|MadSci Network: Physics|
The condition referred to as weightless occurs once the space shuttle has achieved orbit.
The force between two objects is inversely proportional to the square of the distance between the objects' centers. Considering this relationship, we can never achieve a condition of true weightlessness (zero force). In reality, what is referred to as weightlessness on a Shuttle mission is actually a microgravity condition.
There are two ways to create a microgravity condition.
To reach a point where Earth's gravitational pull is reduced to one-millionth of that at the surface, you would have to travel into space 6.37 million kilometers from the Earth (almost 17 times farther away than the Moon). This is not very practical.
A condition of microgravity also comes about whenever an object is in free fall; that is, it falls faster and faster, accelerating with exactly the acceleration due to gravity. The state of continuous free-fall is called orbit. Objects in a state of free-fall or orbit are said to be 'weightless.'
The Space Shuttle is launched in a trajectory that arcs above Earth so that the orbiter is traveling at the right speed to keep it falling while maintaining a constant altitude above the surface. For example, if the Shuttle climbs to a 320-kilometer-high orbit, it must travel at a speed of about 27,740 kilometers per hour to achieve a stable orbit. At that speed and altitude, the Shuttle's falling path will be parallel to the curvature of Earth. Because the Space Shuttle is free-falling around Earth and upper atmospheric friction is extremely low, a microgravity environment is established.
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