Of course, falling all the way through the Earth is impossible, since its core is molten. But it's a fun thought experiment nevertheless, and one which The Straight Dope's Cecil Adams tackled some thirty years ago -- but which I still find fascinating. So there's this hypothetical tube. Made of some indestructible material that doesn't get melted from the molten-ness of the Earth's core and protects you from getting cooked on your way down (and back up). If you ignored all the KEEP BACK signs and clumsily fell into the hole, what would happen?
You'd fall, obviously, picking up momentum as you went. As you approached the center of the earth the pull of gravity would decline and eventually (at the center) cease, but inertia would keep you going.
Once past center, though, the pull of the earth's mass behind you would begin to slow you down, at exactly the opposite rate that you'd accelerated. You'd come to a complete stop just at the brink of the Antarctic end of the tube, where you'd have an opportunity to wave gaily to the bunny rabbits or whatever they have out there before beginning to fall back in the opposite direciton. This process would continue forever.
Once we start figuring for the effects of atmospheric friction, of course, the situation changes. After a certain point in the course of falling you'd reach a top speed called "terminal velocity," where air resistance would counteract the accelerating effects of gravity. With less momentum, you'd only fall a relatively short distance past the center of the earth before you stopped and started heading in the other direction. Eventually you'd reach equilibrium at the earth's center.
Okay, well that would never happen, obviously. How about another burning question: what would happen to an astronaut who took off their helmet in space?
After all, this is something that could actually happen, though NASA has a pretty rigorous psychological testing regimen that would theoretically weed out anyone wacky enough to rip off their space suit helmet in outer space (if this is even possible; astronaut X would probably need a friend's help). (Of course, there was that cross-country-driving, diaper-wearing astronaut -- but that's another story. I'm sure she's perfectly sane.) In any case, here's how it would go down, according to Damn Interesting:
For about ten full seconds"“ a long time to be loitering in space without protection"“ an average human would be rather uncomfortable, but they would still have their wits about them. Depending on the nature of the decompression, this may give a victim sufficient time to take measures to save their own life. But this period of "useful consciousness" would wane as the effects of brain asphyxiation begin to set in. In the absence of air pressure the gas exchange of the lungs works in reverse, dumping oxygen out of the blood and accelerating the oxygen-starved state known as hypoxia. After about ten seconds a victim will experience loss of vision and impaired judgment, and the cooling effect of evaporation will lower the temperature in the victim's mouth and nose to near-freezing. Unconsciousness and convulsions would follow several seconds later, and a blue discoloration of the skin called cyanosis would become evident.
Though an unprotected human would not long survive in the clutches of outer space, it is remarkable that survival times can be measured in minutes rather than seconds, and that one could endure such an inhospitable environment for almost two minutes without suffering any irreversible damage.
Any other burning questions? Let us know in the comments!