From: Monte Davis 

"Geo"  wrote:

>I thought you could go into orbit at
>150 miles?

Yes, you can -- and in that orbit, you weigh >95% of ground weight,
meaning you're falling at ~31 feet/sec/sec. You don't hit because you're
also moving sideways at 17,500 mph... so as you fall, the earth's surface
below is curving "down and away" at a rate that exactly
compensates. You're falling *around* the earth.

It's simple accuracy, not pedantry, to describe orbit as "free
fall" rather than "weightlessness" or "zero-g."
The floating-around part is because the astronaut, vehicle and everything
in it are all falling at the same rate. (the old broken-elevator-cable
trick, it just doesn't stop)

Think about it: you're 3960 miles from the earth's center right now. Even
with an inverse-square force, 150 miles up is a pretty small increment;
you'd need to be 3960 miles up for 2x the distance, 1/4 the gravity.

So a low orbit is mostly about velocity and only a little about weaker
gravity. As you go up, that balance shifts. A satellite in GEO needs about
7000 mph, and the moon lazes along its orbit at maybe 2000 mph. They are
ALL FALLING, ALL THE TIME.

(There, don't you worry less about terrorism now?)

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