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Can satellites also have natural satellites? No, because the satellite's Hill ball is too small.
Someone asked an interesting question: Can satellites also have natural satellites?
Considering gravity alone, satellites can certainly make objects close enough to themselves to become their own. However, it is strange that no satellite in the solar system has its own satellite, which is too unscientific!
Spoilers for the correct answer to this question: The satellite's Hill ball is too small.
▲ There are so many satellites in the solar system, why are there no "satellites"?
Hill Ball? What's this?
This is a concept proposed by the American astronomer George Hill in the middle of the 19th century on the basis of Roche, hence the name "Hill Ball". There are many planets in the solar system, after an early state of chaos, the survivors are heroes, they divide their own territory, each territory is roughly a sphere, which is the "Hill sphere" of each planet. Objects within a star 'Hill sphere' will be primarily subject to the gravitational pull of that star.
▲ American astronomer Hill, "Hill Ball" is named after him.
How big is the Hill Ball?
Consider the system of two celestial bodies, and the radius of the "Hill sphere" of each celestial body is the distance from their center of mass to point L1. So if you know how to calculate lagrange points, you will naturally calculate how big the Hill ball is. Obviously, the larger celestial hill sphere is more "strong", while the smaller mass celestial body can only be bullied by the gravitational pull of the larger body and dwell in a small space.
▲ Or this Lagrange point map, only consider the helio-terrestrial system, the distance from the sun to L1 is the sun's hill sphere radius, the central large L3 to L1 is the sun's hill sphere, and the right between L1 and L2 is the earth's hill sphere.
You can understand that if there is only one celestial body in the universe, its gravitational equipotential plane will form layer after layer of perfect spheres, like layers of bubbles. But the universe is not lonely after all, only two celestial bodies are considered here, and the gravitational fields of the two sides will be divided after a battle with each other. Just as the layers of bubbles on both sides collide with each other, some squeeze each other, some merge, and the ability of larger celestial bodies to maintain their own bubble layer is stronger, and the bubbles that are completely their own are of course larger.
▲ You can use bubbles to understand Hill balls.
In the solar system, the sun occupies 99.86% of the mass, and its gravitational force is pervasive, controlling almost the entire solar system. The gravitational bubbles of the eight planets are squeezed by it, and the closer to the Sun, the smaller the Hill sphere, such as Mercury, Venus, Earth, Mars and other terrestrial planets; and far from the Sun, the Larger the Hill Sphere, although the mass of Jupiter is much greater than Neptune, but because Neptune is the farthest from the Sun, it has the largest Hill sphere.
On the other hand, hill spheres are also related to the average density of celestial bodies, you see although the mass of terrestrial planets is much smaller than jupiter, Saturn and other behemoths, but because their density is about 4 times that of Jupiter, the hill sphere is not much smaller.
▲ The radius of the Hill ball of the eight planets and some asteroids and dwarf planets. From left to right: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Ceres, Pluto, Anderson.
Someone started to scratch their heads: "Do I have a Hill ball too?" ”
The answer is: "Yes, but it's too small to make sense." ”
So to upgrade: "Does the space shuttle have a Hill ball?" This way the astronauts can circle the shuttle. ”
Assuming that the space shuttle weighs 100 tons and flies in space 300 km from the Earth's surface, it can be calculated that its hill sphere radius is 1.2 meters, which is smaller than itself.
If there's only one Tesla in space, it's easy to get the golf ball to spin around it.
According to the previous description, density and gravitational field are important factors in determining the size of the Hill sphere, so it is not absolutely impossible, on the one hand, it is farther away from the Earth, on the other hand, to increase the density of the object, you can let it rotate around the Earth on one side, and also have a larger Hill ball. It has been calculated that if you put an object in geosynchronous orbit (36,000 km) with a density of 55g/cm3, it is possible to have its own hill ball and let other objects rotate around itself. But where to find such a heavy ordinary substance? Among the common substances on Earth, the highest density of osmium is only 22.59g/cm3.
Don't tell me about white dwarf super-solid, neutron star neutron state or whatever, there is no such thing as on Earth!
