Tiny feet can't pitter-patter in zero-G.
Over time, microgravity does lots of nasty things to organic life systems; it forces muscles to atrophy, weakens bones, impairs vision and lowers blood volume, amongst other things. According to a recent symposium on interstellar travel, these physiological changes mean that the creation of space-babies in zero-g is more or less impossible, effectively ruling out any multi-generational space journeys until science masters artificial gravity. Given that a trip to even our nearest star would take hundreds of years, this problem represents a significant hurdle.
At their last meeting, experts from the Defense Advanced Research Projects Agency (DARPA) 100-Year Starship Symposium highlighted this issue as one of the biggest hurdles standing between humanity and deep space exploration. While making sweet space love is impractical, it is technically possible to procreate without the help of gravity (especially if you have one of these cosy love suits). Sadly, the next stage is where things take a turn for the worse: The effects of microgravity on a developing foetus are thought to be severe, ranging from brittle bones to circulatory problems and underdeveloped internal organs. There's also the matter of bringing your little bundle of space-joy into the world (or, well, ship).
"Giving birth in zero gravity is going to be hell because gravity helps you [on Earth]," said Athena Andreadis, a biologist from the University of Massachusetts Medical School. "You rely on the weight of the baby."
"The distances to the stars are vast. Large starships will have to be self-sustainable. We don't have such technology yet," continued Andreadis. "We will have to grow up and do self-directed evolution, realizing that what comes out of the other end may not be human. If we stake our future among the stars, we must change for the journey and the destination."
All of this means that we're not going anywhere, perhaps not even Mars, until we master either artificial gravity or some seriously speedy travel methods. Although this news won't come as a surprise to anybody who's put serious thought into interstellar travel, it is humbling to be reminded of these things from time to time. Humans are perfectly adjusted for life on Earth; as Andreadis noted, we'll have to adapt to both the journey and the destination if we're ever to leave.