[King Prout]

indeed. I left out the liftoff, drive, and rotational stresses as relatively minor in comparison to the gravitic and PSI stresses... but "relatively minor" does NOT equate to trivial.

[NicknamedBob]

For me, the primary difficulty is trying to do orbital maneuvers, takeoffs and landings, without subjecting the passengers to zero gravity.

When you consider that all of NASA's previous plans seemed to involve months of no gravity on a trip to Mars, you can appreciate that it is tricky to have none.