Why do planets remain in stable orbits?

25 June 2013


In appearance a simple question: why do planets remain in their orbits, almost forever, in an apparently magic equilibrium of gravitation and centrifugal forces?
I understand about these forces (or so I think ;-), but what is it that makes planets *remain* in their orbits, even given the gravitational disturbances of other planets, comets or whatever else. It seems to me that, once launched in their orbit, they should either fall down to their Stars fairly quickly (gravitation > centrifugal force), or go away from their star forever. It cannot be that they have just the right mass, speed, distance from their star by chance. So, what makes them stay there? In the same way, I understand that, fairly regularly, the ISS has to be powered up so that it remains at an acceptable altitude and doesn't gradually fall down. So then, why can't we make the ISS turn around the earth for 100000 years without intervention in the way planets do?
Thank you, and thank you for this always interesting and enjoyable podcast!


Transcript to follow.

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