We have this (literally) lofty goal of wanting to put humans on Mars. The problem with this one is rather huge:

Cells, especially in a complex organism, don't function properly under the different gravity. Eventually, if you succeeded in such a manned mission, everyone would just get sick and die faster than normal. Furthermore, even if everyone didn't perish, no one would be able to reproduce. That's kind of a downhill bummer for any sort of long-term goals, isn't it?