Re: gravity and humans

The amount of gravity or G-force tolerable by humans really depends on how 
long they're expected to tolerate it. In a general sense the shorter the 
time you have to tolerate it, the greater a force you can stand.

Fighter pilots routinely pull 8 or 9 Gs (8 or 9 times the force of gravity 
on Earth) fow a few seconds at a time when manuvering or dogfighting, 
they're aided by "G-suits"--inflatable trousers which inflate during high-G 
manuvers and prevent blood from pooling in the legs.

Early NASA research had astronaut volunteers briefly experience 15 Gs or 
more in centrifuges. About 20 Gs is probably the maximum for brief periods 
for a physically fit, healthy individual. Body position influences how much 
can be tolerated.

For longer periods (minutes or hours), researcher Stephen H. Dole in his 
book "Habitable Planets for Man" cites an Air Force study which implies 
that anywhere above 2.2 Gs a healthy human will quickly fatigue and be 
unable to perform useful work. This limits how high an acceleration can be 
used for long-duration space missions as well as how much gravity an 
otherwise habitable planet can have.

At the other extreme, the Guinness Book of World Records lists a man who 
fell off a cliff onto a rock shelf and thus experienced a _momentary_ 
estimated G-force of 209 on impact. However, he was vary badly injured and 
was undoubtedly extremely lucky to survive.