Bungee jump from a bridge or stall an aircraft, and you will feel as if your weight is reduced to zero. Astronauts experience the same sense of apparent weightiessness during space missions, where it is a source of both enjoyment and discomfort. The term "zero gravity "is sometimes applied to this state. Like the term "weightiess," however, it is somewhat misleading. Gravity is still present—gravity is what keeps the spacecraft in orbit. And although astronauts do not feel their own weight-in space, they are still attracted by Earth's gravitational field. What causes the sense of weightlessness is the fact that the astronauts and the spacecraft are falling together toward the Earth.
Under such conditions, you do not walk, but float from place to place. Objects drift about and heavy items are easily lifted. Astronauts may experience temporary nausea, called space sickness, but more serious is a loss of muscle tone and bone strength, and a decrease in white, blood cells, which fight diseases. To deal with these effects, astronauts must perform special exercises during long space flights.
