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The Question

(Submitted April 19, 1997)

Do the laws of motion apply in space. Why is it that when in a space shuttle with no gravity, the people in the shuttle move with the shuttle?

The Answer

Yes, the laws of motion do apply in space. When the shuttle is launched it is maneuvered into a low Earth orbit-it is orbiting once around the Earth about every 90 minutes. It is following Newton's laws in that orbit around the Earth, just as all other Earth satellites, including the Moon, do. The period of the orbit squared is proportional to it's orbital semi-major axis (it's distance from the center of mass of the system) cubed. Geosynchronous (one orbit a day) satellites are therefore in a higher Earth orbit, while the Moon is still further from Earth. The shuttle and the astronauts are still gravitationally bound to the Earth, or they would not orbit. So it is not correct to think of the shuttle with "no gravity". The shuttle, the astronauts, and any equipment they have out are all orbiting the Earth in the same orbit, so the astronauts do not feel gravitationally bound to the shuttle, though they are still gravitationally bound to the Earth. This is why they float freely within the shuttle, and why they move with the shuttle.

Padi Boyd,
for the "Ask an Astrophysicist Team"

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