Space is filled with cosmic rays that are high-energy particles. If we could convert that energy into electric (or any other) energy, then we could use it to power a rocket anywhere in the universe. If that is not possible, why not?
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The problem is with the low energy density of cosmic rays in the universe. Individual cosmic rays are indeed very energetic, but there just aren't enough of them to be a significant power source for interplanetary or interstellar vehicles. The Wikipedia article about cosmic rays (https://en.wikipedia.org/wiki/Cosmic_ray) quotes the energy density of cosmic rays in interstellar space as roughly 1 eV per cubic cm, or 10^6 eV per cubic meter, which is only ~1.6 x 10^-13 Joules per cubic meter. Cosmic rays are so energetic that they're all traveling at nearly the speed of light, so if you multiply 1 square meter by the speed of light (~3 x 10^8 m/s), then multiply by the volume energy density just stated, you get the average energy flux density, which winds up being ~5 x 10^-5 Watts per square meter. Wow, that's not much! If you could convert that energy to electric power with 100% efficiency, it would take a collecting area of nearly 21,000 square meters (a square ~144 meters on a side) to collect one Watt.
The Wikipedia article also states that the cosmic ray flux inside our solar system's heliopause is about a tenth that in the interstellar medium.
Using cosmic ray energy sounds like a good idea at first, but the realities of the universe say no.