The International Space Station orbital speed

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The International Space Station orbital speed


Let's determine the orbital speed and period for the International Space Station (ISS):

ISS orbits about 400 km above the Earth's surface, which means that radius of its orbit is RE (Earth's radius) + 400 km. 

It is easier to determine orbital speed and orbital period of satellite for circular orbits. We have satellite of mass m in a circular orbit about Earth at distance r from center of the Earth. It has centripetal acceleration directed toward the center of the Earth. Earth's gravity is the only force acting, so Newton's second law gives 


Which re-arrange into vorbit = (GME / r)^1/2

To find the period of a circular orbit, we note that the satellite travels the circumference of the orbit 2πr in one period T. Using the definition of speed, we have vorbit=2πr/T, which gives T = 2πr/vorbit. 

Let's calculate orbit speed for ISS. 




so, the orbital speed is 767 m/s (compare to car on the highway 120 km/h = 33.3 m/s).
Orbital period is


which is 555 s, which is over 90 minutes. 


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