->  DW> I have seen it suggested that the Earth-Moon sustem would be 
->  DW> considered a double planet if its centre of gravity were 
->  DW> outside the Earth. But it isn't. 
 
-> Are there pairs of asteroids that might satisfy this criteria? 
  
Criterion. (We are being picky about language.) 
  
I don't know of any for sure, but I imagine that there are orbiting 
pairs of asteroids in which the centre of gravity of the pair is 
somewhere in space between the two bodies. So I suppose they should be 
called double asteroids, according to this criterion. 
  
I thought a bit more about the conditions for a satellite having an 
orbit that is always concave toward the sun. Of course, it boils down 
to the strengths of the gravitational pulls of the planet and the sun 
on the satellite. If the sun attracts it more strongly than the planet, 
then the satellite's path will bend toward the sun, and away from the 
planet, when the satellite passes between the planet and the sun. So 
the condition is: 
  
M / (R^2) > m / (r^2) 
  
where M is the mass of the sun, m is the mass of the planet, R is the 
radius of the planet's orbit around the sun, and r is the radius of the 
satellite's orbit around the planet. 
  
The Earth-Moon system system satisfies this condition. So do some of 
the outermost satellites of Jupiter. But the Pluto-Charon system does 
not. 
  
                          dow 
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