The space elevator for The Wandering Earth 2 has been studied by the physicist for 20 years, and he has something to say

The space elevator system in "The Wandering Earth 2" is one of the "future infrastructure" that often appears in science fiction, but this seemingly illusory device is indeed a serious topic for a group of scientists. Stephen Cohen, a physicist at Vanier College in Canada, has a lot to say about it: "I've been working on space elevators for almost 20 years, and although there are still many problems to be solved, we are getting closer and closer to a dream come true. (No spoilers in this article)

Stephen Cohen (Physics teacher, Vanier College, Canada)

Translated by Qiyi

Edited by Wei Xiao

Space elevators are often dismissed as science fiction pipe dreams, but I believe it will come true soon—perhaps within the next two or three decades. Throughout my career as an aerospace engineer and physics professor, I have always thought about the idea of a "cable stretching from Earth to space" along which people and cargo can easily travel to and from. In recent years, I and other researchers have found new ways to improve the design of space elevators and make them a reality.

There are many reasons to build a space elevator, the most obvious of which is the huge energy and cost savings. Compared to rockets, space elevators are a more practical way to enter orbit. Another reason that is often overlooked is ease of use: the term "space mission" will be replaced by "transport" because access to space will be more routine and almost independent of weather conditions. Compared to the current "space missions" in which astronauts risk their lives at great risk at every launch, "transport" personnel will be safer. A space elevator could be a bridge to the entire solar system: releasing cargo loads in lower positions to orbit Earth, and cargo orbiting the sun at higher locations, all without the help of fuel.

Although I may seem like a proponent of space elevators, I'm actually just happy to study their mechanics. In a world full of problems, thinking about projects like this allows me to imagine scenarios in which we have become responsible caretakers of the planet.

My story unfolded in 2004: I was sitting as a master's student in Professor Arun Mistra's office, expecting him to supervise my thesis. Misra was a top aerospace expert in the mechanical engineering department at McGill University in Canada, so I was a little nervous. Our conversation at that time was as follows:

Me: What research do you think I should do?

Misra: Have you ever heard of a space elevator before?

Me: No. What is that?

Misra: Imagine a cable extending up to 100,000 kilometers high from the Earth's equator, with the far end fixed to a satellite. This system rotates with the Earth. The "climber" on the cable can adjust the load of cable transportation and release it in space. I was thinking you might be able to study the dynamics of this system.

Me: That sounds... It's difficult

Misra: Your job won't be hard. Really want to build an actual space elevator on Earth... It's more difficult.

Fast forward the memories to a few years later. I had just published my master's thesis, "The Dynamics of a Space Elevator," and I was working as an engineer on a satellite design project. On a weekend trip, my friend referred to me as "the space elevator guy" when he introduced me to his buddy, Colin. My wife rolled her eyes. I explained to Colin how the space elevator works.

Me: If you stand at the equator and stare at a satellite in geosynchronous orbit (about 36, 000 kilometers above sea level), it will appear to be stationary in space, because it is just the right speed, orbiting the Earth exactly once a day. Now, the satellite drops a cable on Earth while using fuel to rise higher, and they still rotate around the Earth. This cable becomes the track used by mechanical climbers, similar to a train on a vertical railroad that can carry cargo loads into space.

Colin: But what keeps cables tight?

Me: This is a combination of gravity and centrifugal effects, which fight each other and change with the length of the cable. Under geosynchronous orbit, gravity is even better; Above the orbit, centrifugation is more pronounced. This results in tension throughout the cable, with its maximum being exactly where the synchronous track is.

Colin: It's Friday night! Let's explain it simply.

Me: In order to build a space elevator, we need a material that is about 50 times stronger than steel. But at the same time, I and some of the world are assuming that this problem will be solved later, and that we will solve other engineering problems of the space elevator in the process of waiting.

Colin: Cool.

Space elevator in The Wandering Earth 2 Source: Movie trailer

In 2014, my wife and I met Colin again, and he asked, "What is that space elevator like now?" ”。 My wife closed her eyes and had an "oh no" look on her face.

Colin: What I don't understand is why when a passenger is loaded up from the bottom of the cable, the whole cable is not ripped off.

Me: If a climber is below geosynchronous orbit, especially in near-Earth regions, the tip of the cable will move down slightly and the overall tension of the cable will change. The real problem is that the strain on the cable between the climber and the earth will be reduced (imagine you lift a rubber band vertically and then add an object in the middle of it). If this tension is reduced to zero, then the cable will not be taut and its structure will lose its intrinsic stability. As a result, the climber (including the load it carries) will have a maximum mass limit, which is about one percent of the mass of the entire cable. Since the quality of the cable is estimated to be hundreds of tons, this maximum mass limit will still be a large number.

Colin: So what about the cable material?

Me: I've already told you, it's not something I need to solve.

Colin: Come on, buddy!

Now, in 2022, I'm holding a workshop at Vanier College in Canada, where I teach physics, where I summarize some of my research on space elevators over the past two decades. When the presentation is over, the Q&A section begins.

Student 1: When will the ideal material for the construction of space elevators appear?

Me: Although the synthesis of some potentially suitable materials has advanced in recent years, we are still at least 10 years away from the ideal material, which needs to have both good enough performance and efficient production at a reasonable cost. It's not uncommon for new technologies to emerge and wait for better materials to emerge. Fortunately, the driving force behind materials research itself is not directly related to the space elevator topic.

Student 2: This sounds really cool. But why are we building it?

Me: You can think about it, rockets as a mode of transportation are extremely ridiculous. For a typical space mission, up to 90 percent of the mass on the launch pad comes from fuel. It's like a car with only a tank compressed by 100,000 liters of fuel, but no engine. We need to replace this inefficient way of getting out of Earth's gravity and going to space in a greener way.

NASA plans to send humans to Mars by 2040. Before we had a steerable space elevator, I was skeptical that people would actually be able to walk on Mars (at a cost of hundreds of billions of dollars). But if this is to be a sustainable event, we will need to have a device like a space elevator, and the sooner the better.

Student 3: So, when do you think it can be created?

Me: There is a famous author and engineer, C. Clarke, whose novel The Fountains of Paradise chronicles the construction of the first space elevator. When he was asked this question in the early '90s, his classic response was: "Maybe it's about 50 years after people stop laughing at the idea." In a more modern way, this might be this: When Elon Musk starts to be appreciated for this, we will know that we are not far from achieving it.

To this day, my feelings are very similar to the tension I had when I sat in Arun's office (yes, we still work together, and it always seems strange for me to call him that). This exquisite avenue into space stimulated my imagination and filled me with hope.

Original link: https://www.scientificamerican.com/article/space-elevators-are-less-sci-fi-than-you-think/