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.
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.
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
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."
Luna is, of course, unfeasibly large, relative to its planet. LeeE, 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.