I don't know, I think we're pretty close, or actually there.    Getting to the moon or orbit for rendezvous is expensive and tedious, but not hard at all.  The real challenge in all of it is getting a payload of significant mass to land safely on mars.  The air is so thin that parachutes won't work how you expect them to, and wings don't provide anywhere near the lift they do here on earth. There's still enough gravity that you can't use a rocket-assisted vertical landing like they did on the moon (at least, you can't do that and carry enough fuel to leave again), and in inflatable landing shock absorber (i.e. wrap it in balloons) will cause too much point acceleration for humans to tolerate.  And there's no magnetosphere that we could play funny games with. We have a few good things to try together, but I think we're going to need a few more experimental craft landing attempts before we're ready to drop people on the surface.  But we could get lucky and find a good method on the first shot or two.