Patrick O'Brian Discussion Forum

Re: ‘In relation to the planet Saturn, what is the Cassini Division?’ . .

Bob Bridges

Happens I'm currently reading a series of Asimov essays (collected in The Road to Infinity, most of the essays written in 1979), in which last night I read his comments on the Roche limit.  I'd learned of the Roche limit while reading one or more of Robert L Forward's novels, at least Rocheworld but I think one other as well mentioned it.  So I was aware of the fact that if two large bodies orbit around each other too close, one or both of them may break up, pulled apart by tidal forces.  I wouldn't have remembered the actual number, but Asimov says it's 2.44 times the radius of the other body.  So if our moon were only 15.5km away from earth, it would break up.  And if we were only 4200km apart, both bodies would be torn apart.

What I never thought about until reading that Asimov essay is that the same limit works the other way too:  Any collection of small bodies that might coalesce into a single moon won't have the chance if they're too close to the planet they orbit.  And it happens that all of Saturn's ring structure is inside Saturn's Roche limit.

On Tue Feb 13, Chrístő wrote
>. . visit: to find the answer.

>An easy one for amateur astronomers

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