What If the Earth Had Two Moons?

By: Karen Kirkpatrick  | 
Earth's second moon might already be here.
What would it be like if we had two moons? Pretty dangerous, as it turns out!
Ros Roberts/StockTrek/Getty Images/D.Fagan

"The tide is high, but I'm holding on" is the first line of a Blondie song from 1980 (which was actually a cover, but it considerably outsold the 1967 original). It could also be society's new theme music if the Earth suddenly found itself with two moons. This makes us wonder: Could humankind survive in a scenario that saw the arrival of Earth's second moon?

The song perfectly represents what would likely be Earth's new reality — rising and unpredictable tides — according to Neil Comins, a physics professor at the University of Maine and author of the book "What If the Earth Had Two Moons." Comins is one of only a few science types — as opposed to science-fiction types — to have addressed this question, which means there's not a lot of science-based theorizing out there for what is, really, an interesting — and potentially apocalyptic — scenario.


Humanity could very well survive the arrival of Earth's second moon, but it wouldn't be pretty. In this article, we'll detail the geological effects of having a second moon in Earth's orbit, from volcanic activity to earthquakes. We'll also share details about NASA's recently discovered quasi moon, named 2023 FW13.

How the Actual Moon Came to Be

First, a little background: NASA scientists believe our current moon formed 4.5 billion years ago when an enormous Mars-sized body banged into us. Debris from the impact hurtled into space, began orbiting the Earth, and eventually formed into the moon as we know it [source: NASA].

This is not something most of us would want to see repeated, and the gravitational effects of a second moon — even after its dramatic formation — wouldn't be any better. Of course, if it arrived in the same cataclysmic fashion as the first moon, we likely wouldn't be around to experience it anyway.


A Rocky Phase in the Orbital Period

In his book, Comins goes a different route for the formation of Earth's second moon — one that wouldn't doom all of Earth's inhabitants to extinction via a collision course. His second moon, Lluna, would be captured by our first moon long after the Earth and our current moon have settled into a working relationship with each other, with Lluna orbiting Earth on a path halfway between the Earth and the moon.

As Lluna settled into its orbit, however, we would experience hell on Earth. The gravitational pull of the new moon would create tides up to eight times higher than our current tides, with enormous tidal waves larger than anything we've ever seen before. The tidal waves would result in earthquakes and awaken any dormant volcanos on our planet. Worse yet, this geological activity would continue for years, ultimately causing a mass extinction of marine life [source: Comins].


Once things settled, and this new space rock had eased into an earth-like orbit, life would be very different. Light at night would be much brighter with two full moons, and we would have fewer hours of real darkness. Cities built along the water — think New York, San Francisco or even London — would erode with the tides, and buildings would be destroyed.

Near Earth Asteroids

While Comins's book is hypothetical, some researchers theorize that the Earth already has two — or sometimes more — moons. These scientists argue that we pick up small asteroids that make several orbits around the Earth over a period of a few weeks or months before taking back off into space. They also believe that this happens over and over with no ill or positive effects, since these quasi satellites orbiting our planet are so small [source: Wolchover].


Is 2023 FW13 Earth's Second Moon?

Given our spot in the universe, it's not a huge stretch to speculate about having two moons. After all, Saturn and Jupiter each have more than 50, and even our closest galactic neighbor, Mars, has two [source: NASA]. So it might be overdue that NASA has discovered 2023 FW13, a space rock that scientists are calling a quasi moon or quasi satellite.

This space rock orbits the sun much like our planet -- or, for that matter, any natural satellite. The asteroid is approximately 50 feet in diameter and maintains a safe distance of about 9 million miles from Earth. Such an orbit means there's little risk of an impacting trajectory.


Could this quasi moon have formed from lunar ejecta, which are the particles dispersed after a meteor collision? It's possible. Incredibly, some scientists believe that this quasi moon has been orbiting Earth for thousands of years, even though it was only recently discovered.

So, all things considered, maybe Blondie's "The Tide is High" isn't the right theme song for the Earth having two moons. How about a chorus of Creedence Clearwater Revival's "Sweet Hitchhiker?"


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More Great Links

  • Comins, Neil. "What if the Earth Had Two Moons?" St. Martin's Press. 2010. (April 16, 2015) http://us.macmillan.com/excerpt?isbn=9780312673352
  • NASA. "Our Solar System: Moons." (May 20, 2015) https://solarsystem.nasa.gov/planets/profile.cfm?Display=Moons
  • Saintonge, Amelie. "What Would Happen if Earth Had More Than One Moon?" Ask an Astronomer. (April 16, 2015) http://curious.astro.cornell.edu/our-solar-system/37-our-solar-system/the-moon/the-moon-and-the-earth/38-what-would-happen-if-earth-had-more-than-one-moon-intermediate
  • Wolchover, Natalie. "Earth Has Two 'Moons' Right Now, Theorists Say." LiveScience. Dec. 22, 2011. (April 18, 2015) http://www.livescience.com/33641-earth-moons-theorists.html