Every time you read about a shuttle landing they mention the double sonic  
booms.  Having taken various relevant classes, I have several ideas of where  
they come from, but none of them are very convincing.  Exactly what causes  
them?  Are they a one time pheneomenon, or a constant one like the supersonic  
shockwave that is constantly produced by a plane, but you hear only when it  
goes over you?

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