Steve Schear wrote:
My preference is the space elevator. In simple terms, the space elevator is a ribbon with one end attached to the Earth's surface and the other end in space beyond geosynchronous orbit (35,800 km altitude). The competing forces of gravity at the lower end, and outward centripetal acceleration at the farther end, keep the ribbon under tension and stationary over a single position on Earth. This ribbon, once deployed, can be ascended by mechanical means to Earth orbit. If a climber proceeds to the far end of the ribbon and releases, it would have sufficient energy to escape from Earth's gravity and travel to the Moon, Mars, Venus and the asteroids.
http://www.highliftsystems.com/
"Reality must take precedence over public relations, for nature cannot be fooled." -- Richard P. Feynman
It's a nice idea, but it needs a tensile-strength-to-mass ratio equivalent to holding a girl and her mother up by a single thread of her 10 denier stockings. Not easy to achieve. You'd need carbon nanotubes or the like, and at the moment we can't build it. You also need 45,000 km or so of tether. Expensive. Huge investment, fragile. Unrealistic, imo. Rotating tethers on the other hand can use hi-test fishing line. Really, no kidding. You only need a few hundred km, or at most a few thousand km, of tether. Cheap. There are two types, landing takeup and hypersonic takeup. They work a bit like this (here goes a try at some ascii art...) [] orbiting mass--> \ \ rotating tether \ \ <-\ space -------------------- atmosphere ________________________ earth (on this scale a space elevator cable would be roughly six feet long) The tether, whose centre of gravity is in a fairly low orbit, dips it's end into the earth's atmosphere every so often. Hypersonic takeup tethers catch a 'plane flying at hypersonic speeds in the upper atmosphere, and landing takeup tethers reach the surface. The energy/momentum is replaced by sending current through the tether as it passes through the Earth's magnetic field. Hypersonic takeup tethers are better studied, even the rendezvous techniques apparently work, and can use fishing line except for the short length that enters the upper atmosphere (it would melt). They use a mesh-like tether structure to avoid catastrophic damage from meteorites etc (a patented, but IMO obvious, idea). Landing tethers sort of cast the line a bit ahead, like a fisherman; it hits the ground, is tied on to the spaceship (good knots!) and then the line and the spaceship are dragged up. No-one really has studied them much (except me, and I'm not telling yet), but the strength (and length) of line needed is _much_ (order of mag+) less than a space elevator. And you don't need a hypersonic 'plane. You can also fling things away from the tether when they're going away from the Earth. Can get any (reasonable) speed you like. -- Peter Fairbrother