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International Space Station
Space Shuttle
Astronauts

Why does an astronaut in an orbiting spacecraft float?

Answer

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December 04, 2013 3:06AM

It is a popular misconception that there is no gravity in space. In fact, the gravitational pull on astronauts in the International Space Station (ISS) is nearly the full amount (about 90%) of the gravitational pull on the surface of the earth. While in orbit around the earth, the ISS is free falling, and everything inside the ISS is also free falling. In true free fall you feel weightless, but there is still gravity that is causing you to fall.

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Imagine you are in an elevator and the cables snap, and the elevator starts falling. Disregarding air friction, you and everyone in the elevator will all fall at the same speed as the elevator. You will be able to "float" weightlessly inside the elevator, just as astronauts do inside the ISS.

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In the case of the elevator, we know eventually you will meet an untimely end when the elevator strikes the ground floor. But unlike the elevator, the ISS is also moving sideways at a very high speed, so as it falls downward, it also travels sideways, and so the ISS follows the path of a circle. The shape and size of this circle is such that the ISS never gets any closer to the earth, even though it is "falling". The ISS is free falling in an orbital path.

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To understand how the ISS is free falling, follow this mental experiment. Disregard air friction, and assume you have unlimited strength. Imagine throwing a baseball level to the ground - that is, you don't throw it up or down, but it is travelling parallel to the ground when it leaves your hand. We all know from experience the ball will travel a ways before falling to the ground. Now, throw it twice as hard. It will travel twice as far before hitting the ground. Now imagine being able to throw the baseball so hard that it lands 6000 miles (10,000 Km) away. But 6000 miles away is a quarter of the way around the spherical planet. So your ball did not go in a straight line - it was falling all the time it was in flight, but because the earth was curving away, the ball also travelled in a curved line while in free fell. Finally, imagine throwing the ball so hard that, as it fell downwards, the earth curved away at the same rate, and so the ball whizzed around the earth and hit you on the back of the head.

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We know that a ball cannot hit you on the back of the head, even if you could throw it that hard, because of air friction and obstacles in the way, like mountains. But at 250 miles up, there is (almost) no air friction, and no obstacles to prevent the ball from circling the earth. That is exactly what happens to the ISS - the rockets push (throw) the ISS so hard and so fast in a sideways direction that, as the ISS falls, the earth curves away from it at exactly the same rate, and so the ISS really does fall without actually getting closer to the earth.

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That is why when they make movies (like Apollo 13, directed by Ron Howard), they were able to duplicate true weightlessness. They put the actors in a set inside an jet airplane, and flew the airplane high up, cut the engines, and allowed the airplane, and everyone in it, to free fall for a while. During the airplane's free fall, everyone was weightless, exactly as if they were on the ISS.

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Indeed, we know from Newton's first law of motion that a body in motion moves at the same speed and in the same direction unless a force acts upon it. So we know that the ISS would go in a straight line, NOT in orbit, if no forces were acting upon it. But it does follow a circular orbit, and so we know for certain that that full force of gravity is still acting upon the ISS and all of its occupants.