So we all know that the moon is drifting away from Earth and will eventually leave us, right?
Actually, no! The moon is not leaving the Earth's orbit at all!
The other day I was rewatching some of the old Solarballs educational episodes, specifically Is the Moon Really Leaving Us?, and it reminded me that the information presented in this episode is actually rather inaccurate. It's not wrong, but it stops precisely when things start to get interesting, and in doing so comes entirely to the wrong conclusion.
It's true that the moon is drifting away from Earth at about 3.8 centimeters per year due to the friction caused by the Earth's rotation and the moon's effect on the tides. But this drifting (which is actually extremely variable, having started at a much faster rate and slowing down ever since) will eventually stop once the Earth's rotation slows to match the Moon's orbit - tidally locking each body to the other. It's generally expected that this lock will occur at a 47-day orbit, with the moon about 581 thousand miles away. Compared to it's current 240 thousand mile orbit, that's rather far, but not even close to the eventual ejection that Solarballs is saying will happen.
So in anywhere from 15 to 50 billion years, the Earth and Moon will tidally lock and everything will be fine.
I feel like we're missing something - WOAH
So it turns out that the Earth and the moon don't exist in a vacuum. Long before we tidally lock, the Sun's going to run out of fuel and become a red giant, and that will throw everything into wack.
This is when things get speculative. All of this is actively debated in the scientific community, so don't take it as solid facts, but as what some scientists think will happen.
First off, will the Earth survive the red giant Sun? The unsatisfying answer is that we don't know - you can find research going both ways. Venus and Mercury will certainly be consumed, but the Sun's expansion will probably stop anywhere from 1.2 to 1.5 AU, which leaves the Earth's fate in question.
"Wait!" I hear you cry. "The Earth orbits at 1 AU! That's the whole point of the unit!"
And you're right! 1 AU is the average distance from the Earth to the Sun right now. But as the Sun ages, it loses mass, and by the time it hits its red giant phase, it's going to be significantly lighter than it is today. What that means for Earth is that while it'll be orbiting at the same speed, the Sun's gravity will lessen, causing it (and every other planet) to slowly drift outward. By the time of the red giant phase, the Earth will actually be orbiting at around 1.5 AU, or where Mars is today.
You can see why the Earth's survival is such a debate.
But let's bias ourselves to recent research saying the Earth will survive. What happens to the moon?
Even if the Earth survives outside of the Sun, it's going to be a close call. When you orbit 0.3 AU from your star at best there's going to be a lot of solar wind going around. And as the sun dies, it'll eject its outer layers entirely. What results is an extremely strong solar wind that the moon has to orbit against, which produces drag and slows it down. Slowly the moon's orbit will shrink until it crosses into Earth's roche limit, breaks apart, and dies. The Earth will get some fancy new rings out of it, but they won't last long. They'll quickly disintegrate back onto the Earth's surface.
This fate isn't certain, of course, since the Sun is so variable. It's possible (though less likely) that the Sun ages differently and doesn't eject enough mass for the drag to result in a crash. That doesn't mean the moon is out of the woods yet, though. Even as a white dwarf the Sun will exert solar tides on the (now frozen solid) Earth. Very, very slowly, this effect will act as a reverse to the lunar tides that are pushing the moon away in the present day, resulting in another crash in the far future.
Either way, the moon's fate has a fairly clear course - and it's the opposite of a slow drift into the aether. It's almost poetic, really; the moon goes out the same way it forms, a callback to the original collision with Theia.
(I linked a lot of research here, but the big inspiration for this post comes from this youtube video about the fate of the Earth if it survives the Sun's death. I highly recommend it!)
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