Medary.com

Futurepundit: Too much radiation for humans in Mars trip:

Dreams of a human trip to Mars run up against limits to allowable human radiation exposure.

. . .

One way to get a ship to Mars that would have lots of chemical rocket mass to propel a return trip: Send two space ships. First send one slowly that would carry a lot of fuel. That fuel would enter Mars orbit before humans even left Earth. Then humans could leave Earth on a fast ship and arrive to find another fast ship with lots of fuel ready to take them back to Earth.

Part of the radiation exposure would come while humans are on Mars. How to reduce that exposure? Send robots ahead of time that would burrow down underground to create living quarters in several places that would be within driving distance of each other. The astronauts could move from underground shelter to underground shelter.

Of course, all this requires huge amounts of money and resources.

Well, nobody said it would be easy.