Alex McColgan

A region around an object where its gravitational influence is greater than any other celestial object near them is called its Hill Sphere.

Due to the mass of our Earth, it has a Hill Sphere which has a radius of 1.5 million kilometres, meaning that if you were within this Hill Sphere, you would be pulled more towards Earth than towards the Sun.

If the Earth was closer to the Sun, its Hill Sphere would be smaller, and if it was further away, it would be larger.

The Sun also has its own Hill Sphere, which contains the entire Solar System.

Its Hill Sphere is massive, almost two light years radius, as the nearest celestial object its gravity is competing against are other stars.

Which means in a way, everything orbiting our Sun is the Sun's moon, or rather its satellite.

The term moon is really reserved for satellites of planets.

So why don't moons also have natural satellites?

Well, lots of moons are extremely close to their parent planets, meaning their hill spheres are very small.

Let's take our Moon as an example.

Against the Earth, the Hill Sphere of the Moon is only 60,000 km radius, or only one sixth of the distance from the Moon to the Earth.

Moons like Io have an even smaller Hill Sphere, as it is competing against the gravity of Jupiter.

This makes it quite hard for moons to capture an object, but not technically impossible.

So surely there must be a moon that has a natural satellite somewhere, right?

Well, most moons also have one other major problem.

They tend to be tidally locked to their parent planet.

Because of this, any satellite that orbits a tidally locked object will have its orbit decay from tidal forces until it eventually crashes into the moon.

Now this still takes a lot of time over astronomical standards, but it means if any of the moons in our solar system did have a satellite at one point, chances are that it has since crashed into it.

This leaves one question I think.

Why don't moons of planets also eventually crash?