Ever since humans are able to shoot stuff into outer space, manned missions to the Red Planet are a recurring topic of discussion. Almost fifty years after the first Moon landing, no man or woman has ever been further away than that. On first sight it seems easy to go to Mars.
However, Mars is not just “the Moon a little further away”. Mars is much further away than the Moon. A round trip to the Moon only takes a week, hence it would be a potential destination of terrestrial tourists in the near future, whereas with current technology a one-way mission would take about 8 to 9 months, provided that Mars and Earth are relatively close. A round-trip would be about 2 years.
A second issue of a manned mission to Mars, is the planets escape velocity. If you want to go to Mars, land there and return to Earth, you need the launch a spaceship from Mars. The problem, however, is that the escape velocity on Mars is 5 km/s, slightly less than half of Earth (11.2 km/s). In comparison the Moon has only an escape velocity of 2.4 km/s. In order to get a rough grasp of the fuel required, we should look at the kinetic energy equation:
T = ½mv²
This equation states that kinetic energy is proportional to the velocity squared. Or in order to launch a spaceship from Mars one need about a quarter of the fuel needed on Earth, where on the Moon you only need a sixteenth amount of fuel. However if you have to carry the return fuel from Earth, you are increasing the “payload” mass and hence increases overall launch mass.
From a practical point of view, this fuel requirement is prohibitive. And essentially there are two solutions to this problem: a one-way mission (i.e. the crew does not return to Earth) or producing rocket fuel on Mars from locally available resources. For obvious reasons the first option is usually rejected and would only make sense within the context of colonization. The second option, though feasible, is fraught with technical challenges.
Even if Martian fuel would be available, then there is still another important issue: radiation. On Earth we are protected against cosmic radiation by our planet’s magnetic field, but Mars has no such field. On a short trip radiation is not that big deal, but two years in outer space has severe health consequences.
Considering the above, it is not strange that Mars exploration is done with robots. One should wonder whether manned missions to Mars are actually necessary? Though robotic missions produce good results, there are drawbacks. Because of the distance between Mars and Earth, it takes twenty to forty minutes to send a signal to the Red Planet. Consequently, a rover can only travel a few meters a day. (One should keep in mind that the Martian surface is unknown terrain, and a rover moving at high-speed might get struck in quick sand or collide against a rock.)
A human crew, on the other hand, could easily travel a few kilometers a day and hence being able to explore a larger area. Unlike robots, at least with the currents state of affairs, humans are fully autonomous and can take instant decisions when needed.
Before designing a human mission to Mars, one should make clear what the purpose of such mission is. If you want to colonize the planet, then you could consider a one-way mission as permanent settlement is the main objective. However, if your goal is just scientific exploration then this no possibility.
The good news is that for a purely scientific manned mission to Mars, one does not need to land people on Mars. Instead one could leave the crew in orbit around the Red Planet, while they control robotic rovers from there. This scheme has several advantages.
First one eliminates the need to have a launch from the surface from Mars and hence saving a lot of fuel. The second advantage is that because the crew is in orbit around the planet, real-time operation of Mars robots becomes possible. This makes it possible to explore a larger area than is currently possible.
I know, this proposal is not as “sexy” as an actual landing, but it makes most sense for a scientific mission.