The Sun with a planet with a moon with a satellite

Loren Booda

In our Solar System, what is the best example of a planetary moon having its own satellite? What general properties do such satellites share?

Drakkith

I am not aware of any moons that have their own satellites. But I am definitely not 100% sure. I believe the interaction between the planet and the moon would make almost all orbits unstable. Maybe someone else will have more details.

D H

There are none, at least none that we know of.

For a while it was conjectured that Rhea (2nd largest moon of Saturn) might have a ring system, but this turn out to be false. http://news.sciencemag.org/scienceno...ever-were.html

Since there are none, a better question is why aren't there any? For a moon to have satellites,

• The Moon would have to be large enough so as to have a sizable Hill sphere and to have a fairly spherical gravity field.
• The Moon would have to be orbiting at a significant distance from the planet so as to have a sizable Hill sphere, but not so far as to allow perturbations from the Sun to make the orbit of the moon's satellites unstable.
• The satellite would have to have formed with the moon (unlikely), have been captured ballistically by the moon (unlikely), or have crashed into the moon such that fragments went into orbit about the moon (also unlikely).

phyzguy

How about the NASA Grail satellites currently in orbit about the moon?

D H

Lunar impact on about July 13, 2012.

D H

As an addendum to my last cryptic post, no maneuver will be needed to make that lunar impact occur. The GRAIL satellites will eventually be placed into low lunar orbit so as to better determine the Moon's gravity field. Low lunar orbits are not stable. An object in a low lunar orbit will eventually crash into the Moon thanks to the non-spherical nature of the Moon's gravity field. The Moon's center of mass is offset from its geometric center by a couple of kilometers, and the near side of the Moon is peppered with seven or so mass concentrations ("mascons"). These features collective make low lunar orbits unstable. The orbital eccentricity is not constant. It varies over time, eventually making perilune be inside the Moon.

This article, http://science.nasa.gov/science-news...6nov_loworbit/, makes for an interesting read.

tasp77

The very outer satellites of Jupiter might have satellites in numbers proportional to the Jupiter Trojan asteroids, although none of those at the moment (AFAIK) have been proven to be enmooned. We have seen some pretty small 'rocks' near earth that have satellites, and it seems reasonable some % of Trojans would too.

Note the very outermost satellites of Jupiter are very weakly held to Jupiter, they being in the outer fringe of the Jupiter Hill Sphere. We also see them in retrograde orbits around Jupiter too. Weird little critters.