FORESAIL-1 and ESTCube-2 Cubesat experiments of Coulomb drag propulsion

Coulomb drag propulsion taps momentum from a natural plasma stream to generate propellantless low-thrust propulsion for a spacecraft. The plasma is attached to by means of a long, thin, charged metallic tether. The tether's electrostatic field deflects the motion of streaming plasma ions and transfers momentum from them. The technique can be applied in the solar wind (i.e., outside Earth's magnetosphere) to produce general-purpose interplanetar propulsion. This application is called the electric solar wind sail (E-sail). It can also be applied in low Earth orbit (LEO) to brake the satellite's orbital motion. Here the relevant plasma stream is the ram flow of the ionosphere due to the satellite's orbital motion. This application is called the plasma brake and it is useful for satellite deorbiting for mitigating the growing problem of orbital debris.

Here we report on progress of two CubeSat missions (FORESAIL-1 and ESTCube-2) that are under construction for measuring the Coulomb drag effect in LEO.  Both are scheduled to fly in 2022. Both satellites deploy up to 300 m long tether, charge it up by a high-voltage source and measure the resulting Coulomb drag. The satellites are slowly spinning and the tether is tightened by the centrifugal force. The tether is deployed from a reel which is rotated slowly by an electric motor.  Both satellites use negative tether polarity, which is the case relevant for the plasma brake. ESTCube-2 contains, in addition, a positive mode experiment which is relevant for the E-sail. The plasma environment in LEO differs from the solar wind, so the measured positive mode Coulomb drag must be scaled to yield a prediction of the strength of the E-sail effect in the solar wind.

The Coulomb drag is measured by two independent methods. In the first method we set the tether voltage on and off in sync with the satellite's rotation and thereby accumulate a change of the system's angular momentum. The Coulomb drag is inferred from the measured change of the spin rate per time unit. In the second method we estimate Coulomb drag from the speeded-up lowering of the satelllite's orbital altitude.

The presentation is a status report of the technical progress of these two Coulomb drag CubeSat missions; FORESAIL-1 and ESTCube-2.