[VLM]

> to a fast-moving spacecraft, are all electrons "high energy"?

Define fast. Electrons get to relativistic-ish speeds in the hundred of kilovolts-ish range. If you look at the specs for a x-ray tube thats a pretty intense / hard to shield x-ray source, not like those wimpy tens of kilovolt xrays from a CRT that don't even make it thru the glass.

So if you're going fast enough for relativity to be an issue, smacking a stationary electron is going to hurt and be a pain to shield, so yeah, all electrons are high energy. On the other hand what we puny humans currently think is a fast spacecraft, is going to be a rounding error compared to solar winds and just random stuff out there, so in a different way, all the electrons that matter are high energy compared to our actual deployed slow spacecraft, sorta.