The NASA's space tether experiment in 2001,called for a scientific payload--a large, spherical satellite--to be deployed from the US space shuttle at the end of a conducting cable (tether) 20 km long. The idea was to let the shuttle drag the tether across the Earth's magnetic field, producing one part of a dynamo circuit. The return current, from the shuttle to the payload, would flow in the Earth's ionosphere, which also conducted electricity. The experiment, unfortunately, couldn't be realized at that time.
With the present technology, is it possible to put this into practical use? How much energy can be harnessed on the average from such tethered system typically? Could it be scaled up for gradually reducing the orbital velocity of spacecraft in orbit before reentry?