Given the success, the robustness in hostile environs and the ever increasing capabilities of robotic spacecraft, exploration is most readily done by unmanned probes. Sure, a human on the ground is still a better wide spectrum observer able to follow the unanticipated but many robotic explorers could gain access to more places and more unforgiving environments.

Any utility for humans in space, aside from adventure tourism in suborbital, LEO and cis-lunar, remains actual settlement. Given our increasing understanding of DNA and nanotech, suitable means of dealing with increased radiation exposure in flight and on the surface of Mars is reasonable to expect within a couple decades.

Going back to the moon, just because it's more readily accessable will not capture the imagination of young generations. Proof: where is the public excitement over Bush's return to the moon program? Instead, a new, bold and adventurous goal, with the exciting possibilities of discovering the unexpected is needed. NASA should be building a generic manned space exploration craft, not something special purpose engineered for lunar return. If an incremental approach to a Mars base is deemed wisest, then take initial long flight steps to visit a nearby NEO or 2 where actual really new science will benefit and then follow with missions to Mars' moons before attempting a landing at a site pre-prepared by robotic craft directed from people based on Mars' moons. Zubrin has done an excellent job providing ideas of how to live off the natural resources on site.

Looking back at the 40 years since Apollo 11 shows the complete lack of purpose and vision in the US manned space program. ISS is simply a hugely more expensive 70s SkyLab with no new developments of substance for the $100 Billion.