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<blockquote data-quote="Umbran" data-source="post: 667931" data-attributes="member: 177">That &quot;minimum safe distance&quot; is called the &quot;Roche Limit&quot; for the system.&nbsp;&nbsp; It isn't exactly easy to calculate for &quot;real&quot; moons, especially when the two objects are roughly the same size, mass, and composition.To a first approximation, though - if the planet and moon are of similar composition (meaning similar density and material strength), then the moon's center should not be any closer than about 2.4 planetary radii (R) from the planet's center.&nbsp; Closer than that, and it risks being pulled apart by tidal forces, and you'll get rings rather than a moon.Now, here's where it gets sticky.&nbsp; If the two objects are of the same size (radius R),and there's oly 2.4R between their centers, this means that there's only 0.4 R between their surfaces.&nbsp; Let's say these are planets like Earth.&nbsp; Radius 3960 miles.&nbsp; That means the surfaces at their closest are 1584 miles apart.&nbsp;&nbsp; Only half the width of the North American continent.&nbsp; At that distance, the &quot;moon&quot; would be huge.&nbsp; If you were standing right under it, it would seem to be... something approaching 136 degrees of the 180 degrees of sky.Now, it's reasonable to say that the rough approximation will fail at such an extreme.&nbsp; However, Robert L Forward used some more sophisticated math to show that such a planetary system can actually be stable on geologic timescales.&nbsp; He uses them in the Rocheworld books.The planets are tidally locked, and cease to be spherical.&nbsp; They are more like eggs, and they orbit each other with their points facing each other, always.</blockquote>

[QUOTE="Umbran, post: 667931, member: 177"] That "minimum safe distance" is called the "Roche Limit" for the system. It isn't exactly easy to calculate for "real" moons, especially when the two objects are roughly the same size, mass, and composition. To a first approximation, though - if the planet and moon are of similar composition (meaning similar density and material strength), then the moon's center should not be any closer than about 2.4 planetary radii (R) from the planet's center. Closer than that, and it risks being pulled apart by tidal forces, and you'll get rings rather than a moon. Now, here's where it gets sticky. If the two objects are of the same size (radius R),and there's oly 2.4R between their centers, this means that there's only 0.4 R between their surfaces. Let's say these are planets like Earth. Radius 3960 miles. That means the surfaces at their closest are 1584 miles apart. Only half the width of the North American continent. At that distance, the "moon" would be huge. If you were standing right under it, it would seem to be... something approaching 136 degrees of the 180 degrees of sky. Now, it's reasonable to say that the rough approximation will fail at such an extreme. However, Robert L Forward used some more sophisticated math to show that such a planetary system can actually be stable on geologic timescales. He uses them in the [i]Rocheworld[/i] books. The planets are tidally locked, and cease to be spherical. They are more like eggs, and they orbit each other with their points facing each other, always. [/QUOTE]

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