I get the impression that NASA has done alot to make a crewed landing on Mars possible. Not as a focused mission like Apollo, but piece by piece in spite of the lack of dedicated and lasting political commitment. I think that maybe most people who are lucky enough to work with human spaceflight probably prefer to help a mission to Mars when they can, and maybe manage to do so partly in the shadow of other pretexts. Now, my claims above may be "opinionated", and may be edited out, but as a matter of fact, the question is:
How much of what is needed to land humans on Mars has been investigated and prepared today?
Below are my own impressions to begin with, I hope for better versions of these hypothetical answers:
+) What has been, or is being, done:
- LDSD, a way to land heavy payloads on Mars. For what if not humans?
- Suitports, spacesuits which avoid toxic dust and bio contamination.
- Athlete, the mobile Mars rover/habitat concept.
- Mars 2020 rover oxygen ISRU demonstration. Obviously just for human purposes.
- Orion, together with a habitat module it'd be quite useful for a trip to Mars.
- SLS, huge and expensive rocket, but if financed would be great for a Mars mission.
- Rovers and orbiters to Mars, quite a set of robotic precursors.
- NASA has at least investigated a communication satellite at Mars.
-) What remains to be done:
- Artificial centrifugal gravity, in order for astronauts to arrive healthy.
- Fully (water) recycling life support systems. The ISS is only partly there.
- The travel habitat on route to Mars. But, they do invite Bigalow to dock with the ISS next year!
- The forward bio contamination issue. Would the exobiologists destroy the data they look for upon arrival?
(My linked sources are mostly Wikipedia, because I'm anyway not quite competent to qualify the usefulness of sources on this topic beyond what most who read and comment here can do themselves)