Coulomb drag devices: electric solar wind sail propulsion and   ionospheric deorbiting
  A charged tether or wire experiences Coulomb drag when inserted into flowing plasma. In the solar wind the Coulomb drag can be utilised as efficient propellantless interplanetary propulsion as the electric solar wind sail (electric sail, E-sail). In low Earth orbit (LEO) the same plasma physical effect can be utilised for efficient low-thrust deorbiting of space debris objects (the plasma brake). The E-sail is rotationally stabilised while the deorbiting Coulomb drag devices   According to numerical estimates, Coulomb drag devices have very promising performance figures, both for interplanetary propulsion and for deorbiting in LEO. Much of the technology is common to both applications. E-sail technology development was carried out in ESAIL FP7 project (2011-2013) which achieved TRL 4-5 for key hardware components that can enable 1 N class interplanetary E-sail weighing less than 200 kg. The thrust of the E-sail scales as inverse solar distance and its power consumption (nominally 700 W/N at 1 au) scales as the inverse distance squared. As part of the ESAIL project, a continuous 1 km sample of E-sail tether was produced by an automatic and scalable "tether factory". The manufacturing method uses ultrasonic wire to wire bonding which was developed from ordinary wire to plate bonding for the E-sail purpose. Also a "Remote Unit" device which takes care of deployment and spin rate control was prototyped and successfully environmentally tested. Our Remote Unit prototype is operable in the solar distance range of 0.9-4 au.   The 1-U CubeSat ESTCube-1 was launched in May 2013 and it will try to measure the Coulomb drag acting on a 10 m long tether in LEO when charged to 500 V positive or negative. A more advanced version of the experiment with 100 m tether is under preparation and will be launched in 2015 with the Aalto-1 3-U CubeSat to polar LEO. 
Pekka Janhunen
astro-ph.IM