Brian, Harlow asked:
If a man voyages to Mars. If it takes 4.5 years to get to Mars and 4.5 years back I think it will be impossible for someone to spend 9 years aboard a space ship. What do you think?
That is probably quite pessimistic I think it takes less than 4.5 years to get to Mars, for example Phoenix only took 10 months to get there, and if you were carrying people you would probably travel faster. Probably somewhere around 6 months or so, depending exactly the orbits and the trajectories that you're looking at and how close Mars is to Earth at the time.
The way it's normally done is that every two years Mars does a close swing past of Earth. That's when we launch all our space missions: every 2 years. What you're looking at for a human mission is you're either going to go there and back, quickly while Mars is close or you go there, stay for 2 years and then come back again.