| There are two types of energy at play here: the gravitational potential energy and the kinetic energy of the orbiting object (necessary to maintain an orbit). As you climb the cable, the force of gravity pulling you back to earth decreases, and the centrifugal force pulling you away from earth increases. The difference between these two is the force you need to provide to climb the cable. I believe you are correct for tethers much shorter than geosynchronous orbit. Below geosynchronous orbit, the force of gravity is higher than the centrifugal force. Therefore, an object climbing a space elevator will have to provide energy equal to the integral of the difference between the centrifugal force and the gravitational force across the distance traveled. The remaining energy (the remaining gravitational potential and the kinetic energy of the orbit) will be leeched from the orbiting counterweight (requiring the counterweight to have a rocket to maintain orbit, as you suggested) For tethers that extend beyond geosynchronous orbit, it is possible to for no energy to be removed from the counterweight (instead, all the non-climbing energy will be taken from the rotation of the earth). Imagine that we place a counterweight on a tether beyond geosynchronous orbit. This counterweight and the earth it form an orbiting two body system. The tether will be under tension (the force necessary to keep the counterweight in synchronous orbit) -- let's call that force T. A climber that scales the tether will exert some force T_1 on the counterweight, pulling it towards the earth. However, as long as T_1 is less than T, the counterweight will remain where it is. The force of the table on the earth will become T_2 = T - T_1. In other words, a portion of the force necessary to keep the counterweight in orbit will now be applied by the climber instead of by earth. The energy that the climber must apply is the same as before, but the counterweight is not affected. The remaining energy, by process of elimination, must come from the rotation of the earth. Geosynchronous orbit is 42 km from the center of the earth while the ISS orbits 7k km from the center of the earth. I expect the experiments are being done at the ISS for convenience rather than from a plan to build a space elevator to the ISS. The article also cites speed and distance numbers that imply reaching a geosynchronous orbit. To answer your questions more directly: 1) below geosynchronous orbit, yes. 2) No, the ability to extract energy from the earth's rotation is the main advantage. 3) Yes, but for a counterweight beyond geosynchronous orbit, the tension on the tether will pull it vertical. |