Science Questions

Can a moon have a moon?

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. Geoff

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.

May 2010