Why Not Mars
Some craaazy facts about microbes in there. And I actually came away convinced! Have previously taken the same argument NASA made for space exploration originally - all the incidental inventions and discoveries made make it worthwhile. But the extreme cost (and risk of contamination) make sending humans to Mars seem like a not great idea. Just let robots do it.
Humans in space is more of a biology problem than an engineering problem. Or at least the engineering of life support systems is the bigger challenge than building rockets.