The asteroides belt
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The asteroid belt is usually listed next, and I will adhere to tradition. However, it is very important to note that while a great many of the asteroids are neatly orbiting where they are drawn between Mars and Jupiter, a great many are elsewhere in the solar system, those inside Earth's orbit, those that cross it, the entire stretch between Mars and Jupiter, and many more out beyond. In fact, the most important ones, both in terms of possible resources and in danger to Earth, are the 2000 or so that periodically cross our orbit. In general, the asteroids were formed by unknown and likely varied processes very different than those which formed the planets. In size, most are quite small, with the total mass likely to be less than that of the Moon. They are, however, possibly the best source for raw materials. In the case of the planets, their molten pasts allowed most of the heavy elements to separate out and sink towards the center, thus making them unobtainable to us by any technology yet discovered. For asteroids, this process never occurred. Thus, it is quite possible to find very rich ores of most metals, while at the same time offering a decent quantity of the lighter elements that make up living matter.
However, some scintifics think it is possible that the asteroides were two planets which have destroyed themselves. Probably they were too close one of the other one...
