Four obstacles
The third point surprises people most often. The Martian atmosphere is more than 100 times thinner than Earth's: a parachute can't slow the spacecraft enough. At the same time, it's dense enough to cause intense heating, so the spacecraft can't land using engines alone.
That is why complex landing systems have been used for rovers. But their mass is measured in tons, while a crewed spacecraft with supplies would be tens of times heavier, and there are still no known ways to land something that massive.
The second tile explains the expedition's main feature. The planets come close to each other once every little more than two years, and you can't leave sooner: if something goes wrong on Mars, you'll have to wait more than two years.
The third tile rules out the usual kind of control from Earth. With a delay of tens of minutes, any conversation becomes an exchange of letters, and the crew has to make every decision on its own, without guidance.
What is already being tested
Isolation is being tested in ground-based experiments: groups of people are confined for months in modules with communication delays and limited supplies. That is where conflicts, problems with routines and drops in motivation emerge.
Making fuel on Mars is also being tested: one rover carried an instrument that produced oxygen from the local atmosphere. The amounts are still symbolic, but the principle has been confirmed.
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Sources
- The main obstacles to a crewed expedition are radiation exposure beyond the magnetosphere, loss of bone and muscle mass, landing large masses in a thin atmosphere and the need to produce fuel on site
- launch windows recur about every 26 months