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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$\begingroup$ Shubanshu, this is a question-and answer site, not a general discussion platform. If you have a question that you think can be answered, ask it. $\endgroup$– user10509May 7, 2018 at 13:43
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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.
answered May 7, 2018 at 19:28
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2$\begingroup$ It is not only the low energy density, it is also the very low efficiency of the conversion to electric energy. $\endgroup$– UweMay 7, 2018 at 19:32
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2$\begingroup$ @Uwe , the reason I postulated 100% efficiency is that it shows that no matter what the efficiency is, it isn't a practical source of power. But indeed, even speculative conversion techniques suffer from low efficiency. $\endgroup$ May 7, 2018 at 19:38
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$\begingroup$ Still not that hopeless for sustaining "sleep mode" during interstellar travel, or burst signals, say, a second of communication per month. What are the efficiency rates? $\endgroup$– SF.May 8, 2018 at 10:02
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$\begingroup$ Would it be possible to build a magnetic "lens" to usfully concentrate [some of] this radiation? The lens would be a set of superconducting wire loops, many kilometers across (but only massing as much as a few 10s of km of thin wire) and the hope would be achieve a useful energy density on a collector, which would probably be flying separately, at a focal point which might be thousands of kilometers away. $\endgroup$ May 8, 2018 at 10:51
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1$\begingroup$ @SteveLinton Since cosmic rays have such a broad distribution in momentum, a (magic) magnetic lens that performed say a 1000:1 ratio of diameter to circle of least confusion of one energy and particle type would perform miserably even for the same particle type with even a 1% difference in energy, so you'd have to go after a achromatic electromagnetic lens design. And while solenoidal magnetic lenses do in fact focus, it's a second order effect and therefore a fairly weak lens, which is why quadrupole pairs are used wherever possible, and solenoids mostly get used for low energy electrons. $\endgroup$– uhohMay 8, 2018 at 15:15