If gravity is exerting a force on an astronaut in space, why does he or she experience the sensation of weightlessness?

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There is definitely gravity, and a lot of it, in space.  It is the invisible force that binds huge bodies, like the Earth and its Moon, and that has similar effects throughout the known universe.  Gravitational pull, however, varies widely throughout the expanse of space, and, as we know from the history of rocketry, there are ways to overcome even enormous levels of gravity.  Weightlessness, then, is a product of man’s ability to create machines that are strong enough to overcome the force of gravity, mainly using rates of acceleration that exceed gravity’s pull.  Man-made objects sent into space, rockets, satellites, the International Space Station (and its predecessors, Skylab and the Russian Mir space station) all use high-levels of speed to counter the effects of Earth’s gravity.  By traveling at speeds of around 17,000 miles per hour, these man-made objects are able to maintain a constant distance from the Earth without being pulled down towards it.  This same use of acceleration and speed creates the weightlessness that astronauts experience when in space. 

Astronauts in space are, in fact, falling towards the Earth.  Gravity is pulling them down, using what is known as centripetal force, which provides for the curvature necessary to maintain an orbit around the Earth.  The speed at which they are moving combined with the gravitational pull of a spinning planet produces the balance necessary for that sense of weightlessness.  One needs to keep in mind that space stations like the current International Space Station function in low earth orbit, where Earth’s gravity remains substantial.  Mathematical calculations are used to determine the precise characteristics of an object required to maintain a steady orbit around the Earth.  As noted, the space station has to maintain a steady speed of 17,200 miles per hour to maintain its orbit.  That is the amount of velocity needed to counter the specific amount of gravity at its 220-250-mile high orbit, which causes it to complete an orbit of the Earth every 92 minutes.

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