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If the astronauts perform space missions for a long time, they need to rotate the spacecraft to generate artificial gravity , but this is not as easy as you imagine.
Although it is great for people to live in space, the " weightless " environment brings some serious challenges. Humans are affected by constant gravity on the earth and can perform their best physical properties. However, if exposed to the microgravity environment for a long time, the human body will be affected many times, including bone loss and muscle atrophy.
Therefore, if humans want to live in space, they need to create an artificial gravity environment . There is only one way to do it: build a vehicle that moves at a constant acceleration. The most common concept is to build a rotating spacecraft, but it is not as easy as it sounds, with a lot of difficulties in it.
feel the gravity
First of all, let’s review the basic knowledge of gravity and the meaning of feeling the gravity.
Gratitude is the attraction between objects with mass. Because both the human body and the earth have mass, gravity keeps pulling you towards the earth and keeping it on the ground. But even if this force continues to act on you, you can't feel it because the earth pulls in various parts of your body at the same time, making this effect undetectable.
I know what you are thinking: "I sit in a chair, and of course I can feel the effect of gravity." In fact, what you feel is not gravity, but the force of the chair (and the ground) pushing you upward. We call this upward thrust " attention " (apparent weight).
We can get a good understanding of the concept of attention by taking the elevator. In a stationary elevator, when you press the up button, the elevator starts moving upward. This means it must have an upward acceleration (at least for a short time) until the elevator reaches its running speed. In this upward acceleration, you feel a little heavy. Then, once the elevator approaches its set floor, it must slow down. This means it accelerates downwards, during which time you will feel a little light.
However, your actual weight will not fluctuate. Your real weight measures the amount of force gravity exerts on your body, and it is the result of the interaction between your mass (m) and the distance between the Earth's mass and your distance from the center of the Earth. On Earth, gravity exerts a force of 9.8 Newtons per kilogram. (Mass and weight are different. On planets with different gravity, even if the mass is the same, the weight will be different.)
Elevator will change the attention weight, not your weight or mass. This may seem a bit strange, but this effect is very useful for spacecraft.
linear acceleration
Assuming you are in gravity-free space—or in a low-Earth orbit with microgravity environment (the microgravity here represents weightless environment), what happens if the spacecraft has a giant elevator that keeps accelerating upward? If the acceleration of the elevator has the same value as the gravity field on the earth's surface, the gravity you feel is exactly the same as on the earth.
Of course, a spacecraft with an infinite elevator is unrealistic. It would be easier to accelerate the entire spacecraft, which would definitely produce artificial gravity. In fact, this is the main method of generating gravity on space ships in the science fiction novel The Expanse series.
But there is a problem here: in order to generate continuous gravity, the spacecraft needs to continuously accelerate , the engine cannot be turned off, otherwise the acceleration will drop to zero, which requires a lot of fuel, and artificial gravity will cost a lot of money (in "The World of Sky", they invented the "Epstein Drive Technology", which can be regarded as magic).
For humans of our time, another acceleration method will be needed to generate artificial gravity.
circumferential motion
acceleration is defined as the rate of change of velocity . If a car accelerates from 10 m/s to 20 m/s in 1 second, the vehicle's acceleration is 10 m/s² per second. (We usually write it as 10m/s².)
but the speed is actually a vector. This means that speed can not only indicate how fast the object moves, but also the direction of the object moving.
Assume a car is heading west at 20 m/s, then turns, and after 1 second it heads north at 20 m/s. From the definition of speed, even if the car is traveling at the same speed, it will have acceleration due to changing direction. If we know the radius (R) and speed (v) of the path the car takes during this turn, we can calculate the magnitude of the acceleration:
You don't really need this mathematical operation. Intuitively you can know that a car turns is an acceleration because when you turn you can feel something pushing you to the side of the car, just like you feel acceleration in a moving elevator.
This is why we can use rotating objects to generate artificial acceleration. You don't need to have the spacecraft or space station go around like a car. Instead, imagine a huge rotating object with people standing inside. It looks like this:
Illustration: RHETT ALLAIN
Three people stand inside a rotating cylinder. Because they are all moving on a circular path, each of them feels a bit like the upward acceleration of . (For them, the "upward" direction is towards the center of the cylinder.)
It turns out that we can describe their motion by the angular velocity of the spacecraft (ω) instead of the velocity (v). Everyone will have an acceleration:
The unit of measurement angular velocity (ω) is radians/second. If the acceleration value is —9.8m/s² is the same as the gravity field on the Earth’s surface—then that person will almost feel like he is standing on the Earth’s surface. (We'll talk about the difference soon.)
for a rotating spacecraft or space station The biggest advantage is: Once it is allowed to start spinning, there is no need to use any rocket fuel to keep it running. Unless there is an external force, it will continue to rotate. This is why you often see this artificial gravity method in science fiction TV series and movies (such as " Mars Rescue ", " Babylon V ", "2001: A Space Odyssey", " Interstellar ", etc.).
This equation illustrates the important thing in designing a spacecraft. You can make small vehicles (R is smaller) and make them spin very fast (ω is larger), or you can make a large hull with a smaller rotation rate.
The smallest rotating spacecraft
If the radius of the rotating spacecraft is reduced, must increase its angular velocity to obtain the required acceleration. (Suppose you need to reach 9.8m/s², which is 1 g, which represents the same acceleration as standing on the surface of the earth.)
