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**Physics, Astronomy & Cosmology / Re: How to calculate the orbital speed of the ISS around the centre of the earth?**

« **on:**03/05/2021 18:20:12 »

If you want to work it out via orbital mechanics, the equation is v^2 = u(2/r-1/a), also known as the vis-viva equation.

u is the gravitational parameter or GM (with M being the mass of the planet). For the Earth, it is 3.987e14 m^3/s^2

r is the present distance of the orbiting object from the center of the Earth.

a is the semi-major axis of the orbit, or average orbital distance from the center of the Earth (this allows you to account for elliptical, non-circular orbits.)

For a circular orbit, r=a, and the equation reduces to V^2 = u/r

With the numbers given in the OP, and assuming a circular orbit, you get 7.652 km/sec.

However, the actual radius of the Earth (at the Equator) is 6378 km. making this slight correction gives 7.664 km/sec, which when rounded up to two significant digits, gives 7.7 km/sec

The Wiki article on the ISS lists its orbital speed as 7.66 km/sec, which either of the two above answers would round out to.

Also, using the smaller, more accurate radius of 6378 km, and using chiralSPO's method, we get 7.64 km/sec, much closer to to the given value.

u is the gravitational parameter or GM (with M being the mass of the planet). For the Earth, it is 3.987e14 m^3/s^2

r is the present distance of the orbiting object from the center of the Earth.

a is the semi-major axis of the orbit, or average orbital distance from the center of the Earth (this allows you to account for elliptical, non-circular orbits.)

For a circular orbit, r=a, and the equation reduces to V^2 = u/r

With the numbers given in the OP, and assuming a circular orbit, you get 7.652 km/sec.

However, the actual radius of the Earth (at the Equator) is 6378 km. making this slight correction gives 7.664 km/sec, which when rounded up to two significant digits, gives 7.7 km/sec

The Wiki article on the ISS lists its orbital speed as 7.66 km/sec, which either of the two above answers would round out to.

Also, using the smaller, more accurate radius of 6378 km, and using chiralSPO's method, we get 7.64 km/sec, much closer to to the given value.

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