Osmium metal is the densest of common substances.
Back to the previous question: Can satellites have natural satellites?
Theoretically, of course, it is possible to have, don't you see, didn't humans launch a lot of artificial satellites into the lunar orbit? In fact, there are asteroids in the solar system that have their own natural moons.
There is an asteroid 1999 KW4, orbiting the sun in an elliptical orbit, with perihelion closer to the sun than Mercury's orbit, only 0.2 astronomical units, while the aphelion is very close to Earth, at 1.1 astronomical units, and calculated that it will pass the Earth on May 25, 2036, at a distance of only 2 million kilometers from the Earth, so this is a dangerous asteroid that requires close attention.
Interestingly, although the asteroid is small, only 1.3-1.5 km in diameter, it has a companion - a small object with a diameter of only 360 m orbits the asteroid and becomes its moon.
▲ Asteroid asteroid with satellites 1999 KW4.
However, this is only a case after all, the truth is cold, and none of the 185 satellites in the solar system have been discovered to have their own "satellites". (Is it okay to call it that name?) ）
The universe is indifferent, the law of gravitation is everywhere, and those who follow it will die. The solar system is crowded again, and where jupiter and Saturn go, it is as if a giant aircraft carrier has passed through the strait, and the waves have spread far and wide, torturing the "Hill sphere" of weak and small objects again and again. These weak satellites are not safe on their own (didn't you see that Io is often pulled to "vomit"?). ), under the nest, Ann has the finished egg?
In short, after a long astronomical time, even if there were some "satellites" at the beginning, they were already swept away by the major planets.
The volcanic eruption photographed on Ganymede stems from Jupiter's strong gravitational pull.
It could also explain why Mercury doesn't have moons. Mercury is too close to the Sun, the Hill Sphere is the smallest, even if there have been moons, it has long been penetrated by the Sun's gravity, and the satellites either hit Mercury or the Sun, or wandered the world.
▲ From this chart, Mercury is not as big as Sunspot, so you still want to be a big brother? Save it.
Astronomers have also been troubled by Titan's equatorial ridge, which is 10-20km high, making Titan look very much like a walnut. It has been suggested that this is because Titan once had a "Guardian Moon" that was torn apart by Saturn's gravitational pull, and some of the wreckage fell to Saturn's surface, forming this one that towered into the clouds (curious, is there a cloud on Titan?). ) of the mountains.
▲ Titan eight shaped like a walnut. See that equatorial ridge?
In short, if someone asks, what did the celestial bodies end up pulling and pulling? We can roughly answer it like this: pulled into a ball.
Further reading:
Where did C come from? | Lu Chao
Thanks! Just passing by the sun, here is a review of the earthlings' exploration of me | Lu Chao
The Millennium Feud between Human Beings and "Perpetual Motion Machines" (5): How to Avoid Falling into the Trap of "Perpetual Motion Machines"|? Lu Chao
The Millennium Feud between Mankind and the "Perpetual Motion Machine" (IV): Zero Point Energy? Time crystals? Is it science or science fiction? | Lu Chao
The Millennium Feud between Mankind and the "Perpetual Motion Machine" (III): The Road of "Carrying" Energy Full of "Bumps" | Lu Chao
Build bases, install mirrors, blow up poles... Are these ways to modify Mars reliable? | Technology Yuan Ren
Background: Lu Chao, the author of this article, is the deputy director of the Science Fiction Professional Committee of the Jiangsu Science Writers Association, and a member of the Jiangsu Science Writers Association. Graduated from the Department of Physics of Nanjing University, his representative work "Grimace Chemistry Class - Element Family" was listed as the "IYPT 2019 Excellent Popular Science Book" bibliography of the Chinese Chemical Society, and won the first prize of the 10th Jiangsu Provincial Excellent Popular Science Works. This article was published on the WeChat public account on February 28, 2019 Rational Boundary (pulling out the gravitational ball - Hill ball), and the Voice of the Wind and Cloud was reprinted with permission.
Editor-in-Charge: Xinyue Chen