The premise of Moonfall is simple, deranged, and delightful: Something has knocked the moon out of its orbit, and in a matter of weeks, the moon will collide with the Earth. The only people who can stop it are disgraced astronaut Brian Harper (Patrick Wilson), NASA Executive Jo Fowler (Halle Berry), and conspiracy theorist K. C. Houseman (John Bradley). Over the course of the movie’s runtime, this motley crew manages to uncover secrets NASA has been hiding for years about the moon; take the Space Shuttle Endeavour out of a museum; and put together a rescue mission to figure out why the moon has broken its orbit, and hopefully put it back. “Save the moon, save Earth,” the tagline goes.
It’s perfect disaster-movie fare, wonderful for turning your brain off and just experiencing. There’s no reason it needs to be scientifically accurate; it’s a fictional disaster movie! But if you would like to engage intellectually with the premise Moonfall presents, you’ll be surprised at how much of the science they got right. For all of its absurdity, this movie took its science seriously.
[Spoilers for Moonfall ahead.]
Before we get to the main question posed by the movie—what would happen if the moon did somehow break its orbit and begin falling towards the Earth?—let’s look at what’s actually going on with the moon. Scientifically, it’s important to remember that the moon is moving away from the Earth, without any alien intervention. “The moon is moving away from us an inch and a half a year,” James Greene, NASA’s former chief scientist and current senior advisor, told me.
Scientists have run scenarios to peer into our distant future and see what will happen as the moon moves farther and farther away from the Earth. Eventually, in 50 billion years or so, the Earth and moon will be tidally locked to one another, meaning that the same sides of the Earth and moon always face each other. (Right now the moon is tidally locked to the Earth, so we only see one side of it from our planet, but everyone on Earth gets their view of the moon.)
To dramatize the opposite scenario in Moonfall, in which the moon starts gently and devastatingly orbiting closer and closer to the Earth, disaster geophysicist Mika McKinnon (who consulted on the movie) explained to me that the film’s science consultants simply reversed those models.
“The first thing you think about is in terms of the tides,” McKinnon explained. The moon’s gravity pulls on the Earth’s water, resulting in two high tides and two low tides per day. (The Sun contributes somewhat, too.) So, in Moonfall, as the moon spirals closer and closer to the Earth in a dance of death and destruction, the lunar gravitational pull on our planet increases, leading to massive floods. The movie features an epic flooding of Los Angeles, thanks to these tides.
We normally think of tides just in terms of water. “But the tides actually also impact the air, the atmosphere, and the rocks,” said McKinnon. “As the moon gets closer, those tides will get stronger and stronger. Suddenly, you’d have high tide and low tide for your air. Places that are higher elevation having low [air] tide means they don’t have enough air anymore.” And indeed, we see this in Moonfall when Tom Lopez (Michael Peña), Sonny Harper (Charlie Plummer), and their companions are forced to find oxygen masks to travel through the mountains in their quest for safety.
The structure of the Earth’s tectonic plates and large rocks will also be pulled and released by the moon’s gravity, says McKinnon. “As you’re squishing them in and out, you’re going to see cracks and earthquakes and landslides.” In fact, the Earth itself would be compressed and released with high tide and low tide. As the moon moved closer in and the tides became more frequent, so would this constant cyclical push and pull of natural disasters.
As the moon moves closer and closer to Earth, things get even more dramatic. “If you move the moon inward, the days will speed up,” Greene explained, because the moon’s proximity would make the Earth spin faster. “The moon is currently 60 Earth radii away,” said Greene. (For reference, one Earth radius is about 3,963 miles). “When it’s at four Earth radii, the day on Earth will be five hours long. Two and a half hours will be daylight, two and a half hours will be night.”
And then, when the moon is 11,470 miles away from the Earth (about three Earth radii), it will reach the Roche limit. “The Roche limit is when you have so much stress that it shreds physical objects and pulls them apart,” McKinnon explained. In other words, on its approach the moon would actually start breaking apart, and pieces of it would rain down on Earth.
And then? Crisis averted. At least on screen. In the normal world, there would be nothing we could do to stop the moon—there’s no physics solution that could save us. But this is, of course, an epic disaster movie in which aliens are inside the moon.
In the movie, the moon actually enters the Earth’s atmosphere and is perilously close to impact when our heroes finally manage to avert catastrophe, thanks to their daring plan to launch the Space Shuttle Endeavour to the moon to defeat the alien presence within.
This portion of the movie isn’t even a little scientific. There aren’t aliens in the moon. Plus, Endeavour was retired in 2011, and using it to fly would be “impossible,” Astronaut Mike Massamino, who serviced the Hubble Space Telescope on his two Space Shuttle missions, told me. “We’re not going to be able to use [the space shuttles] ever again, and if we needed to do some kind of emergency flight to space, we have other options that I think we would use.”
If the moon had impacted the Earth, though, the planet itself would stay intact. “The Earth wouldn’t fall apart,” McKinnon said. “It’d just be significantly less habitable.” When something large hits the Earth,“it’s moving at such a velocity that a huge crater is created, typically 10 or 15 times the size of the object,” said Greene. “Everything in that [the crater] is gone. What happens to the material inside the hole is that it gets pulverized.”
The moon’s diameter is 2,159 miles. The Earth’s is 7,917.5 miles. Which means that the entire planet would be a massive impact crater. “We are small and fragile on a crust that is tiny,” said McKinnon. The crust would be obliterated and all life would come to an end.
But hey, Earth would have rings!