Firstly, let us see the reasons for not sending manned missions to Mars.
Crewed missions may very well be more efficient and more preferable for space explorations than crewless missions. But crewed missions also have their disadvantages. They are listed below:
Satisfying Basic needs of man
When we design a spacecraft, we have to take into account all the extra facilities to be added to the spacecraft in order to make it inhabitable by man.
As discussed earlier the basic needs of man such as food, water, light, oxygen, eliminating body wastes of humans must be taken care of. All this adds to the amount of payload, which directly increases the amount of fuel needed.
High cost of mission
When humans are in mars, we have to take care about certain points:
We do not need food supply for robots. If we send a team of people to mars, each person requires 408kg or more of dehydrated food to survive.
With robots we do not have to worry about return from mars to earth.
With robots we do not have to worry about life support systems.
Robots do not require a soft landing on the surface of Mars.
All these increase the cost of the mission and hence the mission is not feasible.
Man may not be very comfortable traveling in space for a long duration of time. That is because man's body is not adaptable to space. Though the conditions at Mars are not as inhospitable as that of Venus, they nevertheless are not designed for human stay. Health disorders are also caused due to the reduced gravitational forces of mars. The disorders are explained in detail in the section “effect of space on man”
Earth's magnetic field blocks most of the cosmic radiation that is directed on it. But Mars has no such gravitational field. Astronauts have to face enormous amount of cosmic radiation during such a long mission and there must be procedures to block it. Radiation is added to the already existing cosmic radiation if nuclear propulsion engines are used.
Poor Track Record
The most important reason why manned missions are not sent to mars is because, till date 30 missions have been sent to mars with less than one-third of them surviving. so we need to increase our chances of success before we plan such a mission.
Living on Mars
It is not just enough to reach Mars safely. The astronauts have to find a means of surviving there. It is practically possible to carry food, water and oxygen for a mission which may be three years long.
Coming back to earth
When manned missions are sent to any place, it is of utmost importance to get the astronauts safely back on earth. But the distance to Mars is really huge and it is not possible to carry enough fuel to get the crafts back onto earth.
All these reasons make success of manned mars missions difficult.