The capabilities they outline seem to vary widely in potential difficulty. I think 3 of them are clearly the most difficult: Mars landing tech - we've never la

The capabilities they outline seem to vary widely in potential difficulty. I think 3 of them are clearly the most difficult: Mars landing tech - we've never landed any really large weights on a planet with gravity on the same order of magnitude as Earth and no atmosphere. This is gonna be tough to build and expensive to test Liftoff from Mars - What kind of craft will we need to get the crew back to Mars orbit from the surface? Will we be able to land it ready for launch, or will it have to be partially constructed and fueled on Mars? How will we build the infrastructure needed to do that? Technology for living on Mars for an extended amount of time - if people are going to be living on Mars for months or more, as seems likely, then we're going to need to build a much more substantial habitat on Mars than anything we've ever built before. How will we go about transporting and constructing that? Everything else sounds like something we've already done, just expanded a bit, like bigger rockets, nicer space suits, better radiation shielding for the interplanetary craft, etc.