ISS IS FALLING?

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The International Space Station (ISS) a symbol of human achievement and cooperation in space exploration, which orbits earth in low earth orbit. It is developed by the collaboration of five space agencies, NASA(USA), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA (Canada).

Credits: NASA

Its purpose is to perform experiments in microgravity and space. ISS is the largest structure humans put into space. It orbits 16 times around the Earth in one day, orbiting the Earth in 90 min with a velocity of approximately  8 Km/sec.

Let’s discuss the title of the blog now. Well ISS is not literally falling, not until 2030 at least.  ISS flies at an altitude that ranges from 370 Km to 460 km, which seems quite far from Earth but when you compare it relative to the distance between Earth and the moon it does not seem that high. If you calculate the acceleration due to gravity (g) at this altitude it comes out to be  8.75 m/s2 (for reference g at earth’s surface is 9.81 m/s2), which means astronauts can still feel 90% of earth’s gravity.

Then how are astronauts we see on ISS experiencing zero gravity?

Well answer to this question is very simple, they are continuously free-falling to the earth but never hit it. Let’s now understand what it means. Imagine you throw a ball in the forward direction. It will travel a certain distance and then hit the ground because of earth’s gravitational force is pulling the ball downward. You can see that the path traveled by the ball is curved, which is the resultant path caused by the combined effect of gravitational force and forward force through which you throw the ball.



Now if you keep on increasing the forward velocity of your ball it will reach a certain velocity at which the curved path traced by the ball will match the curvature of the earth and thus the ball will never fall to the ground. Because of this reason, astronauts on the International Space Station feel zero gravity, they and the ISS are in a continuous free fall towards Earth.

Now you must think that it must require a lot of fuel to maintain this high velocity. Well, the simple answer is no, air at 400 km altitude is very thin which provides very little resistance to our spacecraft (in this case ISS). So, once we provide a certain forward velocity to it, it will maintain that velocity until or unless some external force is applied to it (Newton's first law of motion).  

Credits: NASA

The air is thin which provides little resistance but the resistance is not zero, so the ISS loses its velocity and altitude over the period. The ISS loses up to 5 cm/s (0.1 mph) of velocity and 100 meters (330 ft) of altitude each day. Thrust burners are used to further increase the velocity and maintain the altitude of the ISS.

The upward spike in the picture above occurs when these thrusters are used.

Thank you very much for reading! I hope you learned something new. Stay curious!