Then you've got to protect astronauts from constant bombardment by high-speed subatomic particles. This potentially fatal radiation is 100 times more intense on Mars than on Earth. Finally, you've got to bring these interplanetary pioneers home. Mars's surface gravity is only four-tenths as strong as Earth's. It's possible that settlers could make some of the needed rocket fuel from materials found on Mars.
Why are there more unmanned missions to Mars than manned missions?
 |
Unmanned missions are safer because there isn't a risk of people dying. Also, they are cheaper because you don't need to do as much testing for safety, and much less mass needs to be lifted into outer space. Furthermore, unmanned missions can gather almost as much data as manned missions can. 
|
There are many challenges involved in getting unmanned spacecraft to Mars. (In fact, many of the forty missions attempted so far have ended in failure, with some crashing shortly after launch and others falling silent just as they reached their destination.) But these challenges pale beside the requirements for a successful human expedition. For starters, you've got to supply astronauts with enough air, food, and water to make the nine month journey to Mars—roughly fifty tons' worth—and however much is to be consumed while on the Red Planet. (Air and water can be partially recycled, but even so, they would outweigh each crew member many times over.)