Why Getting to Mars is Difficult
The root of the answer is that we often times don't appreciate the scale of what a trip to Mars entails. And, frankly, everything snowballs from there.
Mars is about 150 times further away from the Earth than the Moon is. That may not sound like a lot, but think about what that means in terms of added fuel. Of course, more fuel means more weight. More weight means bigger capsules and bigger rockets. This is what I mean by a trip to Mars being on a completely different scale.
But really this isn't our problem. NASA has spacecraft designs (like Orion and Nautilus) that would be capable of making the trip. Though neither is actually ready quite yet, the technology is certainly available.
No, the biggest problem is temporal. Since Mars is so far away, and orbits the Sun at a different rate than Earth, NASA must time their launches very precisely. This goes for the trip there, and the trip home (unless the astronauts just stay there permanently).
So while it may be possible to cut the travel time down to a month or two using advanced propulsion technology currently under development, once on the surface of the red planet the astronauts will need to wait until Earth and Mars are correctly aligned again before returning. How long will that take? A year and a half. At least.
Dealing With the Issue of Time
This sort of time scale causes problems in other areas as well. How do you get enough oxygen? What about water? And, of course, food? And how do you get around the fact that you are traveling through space, where the Sun's energetic solar wind is reigning down harmful radiation upon your craft? And lest we forget about the micrometeorites that threaten to puncture the spacecraft or spacesuit of an astronaut.
The solutions to these problems are a bit trickier to overcome. But it's all doable. Given enough time and money of course.
Protecting the astronauts while in space means building the spacecraft out of robust materials and shielding it from the Sun's harmful rays.
The problems of food and air will have to be solved through creative means. Growing plants that produce both food and oxygen is a good start. But this means that should the plants die, things go horribly wrong. And that is all assuming you have enough room to grow the volume of planets needed for such an adventure.
You can take food, water and oxygen with you, but that of course adds weight and size.
NASA is confident that it can overcome these problems, but we are not quite there yet. However, over the coming two decades we hope to close the gap between theory and reality. And maybe then we can actually send astronauts to Mars.