Speed of International Space Station

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Discussion Overview

The discussion centers on the speed of the International Space Station (ISS), specifically why it operates at approximately 7.7 km/s and the implications of reducing its speed. Participants explore the relationship between orbital speed and altitude, as well as the consequences of altering the ISS's velocity.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the speed of an object in orbit is determined by its orbital height, referencing the formula v ≈ √(G M/r).
  • Others argue that if the ISS were to slow down, it would need to be at a higher orbit to maintain a stable trajectory.
  • One participant notes that the speed required to escape Earth's gravity differs from the speed needed for a stable orbit.
  • Another participant questions the necessity of stating "negligible mass" in the context of the orbital speed equation.
  • There is a discussion about the implications of the ISS firing its thrusters to slow down, suggesting it would fall into the Earth's atmosphere.
  • Some participants clarify that the equation used is specific to stable orbits and may not apply universally to all orbital scenarios.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the implications of changing the ISS's speed and the technical details of orbital mechanics. The discussion remains unresolved with no consensus reached on all points raised.

Contextual Notes

Participants note limitations in the discussion, such as the dependence on specific assumptions about mass and orbital behavior, and the need for caution when generalizing the orbital speed equation.

Israr
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Why ISS need to run on such high speed 7.7km/s ?
What will affect if it made to run on very slower speed.

thanks
 
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Hi Israr. :smile: http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Good luck with your study of science.

Technical questions belong in the technical forums. I've moved your thread to a suitable forum where it can be appropriately discussed.
 
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Israr said:
Why ISS need to run on such high speed 7.7km/s ?
What will affect if it made to run on very slower speed.

thanks

Hi Israr, welcome to PF.

The speed of any object in orbit in a gravitational field (planets around stars, moons around planets, the ISS around the Earth) is determined by the orbital height of the object.

For a circular orbit, and something of negligable mass compared to the other body (e.g. the ISS around the Earth, or the Earth around the sun) this is roughly equal to

##v \approx \sqrt{\frac{G M}{r}}##.

Where G is the gravitational constant, M the mass of the body being orbited around, and r the radius. If you plug in the mass of the earth, and the radius of orbit of the ISS (~412 km above sea level) into that equation, you see that you get 7.7 km/s!

So, you see that if you wanted the ISS to go at a much slower speed, you'd have to push it much further away from the Earth!
 
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Israr said:
What will affect if it made to run on very slower speed.

If the ISS were to fire its thrusters and slow down, it would fall into the Earth's atmosphere.
 
paisiello2 said:
https://en.wikipedia.org/wiki/Escape_velocity

The speed to completely escape Earth's gravity is something very different from the speed to be in a stable orbit.

@Israr , the ISS *could* fly slower, but in order to still have a stable orbit (i.e. neither falling towards Earth nor flying away from it) the station would have to be in a higher orbit. The higher the orbit, the slower the speed necessary (as evidenced by the formula e.bar.goum quoted)
 
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rumborak said:
The speed to completely escape Earth's gravity is something very different from the speed to be in a stable orbit.
You're absolutely right, I erroneously thought they were the same thing. Why do you use ≈?
 
I don't, that was e.bar.goum.
 
paisiello2 said:
...Why do you use ≈ ?

Did you read the sentence immedialy prior to that equation that e.bar.goum wrote ?
it answers that question

Dave
 
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The equation is derived by equating the centrifugal force to the gravitational force. The small mass m cancels out so I don't why it needs to be "negligible mass".

For a circular orbit, which the ISS is pretty close to, this is exact.
 
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paisiello2 said:
The equation is derived by equating the centrifugal force to the gravitational force. The small mass m cancels out so I don't why it needs to be "negligible mass".

For a circular orbit, which the ISS is pretty close to, this is exact.

Yes, but you can only make that equality for stable, well behaving orbits. If the masses of the bodies are similar, and the orbits are eccentric you'll get into trouble.

As you say, for the ISS it's pretty much exact, but I didn't want to give the impression that this is true in general for all orbits, hence the caveats.
 
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  • #12
e.bar.goum said:
Hi Israr, welcome to PF.

The speed of any object in orbit in a gravitational field (planets around stars, moons around planets, the ISS around the Earth) is determined by the orbital height of the object.

For a circular orbit, and something of negligable mass compared to the other body (e.g. the ISS around the Earth, or the Earth around the sun) this is roughly equal to

##v \approx \sqrt{\frac{G M}{r}}##.

Where G is the gravitational constant, M the mass of the body being orbited around, and r the radius. If you plug in the mass of the earth, and the radius of orbit of the ISS (~412 km above sea level) into that equation, you see that you get 7.7 km/s!

So, you see that if you wanted the ISS to go at a much slower speed, you'd have to push it much further away from the Earth!
Thanks
 
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