A snippet from an abstract I’m working on – further details about ‘falling up’:
Although they have many advantages over present technology, eddy-current actuators are not a killer app for generating 6-degree-of-freedom forces. Eddy-current forces are small and drop off quickly with distance. The necessary electromagnets and motors both generate thermal loads. While possible, it is difficult for an eddy-current actuator to generate a force that pulls the target and inspection vehicle closer together.
Despite these flaws, there are a number of applications which can take advantage of this technology’s strengths (electric-only, propellantless, contactless forces with an uncooperative target) and minimizes its weaknesses.
The magnitude and direction Earth’s gravitational pull varies around the exterior of a large satellite like the ISS. So two test objects – one placed on the ‘bottom’ of the ISS and one placed at its center of mass will experience different accelerations and thus a relative ‘force’ between them. On earth, this relative acceleration between objects at different altitudes is hardly noticeable compared to random ‘noise’ forces like wind. However, there are much fewer disturbance forces in space and the gravity gradient is enough to ‘pull’ an inspection craft towards the surface.
On the exterior of an object the size of the ISS, the ‘force’ is on the order of 0.1 mN. Eddy-current actuators have been able to produce forces on the order of 0.01 – 10 mN so they are well-suited to oppose the gravitational forces, allowing the inspection craft to keep a safe distance while moving laterally along the surface.
[Placeholder for a diagram of this totally unintuitive situation]