estimated this my first year of grad school, but I forget the numbers exactly), so you could only rely on the metals delivered in the past few hundred million years. Asteroid impacts are getting rarer all the time, especially big ones.
Also, recall that a given asteroid is as likely as much rock as metal. In fact, Earth is more metal per mass than the average asteroid. (A lot of our silicates ended up in the Moon instead.) However, some asteroids are definitely mostly metallic and for mining purposes, that's a mad bonus. (For metals raining down from heaven, however, you have to factor in the fraction of the asteroids that isn't metal.)
Also, you're not factoring in the costs of bringing metals back to the Earth (if that's your goal). It's far more expensive to do that than to mine them here and will be for the foreseeable future. Of course, if your goal is to use them in space anyway, then it might be better to mine them there. (On the other hand, then you have to build the refining and construction infrastructure in space, which has a lot of challenges of its own.)