Okay, I tried searching for this in the forums and didn't really find what I was looking for. So, I'll start a new thread and if anybody has seen this before, feel free to drop a link to an old thread or whatever. Many of our older space probes use/used thermoelectric radioisotope batteries to power their electronics as they move/moved through space. Solar sails are a good option when it comes to free propulsion in space near to the sun, but further out they stop working and lose their effectiveness if further acceleration is desired. Besides that, there are plenty of material science challenges with development of sails (though I think a lot of them have been solved and some space probes recently have made use of solar sails). As an alternative, why not use a thermoelectric radioisotope battery to power an electron accelerator to allow for continued propulsion beyond the known solar system, to perhaps make it to a neighboring system? On that note, I wanted to do a simple analysis, but I'm having trouble figuring out how the momentum equations change to accommodate relativity, assuming the electrons are accelerated to relativistic speeds. Other than requiring a ridiculous length to accommodate an accelerator inductor system, I think this might work. Any problems anybody wants to point out before I waste too much time on this (if there are major problems I'm probably overlooking). Would there be enough electrons in deep space to keep the craft close to a neutral charge (as this would quickly eat into the power required per electron I'm thinking).