>>9
The space fountain concept is a proposed form of space elevator that does not require the structure to be in geosynchronous orbit, and does not rely on tensile strength for support. In contrast to the original space elevator design (a tethered satellite), a space fountain is a tremendously tall tower extending up from the ground. Since such a tall tower could not support its own weight using traditional materials, massive pellets are projected upward from the bottom of the tower and redirected back down once they reach the top, so that the force of redirection holds the top of the tower aloft. Satellite payloads ascend or descend by coupling with this stream of pellets or by climbing up the side of the tower. The space fountain has several key advantages over a space elevator in that it doesn't require materials with extreme strength, can be located at any point on a planet's surface instead of just the equator, and can be raised to any height required. Its major disadvantage is that it is an active structure, and so requires constant power input to remain aloft.
[wiki]
http://www.strangehorizons.com/2003/20030714/orbital_railroads.shtml
harvesting uranium would be enough to offset the cost of mining other useful minerals an asteroid may contain.. sure, calculational error could cause a disaster, but if minerals are mined on the asteroid itself, mass can be easily calculated securely. once we got an elevator or fountain, the energy cost would be minimal compared to the net gain of mining asteroids