Why Does the ISS Not Fall to Earth?
The ISS doesn’t fall to Earth because it is moving forward at the right speed that matches the curvature of the Earth.
How do you explain in a simple language why the ISS doesn't fall to the earth? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.
How do you explain in a simple language why the ISS doesn't fall to the earth?
One of the simplest explanations is provided by Sir Isaac Newton in his book Treatise on the System of the World. Sir Isaac describes a thought experiment involving a projectile, such as a cannon ball. He says that if we propel that cannon ball sideways, parallel with the ground, the cannon ball travels a curved path towards the ground, because of gravity. He then says, suppose we increase the velocity at which we eject that projectile. The more we increase that initial speed, the farther the projectile will be able to travel before it hits the ground.
Sir Isaac then imagines we take our cannon up to the top of a very high mountain, so that not only will there be no obstacles in the path of the projectile, but the air will be so thin that it will offer little resistance to the projectile. Does it not then make sense that we could fire our projectile at such a speed that the curvature of its path would match the curvature of the Earth and the projectile would never fall to the ground?
That’s the conceptual essence. The ISS doesn’t fall to Earth because it is moving forward at exactly the right speed that when combined with the rate it is falling, due to gravity, produces a curved path that matches the curvature of the Earth.
Newton went a bit further, though. He figured out the math. Stop here if math gives you hives. He said that for this scenario, the force of gravity on the ISS equates to the centripetal force of the ISS traveling in a curve around the Earth.
Gravity pulls the object towards the center of the planet and also provides the acceleration that forces the object to travel in a circular path. The result being, that an object with a certain velocity will achieve stability when it is at a distance from the center of the planet where the equations balance.
So,
v is the velocity at which the ISS has to travel to stay on that curved path that just keeps missing the Earth. The variable in that equation is “r” which is the distance between the ISS and the center of the Earth.
The higher a spacecraft is, the weaker the pull of gravity and thus the slower it has to travel to not fall to Earth. The lower a spacecraft is, the stronger the pull of gravity and thus the faster it has to travel to not fall to Earth.
So at the altitude of the ISS (250 miles (~400 km) above the Earth’s surface or 4210 miles (~6770 km) above the center of the Earth), the required velocity is about 4.7 miles per second (7.6 km/s). By traveling forward at 4.7 miles per second (7.6 km/s) and pulled by gravity into a curved path, the ISS travels around the world at a fixed distance.
But, remember that Sir Isaac said we needed to be high up to avoid the drag of the air, well even at 250 miles (400 km) there is still a tiny bit of air and it has a small decelerating effect on the ISS. The ISS loses up to 0.1 mph (5 cm/s) of velocity and 330 ft (100 meters) of altitude each day, because of the continual collisions with gas particles. To compensate for this, about once a month the ISS briefly fires its thrusters to regain the lost altitude.
This question originally appeared on Quora. More questions on Quora:
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