Pluto Questions

[David_Simmons]

@Podkayne

Sorry to be a nit-picky dynamicist, but…

Actually, the Moon would escape from the Earth before this happens—but this would only occur well after Sun evolves into a red giant.

That’s why I stuck in the proviso "if the earth-moon system lasts long enough. I have no idea how long it would take for tidal lock to happen and I am not about to try to compute it.

I’m afraid this is entirely untrue. The orbits of Phobos and Deimos certainly are evolving tidally. Their small mass means that they raise a small tidal bulge, yes, but their smaller inertia means they are, in turn, more easy to move. Because Phobos is inside the synchronous orbit (the distance from Mars where a satellite’s orbital period equals Mars’ rotational period), tidal evolution actually decreases the size of Phobos’ orbit, and because Phobos is so close, this process is quite rapid, a couple of meters per century. Phobos will not survive long. In about 50 million years, it will be so close to Mars that it will be pulled apart by tides.

Yep. I spoke too soon about Mars moons not being affected by their tidal bulge. Ah me, it pays to think things through a little longer…

Interesting about Phobos. Of course the moon is receding faster than Phobos’ rate, at present. 3.8 cm/year or 3.8 meters/century.

Anyway thanks for the straight dope on the recession of other solar system satellites.

[David_Simmons]

@Podkayne

Sorry to be a nit-picky dynamicist, but…

Actually, the Moon would escape from the Earth before this happens—but this would only occur well after Sun evolves into a red giant.

That’s why I stuck in the proviso "if the earth-moon system lasts long enough. I have no idea how long it would take for tidal lock to happen and I am not about to try to compute it.

I’m afraid this is entirely untrue. The orbits of Phobos and Deimos certainly are evolving tidally. Their small mass means that they raise a small tidal bulge, yes, but their smaller inertia means they are, in turn, more easy to move. Because Phobos is inside the synchronous orbit (the distance from Mars where a satellite’s orbital period equals Mars’ rotational period), tidal evolution actually decreases the size of Phobos’ orbit, and because Phobos is so close, this process is quite rapid, a couple of meters per century. Phobos will not survive long. In about 50 million years, it will be so close to Mars that it will be pulled apart by tides.

Yep. I spoke too soon about Mars’ moons not being affected by their tidal bulge. Ah me, it pays to think things through a little longer…

Interesting about Phobos. Of course the moon is receding faster than Phobos’ rate, at present. 3.8 cm/year or 3.8 meters/century.

Anyway thanks for the straight dope on the recession of other solar system satellites.

[Podkayne]

@David Simmons

Of course the moon is receding faster than Phobos’ rate, at present. 3.8 cm/year or 3.8 meters/century.

Good point! I guess it just seems more urgent in Phobos’ case, because it is heading for dum-dum-DUMMMM CERTAIN DOOM!

[David_Simmons]

@Podkayne

Good point! I guess it just seems more urgent in Phobos’ case, because it is heading for dum-dum-DUMMMM CERTAIN DOOM!

It sure would be a good prediction that Phobos will be gone before the moon is.

[David_Simmons]

@Alive At Both Ends

Tidal forces exerted by the Moon are slowing the Earth’s rotation. This causes the Moon to move further out to conserve the angular momentum of the Earth-Moon system.

Nah. The earth’s tidal bulge speeds the moon up which drives is away from the earth thereby increasing its angular momentum. The earth’s rotation has to slow to conserve the system angular momentum.