On 2010-10-11 06:21:06 -0500, Vince M Hudd said:   
   > Jeffrey Kaplan  wrote:   
   >> Previously on rec.arts.sf.tv.babylon5.moderated, Andrew Swallow said:   
   >    
   >>> Is a spacestation at L1 a good idea? - Yes.  EML1 is the natural   
   >>> crossroads in space.  This makes the EML1 spacestation a good place to   
   >>> change spacecraft.   
   >    
   >> Why is a Lagrange Point a "natural crossroads"? What is it about one of   
   >> these points that make it better for such use than merely someplace in   
   >> orbit?  For that matter, what IS a Lagrange Point? I've heard the name,   
   >> but never understood what it is or why it's a good thing.   
   >    
   > A Lagrange Point is a point in space relative to an orbital pair - such as   
   > Earth and Moon, or Sun and Earth - where the various mutual forces cancel   
   > each other out such that a much smaller object can remain there unaffected.   
   >    
   > I think there are five for each pair:   
   >    
   > One sits between the two (closer to the smaller of the pair than the   
   > larger). This one is probably the easiest to understand, because the biggest   
   > factor is the two gravitational pulls. This is the one Andrew was referring   
   > to as EML1 - Earth/Moon Lagrange Point 1; being directly between the two   
   > bodies (obviously closer to the Moon than Earth) it's a logical place to put   
   > a 'stepping stone' between Earth and Moon.   
   >    
   > Another sits *beyond* the smaller of the two bodies. Here, anything in the   
   > Langrange point is orbiting the larger object on the same orbital period as   
   > the smaller one - and the gravitational pull acting on it is the combined   
   > pull of both of them, which is counter-acting the centripetal force brought   
   > about by its orbital speed.   
   >    
   > Another sits on the same line, but on the other side of the larger object,   
   > and the same principal applies as with the second one.   
   >    
   > The other two sit at points along the orbital path of the smaller object,   
   > one ahead of it, and one following it. IIRC, their positions effectively   
   > form two equalateral triangles with the centres of the two bodies, and I'm   
   > not quite sure I understand the physics of these two. If I try to make sense   
   > of one, the other makes my head explode because it just seems wrong, so I   
   > just trust that the men in the white coats know what they're talking about.   
   I was a physics major way back in college and I studied this stuff. You got it.   
   The cool thing about L4 and L5 (the ones that make an equilateral    
   triangle) is that while L1, L2, and L3 are stable, if some outside    
   force, e.g. another planet, disturbs the object it won't stay in the    
   Lagrange point, in L4 and L5, they will simply end up "orbiting" around    
   the Lagrange point. It's not a true orbit in that there's no mass    
   necessarily at the Lagrange point and the orbit isn't Newtonian.   
   So while there can't be very much stuff at the L1, L2, and L3 points,    
   the L4 and L5 points can have a lot of different objects, each    
   displaced from the actual point and moving around it. It makes a nice    
   setting for space stations.   
   -K   
   --- SBBSecho 2.12-Win32   
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