NASA recently tested an experimental microwave thruster called the "EmDrive" that is a microwave thruster. And it worked!!

Fuel is heavy and takes a lot of space, so it would be a huge breakthrough to use something else. According to Wired UK, the drive generates thrust by "bouncing microwaves around in closed container" without any need for propellant.

Its inventor, Roger Shawyer, formed a company called SPR to promote his EmDrive. Critics "reject[ed] his relativity-based theory and insist[ed] that, according to the law of conservation of momentum, it cannot work."

Shawyer's SPR site noted: "At an input power of 2.5kW, their 2.45GHz EmDrive thruster provides 720mN of thrust." Those numbers indicated that it would be suitable for a satellite thruster.

The site said: the thrusters "could be powered by solar electricity, eliminating the need for the supply of propellant that occupies up to half the launch mass of many satellites."

Chinese Academy of Sciences researchers achieved success with the experiment.