Science Questions

Can a moon have a moon?

Sat, 29th May 2010

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Question

Geoff asked:

Can a moon have a moon?  Do any of the moons in our solar system have moons?  Is it theoretically possible for this to happen, or would the gravity of the planet being orbited make the arrangement too unstable?

 

Thanks for the great podcast.

Answer

Dominic -  Well yes, it could.  Among astronomical bodies, there’s quite a long hierarchy of bodies orbiting around other bodies.  Of course, the moon is orbiting around the earth and the earth around the sun.  But the sun itself is orbiting about the centre of the Milky Way galaxy and that itself we think is orbiting around within a local group of galaxies, and that we think is part of a larger super cluster of galaxies.  So you can certainly add another step to that hierarchy and put a body into orbit about the moon, and that is of course what we did when we went to the moon and we sent the Apollo spacecraft to the moon.

However, each step of the hierarchy tends to be less stable than the previous step.  It would take something quite catastrophic to take the sun out of the Milky Way galaxy but to strip the earth out of orbit from the sun would actually be scarily easy  - if a star were to pass too close to our own.  Stripping the moon off the earth, we think that will probably happen on a timescale of billions of years, naturally anyway.  So I think something orbiting about the moon will probably stay there for a matter of years before being shed into solar orbit.

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Geoff asked the Naked Scientists: Can a moon have a moon?  Do any of the moons in our solar system have moons?  Is it theoretically possible for this to happen, or would the gravity of the planet being orbited make the arrangement too unstable? Thanks for the great podcast. What do you think? Refractor, Thu, 6th May 2010

From the sun's point of view the Earth is a moon, and we have a moon. Bored chemist, Thu, 6th May 2010

The Apollo Lunar Command Module was a moon of our Moon whilst in orbit around it. LeeE, Fri, 7th May 2010

OK, so theoretically moons can have moons, but presumably none of the moons in our solar system have moons (aside from maybe Charon)? Refractor, Sun, 9th May 2010


http://www.thenakedscientists.com/forum/index.php?topic=19243.msg21 RD, Sun, 9th May 2010

Theoretically, moons can have moons, and those can have their own moons too, so long as the ratios of masses and orbital diameters is suitable. Whether such exists is, of course, an entirely different question. Atomic-S, Mon, 10th May 2010

"Two's company. Three's a crowd."

I think Dave nailed it. As long as a moon has insufficient mass to "attract the attention" of the body that its planet is orbiting, all will be well. But if a moon is massive relative to its planet, bad things start to happen. Geezer, Mon, 10th May 2010

Luna is, of course, unfeasibly large, relative to its planet. LeeE, Mon, 10th May 2010



Bummer. Another perfectly good theory goes up in smoke!

Come to think of it, doesn't the Moon actually stabilize the Earth? Geezer, Mon, 10th May 2010

Apart from Earth's Luna and Pluto's Charon, the rest of the moons in our solar system are generally much smaller than their parent planet.

I'm not sure that Luna stabilises the Earth in any particular way but it certainly has a great influence upon it.  The sea and ocean tides are the most obvious effect of Luna upon Earth today but considering its size and closeness to Earth it might be possible that Luna's tidal forces are still warming the Earth in the same way that Jupiter heats Io i.e. via tidal distortion of the Earth itself.  It has been estimated that shortly after the Moon was formed, and while it was still orbiting the Earth much more closely, the actual land surface of the Earth rose and fell by several hundred metres as the Moon passed overhead.  Naturally, with the Moon now being much further away, the degree has lessened considerably, but it should still be having some effect.

It doesn't seem too outlandish to speculate that the heating of the Earth by this mechanism has helped the Earth's core stay molten for longer than it might otherwise have done.  That is just speculation though. LeeE, Tue, 11th May 2010



The moon has kept Earth's axial tilt (and consequently seasons) comparatively constant ...


http://en.wikipedia.org/wiki/Axial_tilt RD, Tue, 11th May 2010



Ta for that - I didn't know that Mar's tilt had ever been so extreme i.e. up to 49° LeeE, Wed, 12th May 2010

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