Applying the Plasma Brake for Active Debris Removal: revolutionarily high downmass/upmass ratio

The Plasma Brake is a propellantless method for deorbiting objects from Low Earth Orbit (LEO). It is based on charging a long and thin metallic tether (a microtether) to high negative voltage (1 kV) so that the electric field around the tether scatters ionospheric ions of the satellite's ram flow, thus braking the orbital motion. The microtether is made of three or four max 50 micrometre thin aluminium wires. It weighs only 23 grams per kilometre and does not pose a risk to other space assets due to its thinness. The tether is deployed initially to about 100 m length by a spring, and then Earth's gravity gradient affecting the reel unit itself deploys the full 5 km length.

A 4 kg Plasma Brake module has been developed in ESA Dragliner project at TRL 4-5, providing nominally 0.4 mN deorbiting thrust, which is enough to deorbit a 250 kg object from 700 km orbit in 2 years. The deorbiting thrust depends on the prevailing ionospheric plasma density and mean ion mass, but the quoted numbers represent average solar cycle conditions and typical 600-800 km starting altitudes. Up to two modules can be installed in a satellite for its end-of-life deorbiting, one deploying its tether downward and the other one upward. In general the Plasma Brake has to lower the altitude only to about 400 km, after which atmospheric drag on the tether and the object itself complete the deorbiting.

Here we consider application of Plasma Braket technology to Active Debris Removal, i.e., deorbiting of old, uncollaborative debris objects. We have a mothership that has a Hall thruster to make a tour of several debris objects.  The mothership attaches a Plasma Brake module to each debris object. The attachment is by a mechanical device such as a flat tape ribbon loop which is opened to e.g. 5 m diameter from a reel by an electric motor. The loop is then passed around the object and slowly tightened by the motor, after which the mothership detaches the Plasma Brake unit and departs to rendezvous the next debris object. Meanwhile the Plasma Brake module opens its tether, first by a spring to about 100 m length and then by Earth's gravity gradient to the full 5 km length. Then it turns on the 1 kV negative voltage on the tether, for which it needs about 1 W of power which it gets from its own cubesat-like surface-mounted solar panels.

Because a 4 kg Plasma Brake module can deorbit a 250 kg mass, if the modules comprise 30% of the launch mass of the mothership, overall we reach a downmass/upmass ratio of nearly 20. This is revolutionary because the current state of the art (chemical propulsion or electric propulsion) yield downmass/upmass ratios not much larger than unity.