As you all know, Earth only has one moon, but many of you may wonder whether there is a second one. Well, sometimes there is, and sometimes there isn’t. Let’s find out more about Earth’s companions in this article.
You may have heard of some claims of a second moon of Earth, but chances are that these are actually co-orbital configurations. From a rotating frame where the Sun and Earth are held stationary, these objects could appear to orbit Earth in a retrograde orbit.
However, these objects are too far away to orbit Earth. Instead, they orbit the Sun in exactly one year, on average. On average, because the orbital period is never the same as that of Earth. The gravity of Earth perturbs the orbit so that the asteroid’s orbit is always in sync with that of Earth, in a phenomenon called an orbital resonance. Sometimes the positions of the asteroid and the Earth relative to the Sun are similar, causing the asteroid to apparently “orbit” Earth because of its larger eccentricity. But again, it’s too far away to orbit our planet, so it’s just an asteroid in a quasi-satellite configuration with Earth.
It doesn’t take an Earth-like orbit to become a companion of Earth. For example, asteroid 3753 Cruithne has an eccentricity of about 0.51, which is as close to the Sun as Mercury and as far as Mars. But it is still in a co-orbital configuration with Earth. In its current orbit, it orbits the Sun slightly faster than Earth. This goes on until Earth catches up and perturbs its orbit, so the asteroid falls behind. As the asteroid falls behind, it approaches the planet again and speeds up against it. This repeating configuration puts the asteroid and the planet in sync, forming a horseshoe orbit. This might not be easy to explain, so we’ll be showing an animation here.
Another co-orbital configuration is a tadpole orbit, in which an asteroid moves around a certain point in the star-planet system. The behavior of moving around and reversing direction is called libration, which is crucial to maintaining orbital resonance. This is common for trojan asteroids, which are trapped in the L4 and L5 points of such systems.
Temporary Satellite Capture
Although most claimed companions of Earth are not orbiting Earth, some do. The most likely scenario, in that case, is an unstable temporary satellite capture. This involves an object moving from deep space into a closed orbit around Earth.
This might seem unreasonable as the inbound and outbound v-infinity speeds are always the same in a flyby. In other words, the eccentricity of the orbit must stay the same. However, in reality, that’s not the case. There are many perturbations, such as the perturbations from the moons or solar radiation pressure. This can modify the object’s velocity so that its eccentricity also changes. If it’s traveling slowly relative to a planet, its eccentricity might drop below 1, allowing a closed orbit around it. That’s how temporary satellite capture works.
If you’re not convinced, there are a few examples of this phenomenon. For example, comet 147P was in orbit around Jupiter between 1949 and 1962. Before that, it traveled in an orbit very similar to that of Jupiter, and perturbations brought it into orbit. It completed a few laps around the planet before more perturbations pushed the eccentricity above 1. You might say that it would only be possible under the immense gravity of Jupiter, but we have seen some asteroids caught by Earth as well. Some prominent examples include 2006 RH120 and 2020 CD3.
Earth sometimes has a second moon that gets captured into an unstable orbit. However, they eventually escape, so we don’t see any other permanent moons, even small asteroids. This shows that the Earth-Moon system is a dynamically unstable place. Earth’s Hill sphere (sphere of influence) is small, in which there is a massive moon whose perturbations can destabilize the orbits of other satellites in a dynamically short time. If you want us to write more about this topic, please leave your suggestions in the comments below.