The only difference is the ISST is 4220 my and the surface of Earth is 4000 my from CM. Centripetal force holds a body to the surface and centrifugal force allows man to stand up. Why does the same mechanism not work in orbit as on Earth? People on Earth and astronauts are orbiting the same CM. At first I thought it might be the weight of the atmosphere; but then I remembered the Moon: it has no atmosphere. Then I thought of the outward pressure due to the heat at the Earth CM; I'm not sure about the Moon.

Not sure if we are on the same wave length here but gravity holds you on the surface of the earth. Centripetal force is Centripetal Force. A reactive centrifugal force is the reaction force to a centripetal force. The moon has almost no atmoshere but it does have gravity, much less than earth so the astronaut will only "weigh" a fraction of what he weights on earth. In out space there is neither gravity or atmosherer but the lack of gravity is what allows him to float. Put that guy in a spinning space station and the spin will create centripetal force causing him to have weight

Astronauts float when they are outside the gravity pull of the earth. Gravity on earth has been set at 1. On smaller or larger orbs it is a variable of mass. Gravity on the moon is 0.6 and on Jupiter is 2.5. On earth 100#, moon 60#, Jupiter 250#. In between there is no gravity per se.

In addition to what ballengerb1 pointed out (that it's gravity, not centripetal force), I should point out to you that astronauts and space stations are very much affected by gravity. As you rightly surmised, since they're only a few hundred miles from the earth's surface, gravity is almost as strong up there as it is a sea level. So why do they feel weightless? Because they are constantly falling toward the earth. They feel weightless for the same reason that a skydiver feels weightless when he or she first jumps out of an airplane (before wind resistance limits their acceleration). Because they're free-falling!

So if it's constantly falling, why doesn't the space station plummet to the ground? Because it's moving sideways so fast! Imagine for a minute that you're sitting in an orbiting space station looking directly down at the surface of the earth through a window, and imagine that gravity suddenly disappeared. The space station would continue onward in a straight line (parallel to the surface of the earth at the instant when gravity was shut off). At first it would seem like you were maintaining your altitude, but since the earth's surface is curved, not straight, you'd find that after a little while the ground beneath you would be getting further and further away. You'd be traveling in a straight line, but the curved earth below you would be "dropping away" until you reached the planet's edge, at which point you'd be looking "down" directly along the horizon a quarter of the way around the planet from where you were first released from gravity and some 7000km below you.

So now if you imagine back to the realistic scenario where there IS gravity, you can see that while the space station is falling toward the earth's surface, the earth's surface is simultaneously dropping out from underneath it as it moves sideways. The two counteract each other exactly, and the space station orbits around the earth at a nearly constant altitude. That's why the space station never actually falls to the ground, but yet its occupants feel weightless.

ma0641, There is DEFINITELY gravity in between. To travel among any of those orbs you'll be fighting against (or benefiting from) gravity the whole way. Also, just for the record, the coefficient of gravity on the moon is around 0.17, not 0.6.

And since you mentioned CM, it's gravity that creates the centripetal force. If gravity didn't held on the moon, the moon would have long been thousands of billions of miles away (maybe even more)

Also, when we say the gravity on the moon is 1/6th that on the earth, it's 1/6 = 0.16666667, rounded giving 0.17 :)

And it's a false conception that there is no gravity in between such and such places. The Sun's gravity pulls the Earth, and it even pulls Pluto and other asteroids from very far away. The centre of the galaxy in turn pulls in every system and stars in our galaxy. Galaxies 'pull' at each other too to some extent.