The Propulsive Small Expendable Deployer system (ProSEDS) is a tether-based propulsion experiment that draws power from the space environment around Earth, allowing the transfer of energy from the Earth to the spacecraft. ProSEDS technology has the potential to turn orbiting, in-space tethers into "space tugboats" -- replacing heavy, costly, traditional chemical propulsion and enabling a variety of space-based missions, such as the fuel-free raising and lowering of satellite orbits. 

ProSEDS will deploy from a Delta-II second stage a 5 km (2.7 nmi) long, ultra-thin bare-wire tether connected with a 10 km (5.4 nmi) long non-conducting tether made of Kevlar and Spectra. The interaction of the bare-wire tether with the Earth's ionosphere will produce thrust, lowering the altitude of the stage. Normally, after a satellite is injected into orbit, the second stage would be slowly pulled back from 400 km (216 nmi) to Earth by atmospheric drag. After 120 days, it re-enters and burns up. ProSEDS might do that in about 15 days.

An electrodynamic tether upper stage could be built using the propulsive tether technology to transfer satellites from a launch vehicle in low orbit to orbits of about 1,500 km (810 nmi). A propulsive tether would weigh about 90 kg (200 lbm). In turn, it would eliminate the need to haul up to 4,000 kg (8,800 lbm) of chemical propellants to the station. Atmospheric drag on the station will be about 0.3 to 1.1 newton (depending on time of year), and the tether could produce 0.5 to 0.8 newton of thrust. Operated the other way around, a tether powered by solar cells could boost a satellite's orbit and keep it from re-entering. The International Space Station (ISS) could use "bare wire" tethers to raise its altitude a few meters every day. 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.

NASA's industry team for ProSEDS includes the University of Michigan, Ann Arbor; Alpha Technologies, Huntsville, Ala.; Electric Propulsion Laboratory, Monument, Colo.; Smithsonian Astrophysical Observatory, Cambridge, Mass.; Tether Applications Inc., Chula Vista, Calif.; and Triton Systems Inc., Chelmsford, Mass. ProSEDS is managed by the Space Transportation Directorate at NASA's Marshall Space Flight Center.