-> Such a condition exists only for Earth's moon, or so I thought.  It may have 
-> been that at the time Clarke made his argument, those outer moons of Jupiter 
-> had not yet been discovered? 
  
Maybe so. The condition, for a jovian satellite, boils down to the 
orbital radius being more than about 23 million kilometres. Satellite 
IX (Hades) exceds this limit, and is listed in my 1971 issue of the 
Rubber Bible. I don't know exactly when it was discovered. I am sure 
that other satellites, even further out, have been discovered since. 
When did Clarke make his argument. 
  
Maybe there are Saturnian satellites that meet the appropriate 
condition, too. 
  
                             dow 
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