Can Moons Have Moons?
Tonight, take a moment to glance up at the sky. There you'll see Earth's celestial constant, our Moon, illuminated by the Sun. Now imagine you're up there, standing in the Ocean of Storms, looking back at Earth. Our home is radiant -- blue, green, and beautiful. But that's not the only close object that specks the Moon's sky. Though it may surprise you, the Moon actually has its very own moon, and we put it there.
Right now, the Lunar Reconnaissance Orbiter (LRO) is circling the Moon, as it has done for the past five and a half years. You'll need a telescope to see it clearly, but it's there. And yes, it counts as a moon. Sure, a moon is technically a natural satellite, and the LRO is manmade. But that's a minor distinction. What matters is that the LRO orbits the Moon. So it's an unnatural satellite. A faux moon, if you will. But a moon nonetheless.
This example serves to show that moons can indeed have moons. The science behind this notion is surprisingly simple. All celestial bodies exert a gravitational force proportional to their mass -- everything pulls on everything else. Over billions of years, this has balanced out quite nicely in our solar system, as all the planets have settled into set orbits around the Sun. More specifically, the planets and sun all orbit around their mutual center of mass, or barycenter.
While any two objects can orbit one another in a stable way, adding more celestial bodies complicates the picture dramatically. Mathematically, there is no general solution for three or more objects to orbit one another. This means that systems of multiple objects can only stably orbit one another in certain special cases.
There are a handful of spots where the gravitational tug of large objects is precisely negated by the pull of another. These are the Lagrangian Points; five of these exist between any two bodies. Asteroids sometimes accumulate at these spots, and we have parked spacecraft in the ones between the Earth and the Sun. But the objects at these points aren't moons as we think of them: they stay in one point in space and don't revolve around either body.
The other special case arises in a system where a planet orbits a star, or a moon orbits a planet. In this case, an even smaller object may be able to orbit either of the two large ones, but only if it is very close to one and very far from the other. For example, our Moon orbits 388 times closer to us than the Sun. In this case, the gravitational pull of the closer body is vastly stronger than the far body (gravitational force decreases by the distance away squared).
The closer the small sub-satellite is to the bigger planet or moon, the easier it is to orbit there. The region of space around a planet within which a moon will orbit on the planet and not the sun is called the Hill Sphere. Within this area, the gravity of the planet alone is king. Moons have their own, much smaller Hill Spheres where they own satellites and don't share them with their planets. Sub-moons have even tinier Hill Spheres, and so on.
The Earth's Hill Sphere extends out 1.5 million kilometers. If the moon were outside of that range, it would orbit the Sun instead! The Moon also has its own Hill Sphere, with a radius of 60,000 kilometers. It is within this range that the Lunar Reconnaissance Orbiter currently orbits the Moon. Even asteroids have their own Hill Spheres. The 19-mile-wide asteroid 243 Ida, which orbits the Sun every 4.8 years, has it's own little pet moon Dactyl, which is only about a mile wide.
While moons can have moons, these sub-moons often don't last long. Many moons that orbit planets are tidally locked, meaning that one side of the moon always faces the planet. This spells trouble for potential sub-moons. The tidal forces associated with tidal-locking cause the sub-moon's orbit to decay in a matter of years, sending it crashing to the surface of the bigger moon. This will be the eventual fate of the Lunar Reconnaissance Orbiter.
The Hill Sphere accounts for gravitational forces alone, but no others. That's why these tidal forces, or other small forces such as the force of light from a star pushing on the body (radiation pressure) and even the body emitting photons that push on it like a miniscule rocket engine can still ruin orbits within the supposedly safe region.
While the relatively tiny Hill Sphere makes it very difficult for sub-moons to form naturally around moons, there may be one fascinating example from our very own solar system. Saturn's second-largest moon, Rhea, may have its very own rings, composed of moonlets! Their existence is in dispute, however, with the weight of evidence currently pointing to their absence. But we can still revel in the glory of possibility!
(Images: AP, NASA)