Absorption of cosmic rays by solar wind particles apparently has been a key factor in manned space missions, preventing astronauts from getting radiation poisoning. But if the Sun is entering a new quiescent era like the Maunder Minimum, there might not be enough solar wind to protect them on long missions. Of course, if we already have deep black bases on Mars, using reverse engineered alien space tech, this is a moot point...

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Trips to Mars may be OFF: The SUN has changed in a way we've NEVER SEEN
Only nuke power can save us from radiation
By Lewis Page, 22 Oct 2014

A drop in the solar wind of a kind never yet seen in the space age has made travel beyond Earth orbit a lot more dangerous, according to boffins – so much so that a manned mission to Mars may not be feasible for many decades.

"The behavior of the sun has recently changed and is now in a state not observed for almost 100 years," says professor Nathan Schwadron. "Starting in about 2006, we observed the longest solar minimum and weakest solar activity observed in the space age."

It's a story we've already reported on here at the Reg: some solar physicists, noting that the current "solar maximum" is an especially damp squib, suspect that the Sun may be entering a so-called "Maunder Minimum", a lengthy spell of low to no activity. Such a minimum occurred from 1645-1715.

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http://www.theregister.co.uk/2014/10/22/the_sun_has_changed_in_a_way_weve_never_seen_mars_trips_may_be_off/