Human space travel! Already difficult because of the gnarly technology and expensive dollars it costs but even more difficult because humans just can't stay alive long enough for interstellar trips. We need to, like, have sex to procreate and raise kids and stuff.
Why is sex so important? Aside from the obvious, space trips to other stars need hundreds of years to complete and the average life span of a human just doesn't cut it. We need generations of humans, a whole civilization of people to make that trip. And that means babies. And gravity.
Yep, gravity! Of which there is none in space. That lack of gravity translates to weightlessness which is terrible for humans over a long period of time: it decreases our blood volume, atrophies our muscles, diminishes our bone mineral content and screws with our vision. But most terrible is that it makes it increasingly difficult to have sex. Biologist Athena Andreadis of the University of Massachusetts Medical School says:
"Sex is very difficult in zero gravity, apparently, because you have no traction and you keep bumping against the walls. Think about it: you have no friction, you have no resistance."
Not only that, scientist have no idea what the repercussions of raising a child in gravity-less space would be. Would it disrupt embryonic development? Will the baby have weaker bones? Can you even have a baby? Seriously, giving birth is aided by gravity (mothers need the weight of the baby to push 'em out), without that gravity and weight, how would that happen? It's messy.