Shulou(Shulou.com)11/24 Report--

Space elevator from Wandering Earth 2 Ladies and gentlemen, the space elevator is approaching the weightless space station. Please be prepared to descend the stairs from the right side. "

"Ladies and gentlmen, we are approaching space station. Please prepare to get off the elevator. The door will be open at the right side. "

Would you believe me if I said that one day you would hear such announcements?

01. Where did the space elevator come from? At the beginning of the 20th century, Russian scientist Konstantin Tsiolkovsky, known as the "father of space," put forward several ideas:

liquid rocket fuel;

Reaction forces in space are the only way to move;

More than two rockets are connected in series to form a multi-stage rocket train to increase the speed of the rocket.

Today, more than a hundred years later, these ideas have become important applications in the aerospace field.

Konstantin Tsiolkovsky, however, had an idea in 1895 that has not yet been realized.

This idea is actually very simple:

He proposed building a super-high tower on the ground until it reached geosynchronous orbit, and setting up elevators in the tower so that we could take the elevator into outer space.

This is the prototype of the space elevator.

Does this tower structure feel familiar?

In fact, that's what Tsiolkovsky was inspired by when he visited the Eiffel Tower in France!

This concept is also closest to our understanding of elevators, but...

Geosynchronous orbit is 3,578,600 meters away from us. At present, the tallest building in the world is Burj Khalifa in Dubai, but its height is only 828 meters...

From this point of view, it seemed that the space elevator was dead?

Take it easy!

You are now a cosmologist in the middle of the last century, so figure out how to solve this problem!

If you don't have any ideas for a while, try answering the following question:

New Year's First Question

Suppose I asked you to put a kite 250 meters high in the air. What else could you do besides running on the ground and constantly releasing the kite into the air?

You can take a helicopter higher up, throw the kite, and slowly release it until it reaches 250 meters in the air.

Don't ask me why I fly a kite, and don't ask me if the kite string will break, that's not the point (I won't admit that this analogy is very rigorous)!

The point is, reverse thinking!

Similarly, we want to build an elevator to outer space, the most important thing is to provide rope track, so, since it is unrealistic to build from the ground up, then we…

Can you "throw" a rope from space, like a kite?

That is to say, we can launch a geosynchronous satellite first, then extend a rope from the satellite to the ground and fix it at one end of the ground to form the orbit of the space elevator.

Space elevator concept map (source NASA) Ha! No towers, just "a few ropes"!

It is this reverse thinking that makes the space elevator seem less illusory, and today's space elevator projects are based on this model.

Among the many space elevator projects, the most notable is the space elevator project announced by the Dalin Group in 2012.

In February 2012, Dalin Group, a famous Japanese construction company especially good at building towers, announced that it would invest US $10 billion to build a space elevator. It is estimated that the elevator will operate at an speed of 200 kilometers per hour and take 7 days one way. It is planned to start construction at sea near the equator around 2025 and be completed and put into operation around 2050.

However, ten years have passed since the start of the project, and the prospects seem to be not optimistic. Even senior engineer Ishikawa Yoji, who has been involved in the research and development of space elevators within the Dalin Group Company, admitted frankly: The more this project is tried, the more difficult it is.

First of all, regardless of all external factors, the space elevator is mainly composed of four parts:

the car of the elevator, the cable tracks required for the car to move up and down, the offshore base for fixing the cables at the earth end, and the counterweight.

The first three parts of the space elevator structure seem easy to understand, but why do you need a counterweight?

In the space elevator scenario just mentioned, we have to "throw" the cable from the synchronous satellite and "hang" it all the way to the earth. However, as the cable is gradually lowered, the gravitational force will be greater than the centrifugal force, so the cable will exert an inward pull on the synchronous satellite. Isn't it that the cable will pull down the originally stable synchronous satellite?

To solve this problem, we must "throw" something upward as we lower the cable, creating an outward pull that counteracts the inward pull of the cable on the satellite. The object "thrown" upward must be heavy enough to stabilize the satellite. We call it a counterweight.

However, a new problem had arisen!

The cable is not actually stationary, but is rotating at high speeds with the geostationary satellite, and the huge centripetal force required may exceed the tensile limit of the material, causing the cable to break itself.

Let's deeply understand how demanding the space elevator is on the tensile strength of materials.

In the geocentric reference frame, the cable is simplified into a cylinder with density ρ and cross section S, one end fixed to the geostationary satellite and the other end fixed to the equatorial offshore base. Consider a small length of cable near the geostationary satellite orbit, and regardless of the extra loads, the tension it receives can be calculated as follows:

If we use steel as the cable of the space elevator, simplify the deformation of the above formula and substitute the density value of the steel, we can estimate that the maximum stress that the steel needs to withstand must reach at least 400 GPa. But in fact, the tensile strength of steel is only 400 MPa!

That is to say, even if steel is used as a cable, it will deform directly under the strong gravitational force.

