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From: jlg@lanl.ARPA
Newsgroups: net.space,net.astro
Subject: Re: L5
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Date: Thu, 24-Jan-85 12:32:13 EST
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Posted: Thu Jan 24 12:32:13 1985
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Xref: watmath net.space:3536 net.astro:473

>      I'll give it a try.  A stable point is where there is no
> acceleration relative to the Earth and Moon.  This requires that
> forces be balanced so there is no net force to produce an
> acceleration.  In the case of the L2 point this is easy to see,
> since it is between the bodies, there is somewhere where the
> attractions are equal and opposite.


An equilibrium point is one where the forces ballance to zero.  Such a
point is not necessarily stable.  A stable equilibrium point is one where
a slight deviation from the correct location will not cause the unballanced
forces to push you away from the equilibrium point.  For orbital systems,
L1, L2, and L3 are unstable equilibria and L4 and L5 are stable equilibria.
This is why L4 and L5 are the locations most often proposed for space
stations - they don't have to continuously expend propellant to keep
their position against orbital purturbations.