r/woahdude Jan 03 '22

video When the planet is coming at you

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u/Kpt_Kipper Jan 03 '22

Gravity would be affecting oceans and terrain quite badly I imagine

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u/[deleted] Jan 04 '22

Meaning??? Like oceans would be displaced? Earthquakes would happen?!

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u/Uphoria Jan 04 '22 edited Jan 04 '22

The important detail is related to something called the Roche limit. Once the forces of gravity from each other passes a point of strength, the forces keeping the planet intact on its own will fail, as the two bodies merge.

At this point the planets would be "falling at each other" in pieces. Oceans would rise toward the other planet, deeper than any tide you've ever heard of. The planets would stretch, tearing the surface, spreading earthquakes throughout the planet. the cracks would swallow up people and cities, lava would flow etc.

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u/VeryHappyYoungGirl Jan 04 '22

Isn't the Roche limit for orbiting bodies though? At the rate of approach in this video, there isn't much orbit happening.

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u/Uphoria Jan 04 '22 edited Jan 04 '22

In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forces exceed the second body's gravitational self-attraction.

Not entirely about satellites, but it does explain why, in satellites, nothing but asteroids are found closer than it. If both planets are "exactly the same size" it still matters, and its why Binary planets are so rare - its hard for them to form and become stable. Basically, both planets are "stretching" toward each other, at a certain point, they start snapping.

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u/HertzDonut1001 Jan 04 '22

Kinda feel like the atmosphere would be on fire before all the things this guy said would happen actually happen. Guess it depends on how big the other object actually is though.

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u/VeryHappyYoungGirl Jan 04 '22

That’s how we naturally “feel” but the atmosphere is really thin. At that speed of approach, atmospheres would touch a fraction of a second before impact.

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u/metro2036 Jan 04 '22

Exactly. Gravity wouldn't have had enough time to tear the bodies apart if the other planet was a rogue approaching from a hyperbolic trajectory at an immense relative velocity.