The Moon. Mars. Asteroids. Deep space. These are all places that humanity would like to explore. So what's the holdup?
Flight hardware, money and politics aside, there's some major problems that need further study if humans are to visit or colonize anything other than Earth, most notably: habitation and survival. Humans are fragile creatures, and the environments found off-Earth are decidedly unfriendly. We've been in space for 50 years. The International Space Station has been occupied continuously for over 10 years. We know a little bit about the physiology and psychology of living in space, at least for a few months at a time.
But a round-trip mission to Mars, for example, would take much longer than even the longest-duration missions to ISS, on the order of seven months in each direction in the best case. That's not including an actual stay on the planet. And what's the point of going to Mars if you can't stay for a while?
Before humanity can leave low Earth orbit, we have to figure out how to feed, house and protect a crew on another world. It'd also be helpful to know more about human psychology and behavior in such foreign environments. Basically, we need to know how to successfully live and work in space for long periods of time. The best-case scenario for figuring out how to do that, and the technologies needed, is simulation of deep-space missions on Earth. To that end, NASA and the European Space Agency, along with some partners, have come up with some analog exploration missions: NEEMO, Mars500, and the Desert RATS studies. Read on for more on these projects: