That's a tricky question.
Here's the real reason that astronauts don't feel gravity. If you
think about it, they're not that much farther away from the center of
the earth than we are when you think of how thick the earth is. But the
astronaut's ship is moving so fast that when it falls due to gravity,
it falls "around" the earth instead of straight down toward it.
Now you night say, "I might buy them falling around the world, but
why don't they feel like they are falling?" Well, they are falling but
the ship is falling too. If everything is falling at the same rate,
then it appears to be weightless.
People on earth can sometimes feel almost weightless too. Some
people go skydiving. Many more ride roller coasters. In these
activities, if we are ever moving down with the same acceleration as
something that is just dropped, we feel weightless.
(published on 10/22/2007)