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Phobos orbits really close to Mars, so close that one Phobos period is shorter than one sol. Due to tidal friction, it is also getting closer and closer to Mars. How much time will it take before Phobos reaches the Roche limit of Mars?

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Sometime before now and "tens of millions of years".

Phobos currently orbits at a radius of about 9400km

The theoretical Roche limit is different for rigid and fluid bodies. If Phobos were a fluid body, it would already have passed the Roche limit at 10500km. This is because fluid bodies will deform into an ellipse pointing towards the planet, and this amplifies the tidal forces and so causes break up a larger radius.

If Phobos is fully rigid body, then the Roche limit is half that at about 5400km. In fact a rubble-pile-with-crust like Phobos is neither fully fluid, nor rigid. Wikipedia gives an estimate of "30-50 million years" before it is fully disrupted.

So Phobos is already a body under stress, and (as anybody who knows a little about earthquakes knows) rock under stress is often unpredictable in how it responds: It can creep, fracture, or hold rigid while stress increases and then fail catastrophically.

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