There is no doubt that humans are going to Mars. It is simply a question of how and when. However, there are many fundamental concerns that must be dealt with. Some of these address crew safety, radiation exposure, long travel times, life support on Mars and return options. We already know that low energy methods of transfer can take eight months each way and minimum Mars surface time between return windows is about two years.
At a minimum, any Martian trip is going to be a major hassle. Transportation costs alone will surely be in the hundreds of millions of dollars. Interplanetary spacecraft will cost more. Slight errors in navigation could result in missing the planet and marooning the crew forever in the vacuum of the Solar System.
Overall risk factors may not be acceptable for government sponsors. Private sector sponsors may not be able to raise enough capital. International partners may be reluctant to take on such expensive missions.
Based on the cost and complexity of traveling to Mars, landing on the surface, sustaining life for many months and returning to Earth, it is logical that early travel to the red planet would entail only relatively simple mission objectives. Of course, the simplest trip is not the most desirable.
For example, a one-way excursion is possible at relatively low cost. This would involve a high-energy launch directly into an eight-month heliocentric transfer coast, ending in a hyperbolic swing by of Mars. After that the spacecraft would enter a permanent elliptic path around the sun with a period of about 16 months.
The next option allows a return to the home planet. This is known as a free-return transfer orbit between Earth and Mars. This option is most likely for early crewed Mars excursions. This type of mission is referred to as "Mars Direct," first discussed in The Case for Mars, a book by Robert Zubrin.
There are several trajectory options under this mission category. In the simplest case, a Hohmann transfer orbit is made free-return and takes about eight month to get to Mars and about 18 months to return. No propulsive maneuvers are assumed after the initial Earth-escape launch vehicle injection.
In 2009, NASA published a Mars Mission Design Reference Architecture, advocating a 174-day transfer to Mars. However, this option requires a delta-v maneuver of approximately 4 km/s for the trans-Mars injection that is needed for the return leg. It appears that this option is very likely for early crewed travel to Mars.
