> It's not because of the humans but because the cost of producing the space ships will be so large that it's not feasible to shoot them to Mars once and leave them there; they have to be reusable.
The problem with the return trip is Earth reentry. Coming in at interplanetary speeds (falling to the sun from a "height" of Mars' orbit to Earth's), it is impossible for humans to reenter the Earth's atmosphere safely. It's barely possible from the moon.
This means that there has to be about as much deceleration before reentry than there was there was acceleration on the way out. So now you have to carry double the fuel, and the fuel to carry that, etc. This vastly complicates the problem with a manned mission to mars.
The problem with the return trip is Earth reentry. Coming in at interplanetary speeds (falling to the sun from a "height" of Mars' orbit to Earth's), it is impossible for humans to reenter the Earth's atmosphere safely. It's barely possible from the moon.
This means that there has to be about as much deceleration before reentry than there was there was acceleration on the way out. So now you have to carry double the fuel, and the fuel to carry that, etc. This vastly complicates the problem with a manned mission to mars.