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Union University Department of Physics

The Science Guys

Science Guys > October 2004

October 2004

The space shuttle astronauts are weightless when they are in orbit. Where does gravity stop?

The Space Shuttle has proven to be a fine laboratory, particularly for experiments requiring a ’zero-gravity’ environment. You commonly hear people refer to the weightlessness of the astronauts and zero-gravity. But, are they truly weightless and is gravity zero? The short answer is no, the shuttle astronauts are certainly not weightless as they orbit the Earth, rather only apparently weightless.

The Earth’s gravitational field extends well into space it does not stop. However, it does weaken as one gets further from the center of the Earth. The Shuttle orbits about 125 mi above the surface, roughly the distance between Jackson and Nashville! When you consider the radius of the Earth is 3800 mi, you realize that Shuttle orbits are relatively close to the Earth in comparison to the Earth’s size.

In fact, at 125 mi the gravitational field experienced by the astronauts is 94% of what they would experience on Earth! A 150 pound astronaut here on Earth would, in the Space Shuttle, have a weight of 0.94 x 150 = 140 pounds! This is by no means weightless, so why do we refer to astronauts as being weightless?

The answer lies with the Shuttle’s orbit. But, to answer that we need to use our imagination. Suppose you take a cannon to the top of a high mountain. You orient the cannon to fire a cannonball parallel to the Earth’s local surface and fire it at very high speed. As the cannonball travels away from the mountaintop, it falls in an arc-shaped path, and the Earth’s surface curves away from it. We normally ignore the curvature of the Earth, but we cannot here. The cannonball would leave the cannon and fall toward the ground but never land because it goes past the ’edge’ of the Earth! This happens because the Earth always curves away from the falling object. The cannonball would perpetually fall toward the Earth but never get to it.

Shuttle astronauts are freely falling, just like the cannonball in the above example. So, how does this produce apparent weightlessness? Consider another scenario. You are in an elevator with your Jackson Sun. The elevator cable snaps and the elevator falls with you in it. You become frightened and release your newspaper. You and your newspaper both fall toward the earth at the same rate, and the newspaper would not fall away from you, but ’float’ beside you. To you the floating paper would appear to be weightless. You would claim that gravity has ceased to act on the newspaper because it did not move away from you, yet gravity would still have you and the newspaper firmly in its grip. In the same way, the astronauts are still in the clutches of gravity and only appear weightless because they are falling continuously around the Earth.