Argument: Long weightless travel to Mars would weaken Astronauts
"Astronauts face bone danger." BBC. May 4, 2000: "Astronauts returning from missions in space may take months to start recovering from dangerous bone-thinning. Living in conditions of near zero-gravity places less stress on bones, and in response, they weaken. This thinning could mean that astronauts are vulnerable to bone fractures." Also because of the lack of work done by the muscles during long durations of weightlessness, the human heart weakens very much from underuse which could endanger a long-term space mission by far.