But the problem comes again - this time it is person . Yes, we have some problems dealing with rotation. (Personally, I can’t play any spinning equipment in the amusement park, such as the crazy tea party at Disneyland , and I feel a little disgusting just thinking about it.) According to laboratory tests, the rotation speed that most people can tolerate is about 1 rpm per minute. Other data suggest that angular velocities up to 4 rpm are also possible. Another study concluded that with constant rotational training, humans may be able to work at 26 rpm.
Assume we have some astronauts who are very good at spinning and don't irritate, and they can tolerate a 26 rpm spin rate.How small a spacecraft do you need to build to generate 1g of artificial gravity?
First, we need to convert the angular velocity from revolutions per minute to radians per second, and get the value of ω = 2.72 radians per second . (Please remember: 1 turn is equal to 2π radians.)
Next, we only use the acceleration of 9.8m/s² and solve R (radius), and we will get a circular spacecraft with radius 1 1.3 meters and 2.6 meters in diameter. This is super small, even smaller than the diameter of the ISS module (the latter is about 4.2 meters). If a more reasonable angular velocity of 4 rpm is used, the diameter of the spacecraft will become 111.7 meters, about 112 meters, reaching the size of a football field or football games.
If you don't want to make a 112-meter-sized rotating spacecraft, you can use a small trick: you can use two smaller containers instead of a large container , and connect them together so that the two smaller parts rotate around a common center of mass. Humans can be in one (or both) of these parts in order to experience the artificial gravitational field . You can see an example of this rotating ship in the movie "Stowaway".
Grain difference
Even under such conditions, there are two differences from what you feel on earth: first of all, the artificial gravity field on the head may have different values from the field strength under the feet. To understand why this happens, let's imagine a person standing in a rather small rotating spacecraft.
Illustration: RHETT ALLAIN
Since people are in the rotating space, their heads and feet have the same angular velocity (ω). However, they do not move in circles of the same size. The head is closer to the center of the rotating aircraft than the foot, so the radius of the circular path (Rh) of the head is smaller than the radius of the circular path (Rf). Remember that the acceleration value (i.e., artificial gravity) decreases with the decrease in the radius of motion, so it feels a little strange that the person's head will be subjected to a smaller field of gravity than the feet.
may even be worse than this. Imagine that super-small spacecraft with a radius of only 1.3 meters. This is smaller than the height of an average person—the astronaut's head will exceed the center of rotation. In this case, their heads will be pulled to one side of the ship (we call it the ceiling), and their feet will be pulled to the other side, which we can call the floor. Even if high-speed rotation is not enough to make astronauts sick, this gravitational difference will affect the astronauts.
This "gravity difference" is not a big problem for larger rotating spacecraft. Let's consider an example of a rotating aircraft with a diameter of 112 meters. It has a radius of 55.8 meters and an angular velocity of 4 rpm. The gravity field at the "floor" is 9.8m/s², the same as on Earth. If the astronauts are 1.75 meters tall, their heads will move within a radius of 54.1 meters. This means that at their heads, the gravitational field will be 9.49m/s². It was only 3.2% less than the venue under their feet—so it was no big deal.
Coriolis
There is another force in the rotation vehicle, called Coriolis . This force is a bit complicated, let's start with an example of using a carousel. Suppose there are two people (labeled A and B) standing on this carousel, one on the edge and one near the middle. The top view is as follows:
Illustration: RHETT ALLAIN
Please note that both people move around the circular path at the same angular velocity. However, B must go further in the same loop time as A, which means B has a larger linear velocity (v) than A.
This is no big deal unless B decides to move towards the center of the circle. By moving to a new circular path with a smaller radius, B must accelerate to a new track, the force that this additional acceleration is called the Coriolis force.
If a ball rolls from B to A, it moves along the curved path as shown below:
Illustration: RHETT ALLAIN
The magnitude of the Coriolis force depends on the speed of the moving object (relative to the rotating object) and the angular velocity of the carousel. The same thing happens on spacecraft.
The calculation of Coriolis force involves many factors and is quite complicated. Here I will write it into an equation:
One thing to note is that this force is always perpendicular to the speed . If the object is still in the rotation coordinate system , the force is zero.
What impact will this have on astronauts in a rotating spacecraft? If the person just sat still, nothing would happen. But what if they stand up? In the process of standing up, they will have a speed towards the center of the circle, because the center of mass of a person moves upwards from sitting to standing.
Coriolis will push them sideways relative to their speed. Depending on the orientation of the chair, this force may push them in different directions. If the chair is in the same direction as the spacecraft, Coriolis pushes the person forward as he stands. If the chair is facing backward, it pushes them backward. If the chairs face one side, they will be pushed to the other side. And it's not just standing up, if you move your hand, there will be a lateral force acting on it. If you try to pour the drink into a glass, there will always be a lateral force on the liquid. Maybe you can adapt to the lateral force of every move, but it is always very uncomfortable.
Is there anything I can do for Coriolis? Yes, you can minimize this lateral thrust by designing a spacecraft with lower angular velocity , which means it takes longer to complete a rotation. It also means less artificial gravity.
If you want your spacecraft to have artificial gravity that simulates the Earth and the smaller Coriolis effect , you only need a larger spacecraft. It's a tough choice: you can build a small, cheap spacecraft to endure annoying Coriolis, or you can build a large, expensive spacecraft that will allow you to enjoy the same comfort as your home — but that will be big and expensive.
Author: RHETT ALLAIN
Translation: Nuor
Review: zhenni
Original link:
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