I pose a question to everyone: How many natural satellites does Earth have? Artificial satellites are not considered here. The question may be more complex than it seems. The answer appears simple—Earth has only one natural satellite, our Moon. But if we change our perspective and look at the area around Earth, the answer could increase to seven. If we expand our thinking further, this number could still increase.
To explain this phenomenon, we first need to understand the concepts of quasi-satellites and horseshoe orbits. Orbital mechanics can be quite unusual in many cases. In systems involving only two celestial bodies, such as a planet orbiting a star, things are relatively simple. The orbit of the planet may be circular or elliptical, or some other mathematical shape. In a fixed orbit, the planet’s orbital period—the time it takes to revolve around the star—is usually constant. In a circular orbit, the planet’s speed remains constant, while in an elliptical orbit, the planet moves faster when it is closer to the star and slower when it is farther away.
Now, suppose there is a third body in the system, such as an asteroid that also orbits the star, moving along its orbit. Suppose the asteroid’s orbit is very close in size to that of the planet and slightly elliptical. At two specific points—the perihelion and aphelion—it is at a distance from the star that makes its orbital period approximately equal to that of the planet’s. When moving around the star, it just so happens to be near the planet. From a global perspective, the asteroid and the planet are each in different orbits, moving at different speeds and periods, but they take the same amount of time to complete a revolution. Sometimes the asteroid moves faster than the planet, and sometimes slower, but on average, its orbital speed is roughly the same as that of the planet.
Specifically, when the asteroid is at a position farther from the star, meaning it is on the outside track relative to the planet, its speed will be slower, and over time, it gradually falls behind the planet. Then, as it moves to a position closer to the star, its speed increases, allowing it to gradually catch up and even overtake the planet. Afterward, it moves again to a position farther from the star, slowing down again, and the cycle repeats.
So far, this story seems quite normal. However, if we look at the situation from the perspective of the planet itself, it is completely different. In this new frame of reference, the asteroid appears to be near the planet at all times, sometimes moving forward on the side closer to the sun and sometimes moving back on the farther side. In other words, it is as if the asteroid is orbiting the Earth! This is similar to the pattern of a satellite’s orbit. However, this is just an illusion from the planet’s point of view, because the asteroid is actually orbiting the star, but its movement matches the pace of the planet’s movement, giving the impression that it seems to be revolving around the planet.
We can make an analogy from a scene in everyday life: Imagine you’re driving in the middle lane on a highway, a car overtakes you from the left lane, then continuously switches to the right lane and slows down to let you overtake it, and after that, it switches back to the left lane and accelerates to overtake you again.
Imagine you’re standing by the roadside, and a car drives around you in complex paths — in your line of sight, it seems to keep circling around you. However, to the bystanders, the car is merely changing lanes on the road intermittently, sometimes speeding up, sometimes slowing down. In astronomy, there is a class of celestial bodies whose orbits are similar to the motion of this type of vehicle, and they are called quasi-satellites. These bodies aren’t actually revolving around the planet but share a similar orbit around the star with the planet, and they are too far from the planet to be firmly captured by its gravity.
Our Earth has several such quasi-satellites. A typical example is asteroid 469219, named “Kamo‘oalewa”. This asteroid is about 50 meters in diameter and has an orbital period of approximately 1.002 Earth years, only 17 hours longer than the orbit of Earth. Its orbit is slightly elliptical, with the closest and farthest points to Earth differing by about 15 million kilometers. Kamo‘oalewa’s orbital inclination is about 8 degrees relative to Earth’s orbit. From the ground observation, Kamo‘oalewa appears to orbit around the Earth, much like the car circling example mentioned at the beginning.
However, Kamo‘oalewa is not unique, and other celestial bodies have slight variations. For instance, if an asteroid is far enough ahead of the Earth, although it will experience acceleration and deceleration due to changes in its distance from the Sun, it will never be overtaken by Earth, like the car in an unattainable position mentioned earlier — asteroid 2020 PP1 is one such “substandard” quasi-satellite.
Another asteroid, named 3753 Cruithne, has even more peculiar characteristics. Cruithne’s orbit around the Sun is slightly less than Earth’s at 364 days. Its orbit is highly elliptical, with the perihelion nearly 75 million kilometers closer to the Sun than Earth, indicating a significant distance difference between the two. Compared to Earth’s orbit, Cruithne’s orbital inclination is as high as 20 degrees, which allows it to appear on the opposite side of Earth relative to the Sun and occasionally approach Earth to a distance of about 11 million kilometers.
From Earth, Cruithne does not appear to actually orbit around the Earth. From the perspective of Earth, it moves along a path shaped like a kidney bean or a horseshoe, changing its position relative to Earth and the Sun over time, completing one orbit approximately every 770 years.
Although currently only seven “true” quasi-satellites of Earth are known, there are many more asteroids nearby Earth following horseshoe orbits. Such orbits are usually unstable; subjected to the influence of planetary gravity, their paths may change, sometimes horseshoe orbits can evolve into quasi-satellite orbits or vice versa. Although we currently know of few asteroids orbiting Earth, over time, their orbits may change, allowing them to enter quasi-satellite or horseshoe orbits, at which point they can be considered Earth’s transient companions.
The presence of quasi-satellites is not unique to Earth. For example, there is an asteroid on Venus’s orbit named 2002 VE68, which has an orbital period almost identical to Venus, thereby being affectionately referred to as Zoozve. Interestingly, this naming originated from a story where an illustrator misread the “2002 VE” in the asteroid’s number as “ZOOZVE”, and since then, the asteroid has been officially called Zoozve. It has been a quasi-satellite of Venus for thousands of years, but its orbit is currently changing, and it is expected to no longer maintain the same relationship with Venus.
Although other planets may also have quasi-satellites, observers on Earth find them difficult to detect due to the great distances involved. With the future use of larger telescopes, we may be able to detect more celestial bodies similar to these. This phenomenon emphasizes once again the idea that the definitions of planets and satellites are more variable than we imagined. In the realm of scientific exploration, we should avoid making overly definitive conclusions and maintain a flexible and open-minded approach. With such an attitude, perhaps your life trajectory will, like a quasi-satellite, reveal a brand new aspect in the long river of history.