At this point, we encountered an extremely difficult problem: how to find a material with low density but high tensile strength?

At present, the most likely to meet the above requirements is carbon nanotubes: tubular nanomaterials composed of carbon atoms, which are currently known to have the highest theoretical mechanical strength and toughness.

The density of carbon nanotubes in carbon nanotube structure is about 1700 kg/㎡. Substituting the above formula for calculation, it is obtained that if carbon nanotubes are used as cables for space elevators, the tensile strength of carbon nanotubes must reach at least 90 GPa.

At present, carbon nanotubes that we have been able to synthesize in experiments have tensile strengths of up to 200 GPa; even for single-walled carbon nanotubes with ideal structures, tensile strengths can reach 800 GPa.

In this way, as long as we produce tens of thousands of kilometers long carbon nanotubes,"hang" them from synchronous satellites and fix them to offshore base stations near the equator, the problem will not be solved!

However, our path to space elevator exploration is bound to be bumpy.

In 1991, Japanese scientist Sumio Iijima discovered and named carbon nanotubes, which injected the most fresh blood into the space elevator idea that was stuck in the bottleneck. Many research teams have picked up the space elevator project again.

However, it was soon discovered that due to the limitations of the preparation process, the actual length of carbon nanotubes that could be prepared was only a few millimeters, and there were a large number of structural defects.

Alas, it seems that we have reached a dead end again…

But as the saying goes, sinking boat side thousands of sails, sick tree in front of ten thousand trees spring.

In 2013, Professor Wei Fei of Tsinghua University successfully prepared carbon nanotubes with a single length of more than half a meter and perfect structure after increasing the catalyst activity probability of growing carbon nanotubes per millimeter length to more than 99.5%.

Currently, they are developing carbon nanotubes that are more than a kilometer long.

Our space ladder seems to have ushered in a ray of hope!

The practical dilemma of space elevators You may have realized that what has just been discussed is the simplest physical model, and once you really want to consider project construction, you need to solve many practical problems.

For example, given that all kinds of high-voltage wires used in life wear out over time, it is natural for us to ask such questions:

How durable is the cable made of carbon nanotubes?

After all, if the cable breaks easily, the elevator will be useless even if it is built.

In order to test the durability of carbon nanotubes, the Japanese Obayashi Group sent carbon nanotube samples to the Japanese experimental module located 400 kilometers above the surface in 2015.

Samples of Japan's Kibo experimental module were brought back to Earth after two years in space. After analysis, the researchers found that the surface of carbon nanotubes had been destroyed by atomic oxygen.

It should be known that 400 kilometers above the atmosphere belongs to the thermosphere, and the air is extremely thin. Even so, two years has destroyed carbon nanotubes.

It can be imagined that cables directly exposed in the lowest troposphere will face even more severe tests.

In addition to being destroyed by atomic oxygen, it also needs to face all kinds of possible wind and rain, and may even encounter lightning, hurricanes and other extreme weather...

It is obvious that the research on improving the durability of cables is full of difficulties, but we will not stop exploring as long as the road is not blocked.

Of course, in addition to durability problems, there are a lot of problems waiting for us to solve.

For example, how to ensure that the elevator car has enough power to support it all the way from the ground to the space station?

If the power system of the space elevator suddenly failed halfway up, it would be like a high-altitude survival thriller scene. Just thinking about it made one shudder.

If the elevator stops at this moment... (Source: Wandering Earth 2 trailer) For example, how to make the space elevator automatically avoid space debris and some satellites that may hit?

Once he did not dodge in time, the consequences would be unimaginable.

It really proved that sentence:

The space elevator, the more you try, the harder it gets.

At this point, you may well ask, if building a space elevator is so difficult, why do we still cling to this seemingly impossible idea?

Because, we yearn for the star sea.

Cough cough cough, let's not talk about this, let's talk about something practical:

In current international commercial satellite launches, transportation costs per kilogram of payload range from $2,000 to $20,000. If you want to travel to space, you need at least $100,000.

Assuming that the space elevator can be successfully built, regardless of the initial construction cost, according to the estimates of the Japanese Obayashi Group, the transportation cost per kilogram of load is about 200 US dollars!

In other words, Xiaobian only needs to spend about 70,000 RMB to travel in space!

With the space elevator built, in addition to making space tourism within reach, we will be able to transport goods between Earth and space at low cost.

This could be the most exciting turning point in the history of human space exploration!

Now, look up at the sky and imagine.

Seemingly weak every second, are in the pro-witness, the birth of history.

In your lifetime, you will see a grand ladder, piercing through the distant clouds, constantly rushing to the surface of the earth with a destructive momentum, and finally traversing the world, beautiful and unique.

Source: Wandering Earth 2 trailer Thinking of this, I really burst into tears.

This article comes from Weixin Official Accounts: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop), by Muller's nanny

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