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What makes a trajectory (orbit) bound?

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What makes a trajectory (orbit) bound?
 

enigma

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Not exactly sure what the question is.

Gravity?
 
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What are the conditions on energy? Does it have to be less than or equal to 0?

Also what makes an orbit closed?
 

HallsofIvy

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Yes, if the total energy (kinetic energy plus potential energy) is negative (In orbital problems, potential energy is typically taken to be 0 at infinity) then there is not enough energy for the vehicle to go "to infinity" so the trajectory must be closed.
 
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The energy by itself determines whether an orbit is bound or not, but not whether it is closed. If the ratio of the orbital period to the period of radial oscillations (something like this...) is not an integer, then the orbit does not eventually have to close in on itself.
 

HallsofIvy

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True, but assuming an inverse square law acceleration (i.e. gravity) the only bounded orbits are ellipses and so are closed.
 

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