If Earth had multiple small satellites in its early history, how did they eventually merge or disappear, leaving only the Moon?

Okay, no problem! The idea that Earth might have had multiple small moons early on is a fascinating topic. Let me explain the process in a way that's easy to understand.

Hey, that's a great question, and it touches on a really cool area astronomers have been studying! While we see only one lonely moon in our night sky now, things were likely very different during the chaotic period just after Earth formed.

Imagine Earth's night sky 4.5 billion years ago, likely hosting several moons of various sizes – that must have been quite a spectacle. So, where did all those "little siblings" go?

The Beginning: A Cataclysmic Collision

First, we need to start with how the Moon itself came to be. The most widely accepted theory is the Giant Impact Hypothesis.

Simply put, early in Earth's formation, a planet roughly the size of Mars (we call it Theia) slammed into Earth at an angle. This collision was incredibly violent, shattering both Earth and Theia into countless fragments. Huge amounts of rock and molten material were flung out into space around Earth.

(A simplified simulation of the collision process)

These fragments didn't immediately form one large moon. Instead, they first created a spectacular ring of rocky debris encircling Earth, similar to Saturn's rings.

"Big Fish Eat Little Fish" in Space: The Fate of a Multi-Moon System

The material in this rocky ring wasn't stable. Under the influence of gravity, particles began attracting and clumping together, like rolling snowballs, gradually forming lumps of various sizes. These were Earth's first generation of "small moons" or "lunar embryos."

Then began a "cosmic battle royale" lasting millions of years. These small moons faced three main fates:

Fate 1: Merging and Growing – "Winner Takes All"

Among these small moons, one or two "lucky ones," due to an initial positional or mass advantage, grew faster and larger than the others. Their gravitational pull became stronger as a result.

This triggered a "big fish eats little fish" scenario:

Gravitational Capture: This largest "proto-Moon" acted like a cosmic vacuum cleaner. As it orbited, it constantly pulled its smaller siblings nearby towards it.
Gentle Collisions: Because most were orbiting Earth in roughly the same direction, collisions were often not as violent as the giant impact. They were more like "fender benders." A smaller moon would collide and merge, becoming part of the larger one.

Our Moon is the ultimate victor of this "merger war." It grew to its present size by continuously "consuming" other small moons. Some research even suggests the Moon's far side has a thicker crust than its near side, possibly because it once "swallowed" a smaller companion moon.

Fate 2: Falling to Earth – "Returning to the Mother Planet"

Not all small moons got eaten by the "big brother." Some had highly unstable orbits.

Orbital Decay: In the complex gravitational tug-of-war (involving Earth, the largest proto-Moon, and the Sun), the orbits of some small moons would gradually lower. They spiraled closer and closer to Earth.
The Final Plunge: Once they crossed a critical distance called the Roche limit, Earth's powerful tidal forces would tear them apart. Alternatively, they might plunge whole into Earth's atmosphere, burning up like a giant meteor or impacting the surface of the primordial Earth. They ultimately became part of Earth itself.

So, early Earth likely experienced a period of frequent "lunar meteor showers."

Fate 3: Getting Kicked Out – "The Eternal Wanderer"

There's an even more dramatic ending.

Gravitational Slingshot: If a small moon approached the largest proto-Moon or Earth at a tricky angle and speed, instead of being captured, it could be violently accelerated by their gravity – like being whipped – and then "kicked" out of Earth's gravitational grasp.

These ejected small moons became wanderers in the solar system, either eventually crashing into other planets or drifting alone through the solar system forever.

Settling Down: Why Only One Moon Remained?

After millions to tens of millions of years of chaos and clearing, this once overcrowded Earth orbital system naturally evolved into its most stable state.

This state is: One sufficiently large moon, orbiting in a relatively circular path at a sufficient distance.

Clearing the Orbit: The "winner" (our Moon) had swept its orbital neighborhood clean of all objects it could merge with, crash into, or eject.
Tidal Locking: Over the vast expanse of time, the powerful tidal forces between Earth and the Moon also synchronized the Moon's rotation and revolution. This is why we only ever see the Moon's "near side." This itself is a sign of a system settling into stability and its lowest energy state.

To Summarize

So, if you could travel back 4.5 billion years, you might indeed see a very lively night sky, with several moons rising and setting. But this system was inherently unstable. After a "cosmic battle royale" lasting millions of years:

One "champion" grew dominant by continuously merging with other small moons.
Many "underdogs" lost the gravitational struggle, their orbits decayed, and they ultimately fell to Earth.
A few "unlucky ones" were flung out of the Earth system by gravitational slingshots.

In the end, a single "victor" emerged, cleared its surrounding orbit, and became the solitary Moon we see today. Within it may still lie the remnants of those long-vanished "siblings."