Drag Compensation for Space Stations
For a spacecraft in a very Low Earth Orbit (LEO), the atmospheric drag is sufficiently strong to cause a re-entry before the intended end of mission if orbit raising maneuvers are not executed in a timely manner. A typical example of this is the International Space Station (ISS) which has an operational altitude above Earth surface of between 330 and 410 km. ISS Aerodynamic Drag Force is between 0.3 N and 0.8 N. Due to atmospheric drag, the space station is constantly losing orbital energy. In order to compensate for this loss, which would eventually lead to a re-entry of the station, it is from time to time being “re-boosted” to a higher orbit. The chosen orbital altitude is a trade-off between time required between re-boost maneuvers and the increased cost of delivering payloads and people to a higher altitude. The upper limitation of orbital altitude is imposed by the capability of the Soyuz spacecraft. Annually, NASA and the Russian Space Agency spend between 5 and 12 t of propellant in order to keep the station on the intended orbit, using chemical boosters ( L. Johnson, M. Herrmann “ISS Electrodynamic Tether Reboost Study”, NASA/TM-1998-208538). SEP-VASIMR® system would reduce the propellant use for ISS or future private space stations drag compensation, because of using higher specific impulse propulsion, by 15 times.