The answer is, of course, gravity. That’s the one thing we can’t increase or decrease without we do some serious messing about with planetary mass. Of the two worlds that offer even the slightest hospitality, the moon is one-sixth gee and Mars is one-third. And we won’t be changing either one anytime in the near future.
(A pause to savor the irony for a moment: of all the worlds in the solar system, the one with a mass, and therefore gravity, closest to Earth’s is Venus, which combines the worst features of just about every other planet. Man, the future really ain’t what it used to be ...)
The problem is that, no matter how much technological advances we make, genetically we’re still just a bunch of naked apes jabbing each other with pointed sticks back on the veldt. We evolved in a one gravity field, and when we spend prolonged periods out of it (such as a few months up in the space station), bad things happen.
Real bad. Prolonged weightlessness will cause, among other things, dehydration, musculoskeletal atrophy, “space anemia,” mineral depletion, vertigo, and a whole host of other problems ranging from unpleasant to downright life-threatening. In the “Pulp Era” of science fiction, back when men wore hats and baggy pants and space flight was still just a dream, one of the common beliefs was that considerable time spent in zero-gee (or microgravity, to placate my astronomer friends), might actually be good for the body. No stress on the joints or the heart would logically mean no wear and tear on our moving parts. Hey, you could maybe outlive Methuselah, just by floating around the asteroid belt.
Unfortunately, like so many rose-colored visions of the future everyone had back then, it wasn’t that easy. In fact, pretty much the opposite holds true; long-term weightlessness resembles, in syndrome, nothing so much as accelerated aging. Spend a couple of years in space (the average time of a trip to Mars), and you wind up looking -- and feeling -- like your grandfather. Oh, sure, the process can be slowed somewhat -- by near-constant exercise. I don’t know about you, but riding an Exercycle all the way to Mars isn’t the future I was promised. Even with flying cars thrown in, the idea pretty much sucks the ol’ Saturn V.
And we haven’t even discussed the physical and psychological effects (which admittedly we don’t know yet, but the probabilities aren’t looking good) of carrying a child to term and raising him or her in a lighter gravity field.
Doesn’t look good, does it? The Earth’s on a fast track to disaster, the only other places we have even a faint chance of reaching aren’t even remotely pleasant, and just getting there can kill you. And even if the Keplerscope finds more New Earths out there, we’re not going to be reaching them anytime soon. A trip even no further than the Centauri System would take centuries. (Norman Spinrad described it best, as far as I’m concerned: Imagine a WorldCon on a submarine -- forever.)
It do make the blood run cold, don’t it?
Next: Spam in a can -- industrial-sized.