Life on Earth has evolved within a very fine set of parameters. We are going to find it a challenge to survive outside the sheltering cocoon of this planet, not the least because our atmosphere protects us from several types of radiation, not the least from our friendly sun.
Now there's another limitation.
Our body's physiological processes are to a great extent governed through the triggering of gene expression, which generates proteins that affect metabolic pathways of chemical reactions. Translated, this means chemical signals trigger the unwrapping and copying of genes (sections of our DNA blueprint) that in turn generate proteins that... make our body work.
For that to happen, amongst other things we need... gravity.
On the one hand, one might intuit that gravity shouldn't be an essential part of our processes. But we are generally pulled in a single direction: towards Earth, the largest mass at hand. From an evolutionary perspective, that amount of gravity is an intrinsic part of the environment in which so many successive iterations (generations) successfully mutated and survived. Our environment tempered the direction of successful mutation.
So it makes sense that our metabolic processes could be so finely tuned that significant change (ie, to zero gravity) could disrupt some of these processes.
And that's what's been found, as reported in New Scientist this week (4 February 2012). Specifically: "weightless conditions... could disrupt the activity of 200 genes linked with immunity, metabolism and heat tolerance."
There is a slight caveat on that: the study used flies, and simulated weightlessness through magnetic fields. Still, the researchers are confident of their results, it sounds plausible, and doubtless the result will be tested by others in other experimental contexts.
Still, just as science can bring the science fiction of space travel crashing to Earth, surely technological solutions will be developed. After all, science fiction has already imagined simulated gravity. It just hasn't filled in the details.