NEW : Faster engines , better shielding under study , NASA official says . Anyone who spends time on Mars could be exposed to more radiation , depending on how long they stay and what the shielding conditions are like . Comment: every five days , study finds . Radiation clocked on Curiosity 's trip to Mars approaches NASA 's career limit for astronauts .
NEW : Faster engines , better shielding under study , NASA official says . Comment: every five days , study finds . Radiation clocked on Curiosity 's trip to Mars approaches NASA 's career limit for astronauts .