Gravity
On Earth, our feet remain firmly on the ground, held there by the downward force of gravity. The magnitude of the force of gravity between two objects is directly dependent on the mass, or amount of matter, in each object, and inversely dependent on the (square of the) distance between them. Because of the Earth's large mass, relative to ours, we stay attached to the surface. However, the further we distance ourselves from the surface of the Earth, the weaker the pull of gravity becomes. When an astronaut is in space, that pull of gravity becomes just slight enough that all it takes to practically eliminate it is for the spacecraft to orbit , or circle, the Earth. This creates a centrifugal force which balances out the gravitational forces on the spacecraft and on the crewmember (the spacecraft and the crewmember are said to be in free fall). If the spacecraft were not orbiting the Earth with enough velocity to balance out the pull of gravity, it would fall back down to Earth.As a spacecraft orbits the Earth, the astronaut experiences a condition known as weightlessness because nearly all of the forces on the body are balanced. If astronauts could somehow step on a normal bathroom scale in space (although they could not do this without tying themselves down), the scale would read zero. Because of weightlessness, many actions that are impossible on Earth become possible in space. For instance, the crewmembers can turn somersaults with ease as they float through the spacecraft. Very heavy objects that might take two or three strong adults to move even and inch on Earth can be moved with a "pinky" in space. If objects such as tools are not securely fastened (Velcro is often used), they drift hazardously about the spacecraft cabin. It is important to note that, in reality, gravity can never be completely eliminated. Therefore, it is not correct to refer to this phenomena as "zero-g." The correct term is microgravity. However, the level of gravity is so small in space that only the most sensitive instruments are capable of measuring the gravitational force present in an orbiting spacecraft.
+ Humans in Space: Environmental Challenges (Back) + Hangman
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