Image credit: pixabay
Let me ask you a question: how many satellites does the Earth have? Of course, I'm talking about natural satellites, not taking into account artificial satellites.
Oh a word of caution, there are pitfalls to this question.
The answer is one – the moon. Simple, right? But think about it another way: if you change the frame of reference and look around the Earth, the answer is more like seven. If you think a little more broadly, there will only be more of them.
How so? To explain this, we must first talk about the concept of quasi-satellites and horseshoe orbits.
The discipline of orbital mechanics is rather strange. If there were only two celestial bodies, such as a planet orbiting a star, the problem would be much simpler. The orbit of the former may be circular or oval (elongated circle) or several other mathematical shapes. The time it takes to orbit a star is known as the planet's orbital period, and it tends to remain the same. (The speed of a planet orbiting in a circular orbit remains the same, while a planet orbiting in an elliptical orbit is faster when it is closer to the star and slower when it is farther away from the star)
At this point, it is assumed that there is a third celestial body, such as an asteroid, that is also orbiting the star. Its orbit may likewise be round or oval. From God's point of view, asteroids and planets orbit in different orbits, at different speeds, and at different cycles.
Strange places are coming. Suppose the asteroid's orbital dimensions are very similar to those of the planet, but slightly more elliptical. It is also at just the right distance from the star at a specific location (perihelion and aphelion) so that its orbital period is almost exactly the same as that of the planet. Not only that, but it is in the immediate vicinity of the planet while orbiting the sun. From God's point of view, it orbits the star and takes the same amount of time to make a circle. Asteroids sometimes travel faster than planets, sometimes slower than planets, and in general, their average orbital speed is roughly the same as that of planets.
Specifically, when the asteroid is farther away from the star, its orbit is also outside the planet, at which point it is slower and will gradually lag behind the planet. After a while, it will come closer to the star, and it will be faster than the planet, and it will gradually catch up with and overtake the planet. Then it comes farther away from the star again, slows down again, and so on.
So far, the story sounds normal. But from the planet's point of view, the situation is very different – the asteroid seems to have remained near the planet all the time, sometimes closer to the Sun and moving forward, sometimes farther away from the Sun and moving backwards. In other words, it's like an asteroid orbiting the Earth!
It sounds a lot like how satellites operate. However, this is only an illusion seen from a planetary perspective, and the asteroid is not actually orbiting the planet, but rather orbiting the star. It's just that the asteroid's motion happens to be close to the planet's pace, making the asteroid appear to be orbiting the planet.
We can make an analogy with a daily scenario, imagine: you are driving along the middle lane on a three-lane road, a car overtakes from the left lane, then successively changes lanes to the right lane, slows down to allow you to overtake it, and then changes lanes to the left lane and accelerates again to overtake you. From your point of view, the car seems to be driving around you, while from the perspective of a passerby, it's just changing lanes left and right while accelerating or decelerating at regular intervals.
Celestial bodies with similar orbits are known as quasisatellites. They are not really moons because they actually orbit stars instead of planets. And they are often too far away from the planets for the planet's gravity to trap them.
And that's where the question at the beginning really gets tough: Earth has a few quasi-satellites that operate in this mode.
For example, the asteroid 469219 Kamo'oalewa, which is about 50 meters wide, has an orbital period of 1.002 Earth years, and an orbital period that is only about 17 hours longer than that of Earth. Its orbit is slightly elliptical, with perihelion and aphelion about 15 million kilometers closer and farther than Earth, respectively. Its orbit has a small angle to the Earth's orbit, which is about 8°. From the Earth's point of view, the oscillating stars are like orbiting the Earth, just like the "circle" car mentioned above.
The Oscillating Star is not alone, but its companions are always different. If an asteroid orbits in a similar orbit but is positioned far enough ahead of the Earth, it will never be overtaken by the Earth, although it will still accelerate and slow down depending on its distance from the Sun. (It's like the other car driving far in front of you, and even if it keeps accelerating and slowing down, you can never catch up to it.) From our perspective, the asteroid appears to stay in a specific region of the sky rather than orbiting the Earth like a real satellite – asteroid 2020 PP1 is one such "substandard" quasi-satellite.
Another quirky example is the asteroid 3753 Cruithne. The asteroid takes 364 days to orbit the Sun, which means that its orbital period is slightly shorter than that of Earth. Krutney's orbit is quite elliptical, and the perihelion is nearly 75 million kilometers from the Earth (the average distance between the Sun and the Earth is 150 million kilometers), which is quite a big difference. Unlike other celestial bodies, Krutney's orbit is 20° away from the Earth's orbit, and it can occasionally come as close as 11 million kilometers from the Earth.
图片来源:Jecowa/wikipedia
From Earth, Krutny does not "orbit" the Earth, so it is not a quasi-satellite, and the reality is even more peculiar: from the Earth's point of view, its trajectory resembles that of a broad bean, or horseshoe, and its position relative to the Earth and the Sun changes over time, about once every 770 years. Although there are only seven known "true" quasi-satellites of the Earth, there are many more asteroids orbiting in a horseshoe-shaped orbit near the Earth.
The orbits of these bodies are often unstable, and the gravitational pull of the planets can change them. Sometimes a saddle-shaped orbit changes to a quasi-satellite orbit and vice versa.
There are not many known asteroids that "orbit" the Earth. However, if the shape of these celestial bodies changes over time, it may be able to enter a quasi-satellite or saddle orbit and temporarily become Earth's companion.
It's not just the Earth that has quasi-satellites. Venus has the asteroid 2002 VE68, which has an orbital period almost identical to Venus, and is nicknamed Zoozve (the story behind it is interesting that an illustrator mistakenly thought 2002VE for ZOOZVE when drawing a poster of the solar system, hence the official name of the asteroid). It has been a quasi-moon of Venus for thousands of years, but its orbit is changing and it will soon leave Venus.
Although other planets may also have quasi-satellites, they are too far away for us to be easily spotted when observed on Earth. Perhaps with the opening of telescopes with larger apertures, we will find more of these strange "fake" satellites.
But this situation shows once again that the old beliefs that were deeply believed to be the definition of planets and moons are easier to change than you might think. In scientific research, we should try to avoid jumping to conclusions too quickly and be more flexible when thinking. This may be like a quasi-satellite, and the trajectory of your life will change over time.
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[1]https://www.scientificamerican.com/article/earth-has-more-than-one-moon/
来源:环球科(ID:huanqiukexue)