The Earth has more than one moon

By Phil Plait for The Scientific American

Here’s a trick question: How many moons does Earth have? And to be clear, I mean natural satellites, not human-made ones.

I remind you, this is a trick question.

The answer is one—the moon. See? Tricky. But there’s another part to this: if you change your frame of reference and squint a little bit, the answer is more like seven. And if you broaden your mind a tad more, the number goes up even higher.

How could that be? To explain, let’s talk about quasi-satellites and horseshoe orbits.

Orbital mechanics is a weird subject. If you have a single object—say, a planet—orbiting another single object—a star—then things are relatively simple. The orbit might be a circle or more elliptical (elongated) or a variety of other mathematical shapes. The time taken to orbit the star once is called the planet’s orbital period, and it tends to stay pretty much constant. (For a circular orbit, the speed of the planet stays constant. But if the orbit is an ellipse, the planet speeds up a bit when it’s closer to the star and slows down when it’s farther out.)

Now, let’s say there’s a second object, such as an asteroid, in orbit around the star. It, too, may travel along a circular path or a more elliptical one. And from the high and mighty view of someone looking at the system from the outside, both objects follow their paths at different speeds and with different periods.

Here’s where the weirdness pops up. Let’s say that the asteroid has an orbit that’s very similar in size to the planet’s but slightly more elliptical. Also, this object is at just the right distance from the star such that its period is almost exactly the same as the planet’s. Not only that, it also happens to be near the planet in space, too, as they orbit the sun. From our outside point of view, we see them both going around the star, each completing one orbit after the same amount of time. Sometimes the asteroid moves a little faster than the planet, however, and sometimes it does so slower, even though, on average, its orbital speed is roughly the same as the planet’s.

In a little more detail, when the asteroid is farther from the star, outside the planet’s orbit, it slows and lags behind. It then drops down closer to the star, speeds up and moves ahead of the planet. Then it falls back outward and slows, and the pattern repeats.

That’s not too strange, but if you’re looking at the asteroid from the planet’s point of view, it seems very different. From that vantage, the asteroid appears to always stick near the planet, sometimes closer to the sun and traveling ahead and sometimes farther out and moving in the reverse direction. In other words, it looks like the asteroid is going around the planet!

That’s a lot like a moon’s motion. It’s an illusion, however, because the asteroid is not really orbiting the planet. Instead it’s moving around the sun. The asteroid’s motion is aligned with the planet’s such that the space rock appears to circle the world.

An analogy to this motion is a scenario in which you are driving down the center lane of a three-lane road. Imagine a car moves ahead of you on the left, passes you, then gets all the way over into the right lane. It then slows, drops behind you, speeds up and passes you again on the left. It appears like the car is circling around you. Someone looking from the side of the road, however, will see it simply moving left and right, faster and slower.

Objects with orbits like that are known as quasi-satellites, or quasi-moons. They aren’t really moons because they actually orbit the star and not the planet—they also tend to be too far from the planet to be gravitationally bound to it.

Here’s where the trick question we started with gets really tricky: Earth has several quasi-moons that act in just this manner.

For example, 469219 Kamoʻoalewa is an asteroid about 50 meters wide in an orbit with a period of 1.002 Earth years—just around 17 hours longer than Earth’s period! Its orbit is mildly elliptical, taking it about 15 million kilometers farther from and closer to the sun than that of Earth. The asteroid’s orbit is also tipped a smidgen, by about eight degrees, with respect to Earth’s orbit. From our terrestrial point of view, Kamoʻoalewa moves as if it orbits us, just like the “circling” car described above.

There are several other objects like Kamoʻoalewa, each with its own variation on this general behavior. For example, if an asteroid is far enough ahead of Earth in its orbit, it won’t fall behind our planet despite its slowdowns and speedups. (This is a little like a car well ahead of you on the road that you never catch up to, even if its speed goes up and down a bit over time.) From our point of view, the asteroid will appear to stay in one area of the sky instead of circling around Earth like the real moon does. The asteroid 2020 PP1 is an example of this kind of quasi-moon.

Another odd example is 3753 Cruithne. This asteroid takes 364 days to travel around the sun, which means it has a somewhat shorter period than Earth does. Cruithne’s solar orbit is quite elliptical, taking it nearly 75 million km closer to and farther out from the sun. Earth’s average orbital distance from the sun is about 150 million km, so this is quite an excursion. Cruithne’s orbit is tilted quite a bit, too, by nearly 20 degrees. Unlike those other rocks, it can actually be on the opposite side of the sun as seen from Earth, yet it also occasionally approaches to within roughly 11 million km of us.

Cruithne doesn’t seem to orbit us as seen from Earth, so it’s not quite a quasi-satellite, but it gets weirder: from the vantage of our planet, the object appears to move over a wide bean-shaped path that changes position relative to Earth and the sun over time, tracing a smeared horseshoe shape every 770 years or so. While seven “true” quasi-moons of Earth are known, many more asteroids have horseshoe orbits near us.

These orbits tend not to be stable over time; gravitational tugs from the planets can alter them. Sometimes horseshoe orbits can morph into quasi-moon orbits, and vice versa.

Only a handful of such asteroids are known to currently “orbit” our planet. If an object’s orbit changes shape over time, however, it can move into a quasi-satellite or horseshoe orbit, temporarily making that object a terrestrial companion.

Quasi-satellites aren’t restricted to just Earth: Venus has the asteroid 2002 VE68, which orbits the sun with almost exactly the same period as the planet. Nicknamed Zoozve (delightfully because of a misreading used in a label on a poster of the solar system), this asteroid has been a Cytherean quasi-moon for many millennia, but its orbit is changing and will soon move away from Venus.

Although it’s likely that other planets have quasi-moons as well, from our vantage on Earth, they’re too far away to be spotted easily. Perhaps as bigger telescopes come online, we’ll find even more of these weird kinda sorta pseudo moons.

But this situation shows once again that the firm ideas we have about things—what’s a planet, what’s a moon—are a lot more malleable than you might think. In science, it’s always best to avoid hard-and-fast definitions and allow yourself to be flexible in your thinking. Like quasi-satellites, it’s possible, over time, to change your path through life.

(Phil Plait is a professional astronomer and science communicator in Virginia.)