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An electric sail harnesses the solar wind for propulsion. Inside the magnetosphere of Earth, the solar wind does not reach. Yet the Aalto-1 satellite intends to deorbit using an electric sail. How can the electric sail be used for deorbiting when LEO orbits are mostly or entirely within the magnetosphere? Does it operate only when the spacecraft is passing through the cusp, limiting its use to deorbiting from specific orbit?

The relevant text on Wikipedia is uncited and confusing at best, containing two mutually conflicting statements:

The electric sail cannot be used inside planetary magnetospheres because the solar wind does not penetrate them, allowing only slower plasma flows and magnetic fields.[citation needed] Inside a planetary magnetosphere, the electric sail may function as a brake, allowing deorbiting of satellites.[citation needed]

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To quote Chevy Chase in the SNL skit about New Shimmer, the Coulomb drag effect is both a floor wax and a dessert topping!

The key difference is that in the sail application, the spacecraft would like to pick up some of the solar wind's speed in the Sun's frame of reference, and so it lets the wind push it forwards.

But in the brake application, the spacecraft is moving much faster than the plasma trapped in the magnetosphere around the earth, and would like to dump some speed by basically dipping a oar into the plasma, producing drag, and therefore letting it pull it backwards.

This is pointed out in Slide 2 of the presentation Electrostatic plasma brake for deorbiting (shown below), which also says that the principle was "Discovered" at the "Finnish Meteorological Institute in 2004-2006".

Both cases represent the use of drag force, despite the use of the word "sail".

You can read further in:

  1. Electrostatic Plasma Brake for Deorbiting a Satellite
  2. Attitude Dynamics Analysis of AALTO-1 Satellite During De-Orbiting Experiment with Plasma Brake 
  3. Simulation study of the plasma brake effect

below: Illustration of an electrostatic "sail", from electric-sailing.fi/technology.

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below: Illustration of an electrostatic brake; Figure 1 from here.

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below: Slide 2 from here.

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