from xkcd, Gravity wells
Of great importance and a subject I never see really mentioned anywhere: after six months in space, humans are really incapable of doing anything under gravity for a little while. It's one thing to land on Mars, it will be another before they can leave their seats. For that reason only and if one is serious about landing people on Mars and unless we find a pill to get space travelers to not deteriorate too much, getting there faster is really the only way. Hence the nuclear rocket option.
- The Soviets flew in low Earth Orbit more than 30 Rorsats that had nuclear reactors in them, The underlying reason for the need for much power was the relative unsophistication of the signal processing that could be done on-board. With no compression, one had to resort to larger power levels to send out all those signals. At the end of the cold war, the US sought to buy a different and advanced technology from the Russians (TOPAZ).
- There are simpler systems however. Most of the probes beyond Low Earth Orbit require power levels that cannot be provided by the Sun. Currently the rather large Curiosity rover is getting its energy from the decay of Plutonium-238 that heats up elements and produce electricity through the reverse of the Peltier effect.
"...Then, after the Kiwi TNT safety reactor test was conducted (to learn how destructive the reactor assembly would be if its control rods went wildly out of control) and the reactor assembly was self-destructed, the Los Alamos NRDS Assistant Director, two of his associates, and again Rocketdyne's NTR Section Chief were exposed to the after-test radiation surrounding the destroyed reactor assembly. The measured environment radiation was approximately 10 rem. Fifty years have passed, and the health of the above-defined test programs participants (as determined by the NTS Medical Surveillance Project Office's evaluation) remain unaffected by radiation...."
Some videos of the tests can be found on Youtube:
h/t Various Consequences
 XE-prime EP-4A startup tests. SPEAR report