Gravity on the International Space Station: or lack of it.


May 10, 2021
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A question that is often asked is one that concerns the conditions of zero gravity, or more accurately micro gravity (10^-6 times the gravity on earth), on the International Space Station (ISS). According to calculations the space station should experience a force of gravity that is about 89% of the gravity on earth. This means there is no way in which objects should be able to float in space. What is happening? Surprisingly, it was Isaac Newton, who in the Seventeenth Century provided the answer.

Newton, in his thought experiment used the analogy of a powerful gun placed on top of a mountain. If the gun was fired with a moderate charge in place, the projectile would travel for a short distance and then descend to the earth in a parabola. If a more powerful charge were used the projectile would travel further before coming to earth. If an even more powerful charge were used the projectile would not come down at all but continue to circle the earth. (i.e., go into orbit). Finally, if the charge were strong enough the projectile would attain escape velocity and leave the earth altogether.


This concept might still be difficult to grasp. Think of a boy throwing a ball into the air. At its highest point, or apogee, the force with which the ball was thrown would come to zero, but before that the two forces, the gravitational force and the propulsive force applied to the ball would cancel out exactly, and at this point there would be zero gravity. Right? Wrong! Instead, it is found that whenever an object is travelling with an acceleration equal to that of the earth’s gravity, conditions of zero gravity would prevail. This in turn means that all objects falling solely under the force of gravity observe zero gravity. What does this have to do with the ISS? A fairly simple calculation using v = u + at shows that the speed of the ISS exactly matches (or more accurately, approximately matches) the speed due to the acceleration of gravity and therefore conditions of zero gravity or micro gravity prevail on the International Space Station.