Electromagnetic formation flight
Electromagnetic formation flight (EMFF) investigates the concept of using electromagnets coupled with reaction wheels in place of more traditional propulsion systems to control the positions and attitudes of a number of spacecraft in close proximity. Unlike traditional propulsion systems, which use exhaustible propellants that often limit lifetime, the EMFF system uses solar power to energize a magnetic field.
Source: Wikipedia — Electromagnetic formation flight (CC BY-SA 4.0)