Johnson, Stone, and Gallagher want to do that in about 15 days. Their plan is for the tether to increase drag by acting as an electrical generator to power batteries on the ProSEDS telemetry package. Among other things, the ProSEDS team wants to study the tether's behavior to see whether the differences between night and day would make the tether swing like a pendulum out of control. This would make up for atmospheric drag and save several hundred million dollars a year in the cost of carrying up chemical propellants for rocket engines. Extra science payloads could be carried instead.
Electrical engines provide more thrust per pound of propellant than chemical engines.
Since a propulsive tether expends no propellant, its efficiency will be measured in its cost compared to rockets. And that can be as little as 8 percent the cost of chemical rockets. The sky is not quite the limit on propulsive tethers. The technique requires an ionosphere, a region of electrified gas which acts as part of the electrical circuit. The solar plasma and magnetic field of the sun are too weak for interplanetary voyages.