This is just my naive guessing. For an environmentally friendly space drive I wonder what would the rest product be. Since the stars produce a lot of radiation and at least our planet have a magnetic field. I guess this is what you use.
So my idea for a space drive is the following. You first create a “slow” moving ground field for which you then can push against. The ground field is the rest product the drive leaves behind.
I saw that the magnetic field component of light can generate a greatly amplified magnetic effect when traveling trough a non conductive material at the right intensity. I also happen to read that particles can self accelerate.
What if you can make light self decelerate instead or self slow down in vacuum. Would this not be similar to light slowing down in a material. If so I guess this could serve as a ground field.
Come to think of it. There should happen something when a particle self decelerate in space. So why could not we take advantage of electrons from a small particle accelerator. Make them self decelerate quickly and use the result for the forward motion.
Another thing is that if we master self deceleration I guess we could perhaps slowdown or stop asteroids.