With so many asteroids hurtling around the solar system, it’s a wonder that Earth only
has one moon.
Because, beyond the inner solar system, the gas giants have a lot more: some of which
are regular moons, plus a lot of irregular moons.
They are called irregular because their orbits do not follow the orbital plane of the planet,
instead following a distant, inclined and eccentric orbit, even sometimes orbiting retrograde
to the regular moons.
The main reason for this difference is that irregular moons are mainly captured objects,
asteroids that were orbiting the Sun before they approached too close to a planet and
got captured by the planet’s gravity.
So, what about Earth?
Well, I can announce that Earth now has a second moon, called 2020 CD3.
But it probably isn’t worthwhile trying to spot this one in the sky.
It’s tiny, only 2-4m across, and also very dark, a type of asteroid known as a C-Type,
or a carbonaceous asteroid.
These types of asteroids are as dark as wet asphalt.
This moon, like the irregular moons of the outer planets, is a captured asteroid that’s
been pulled into orbit around Earth.
It was probably captured as recently as 2017.
Although, before you get warm to the idea of Earth having two satellites, it seems that
this is only a temporary relationship.
Astronomers are expecting it to escape Earth’s gravity again by April 2020.
This is due to its chaotic orbit, it never really got settled into a stable orbit and
so will be ejected again in just a few months.
A very similar thing happened in 2006 with an asteroid called 2006 RH120, where Earth
had a second moon for just short of a year, before again, it was ejected.
And just like then, once this one leaves Earth’s orbit, we’ll be back to just the one natural
satellite.
Or will we?
There is another asteroid orbiting Earth at a distance of about 9 million km, much further
out than the moon, called 2016 HO3.
Except, although it’s circling around Earth, it’s not really orbiting the Earth at all.
Instead, it’s orbiting the Sun, and its orbit is very similar to Earth’s, just a
little more eccentric.
At some points of this orbit, it is ahead of the Earth, and 6 months later it falls
behind Earth.
This is something known as a quasi-satellite, and Earth has 5 of them that we know of.
Moons which circle Earth, and are even influenced by its gravity, but without truly being in
orbit.
And so unfortunately, they aren’t and can’t be classified as true moons.
We aren’t the only planet to have quasi-satellites, in fact they all might considering these objects
are very difficult to spot.
But why do gas giants have so many irregular moons, and Earth only has the one (right now
anyway)?
Well, being big and massive is only part of the answer.
Yes, the more massive you are, the bigger your gravity.
But Earth’s gravity is hardly a slouch, it’s easily comparable to Saturn, Neptune
and Uranus.
So, what’s the difference?
Well, simply put, there’s another force of gravity in the solar system called… the
Sun.
Moons in orbit around planets are constantly being tugged and pulled at by the Sun, however,
they are within something called the planet’s ‘Hill Sphere’.
Within the Hill Sphere of a planet, the planet’s gravity has more of an influence on the object
than the Sun’s gravity.
And this influence will be different depending on how close you are to the Sun, because the
closer you are, the stronger you are under the influence of the Sun’s gravity.
This means that the Hill Sphere around Earth isn’t very big, only 1.5 million km.
If Earth was closer to the Sun, it’s Hill Sphere would be smaller.
If it was further away, it would be bigger.
And now you can see why these gas giants have so many captured moons, their Hill Spheres
are simply much bigger all the way out there.
In fact, Neptune, while not being anywhere near as massive as Jupiter, has a much bigger
Hill Sphere, meaning it can capture stray asteroids which are tens of millions of kilometres
away.
In order for Earth to permanently capture an object, it would have to happen in just
the right way, likely needing some helpful gravitational tugs from our Moon along the
way to tame its orbit.
And as far as we know, that hasn’t happened yet, meaning anything that does come too close
only sticks around for a few orbits before it leaves again.
So, enjoy 2020 CD3 while you still can!
Thanks for watching!
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