  In Coulomb drag propulsion, a long high voltage tether or system of tethers
gathers momentum from a natural plasma stream such as solar wind or ionospheric
plasma ram flow. A positively polarised tether in the solar wind can be used
for efficient general-purpose interplanetary propellantless propulsion (the
electric solar wind sail or E-sail), whereas a negatively polarised tether in
LEO can be used for efficient deorbiting of satellites (the plasma brake).
Aalto-1 is a 3-U cubesat to be launched in May 2016. The satellite carries
three scientific experiments including 100 m long Coulomb drag tether
experiment. The tether is made of four 25 and 50 micrometre diameter aluminium
wires that are ultrasonically bonded together every few centimetre intervals.
The tether can be charged by an onboard voltage source up to one kilovolt
positive and negative. The Coulomb drag is measured by monitoring the spin
rate.
