Can a moon have a moon?
Well yes, in theory, it can.
And yet, we don’t see any examples of this in our solar system!
So why is that?
The first important thing to know is that all celestial objects have gravitational pull.
That includes tiny little asteroids, all the way to giant stars or even black holes.
The mass of an object dictates how strong its gravitational pull is.
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 km,
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 a Hill sphere, which contains the entire solar system.
Its Hill sphere is massive, almost 2 light years in radius, as the nearest big 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 60,000km 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?
The difference is that none of our planets are tidally locked to the Sun as they are
far enough away from the parent star, which means their moons have a stable orbit.
This is one of the reasons we believe that none of the Trappist system planets have moons,
as they are so close to their parent star that we assume the planets are all tidally
locked.
I hope this hasn’t disappointed you too much, so I’m going to leave you with this.
There are some other curious objects in the solar system far away from any other object,
so they aren’t tidally locked to anything.
This particular asteroid called Ida looks like a standard 30 km wide asteroid as seen
by the Galileo spacecraft, but you might notice this little blob here.
This is actually Ida’s moon, Dactyl!
It is only 1.5 km across and orbits only 60km away from Ida.
We don’t know especially how stable this orbit is, but a very cool thing to observe
nonetheless!
Thanks for watching!
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All the best!
And see you next time.