Will a satellite made of wood become a “savior” for space junk?

Recently, the Japanese company Sumitomo Forestry is cooperating with Kyoto University to jointly develop a new type of satellite " LignoSat ", and plans to launch it in 2023.

Its biggest feature is that it uses wooden materials to create the satellite's shell.

▲ Concept image of wooden satellite. Picture from: Sumitomo Forestry

At present, most satellite shells are made of composite materials such as aluminum alloy, titanium alloy or carbon fiber to ensure that the internal components can operate normally under extreme environments.

But once a satellite completes its mission or exceeds its useful life, it is likely to be abandoned directly and become "space junk" floating in low-Earth orbit.

What is the difference between a satellite made of wood?

In an interview with the BBC , former Japanese astronaut and professor of Kyoto University Doi Takao Doi who participated in the project said that when the satellite de-orbits and returns to the atmosphere and burns, it will disperse into many tiny alumina particles. Over time, these particles will Affect the earth’s environment.

Wooden satellites do not have this hidden danger of pollution, and compared to various metals, wooden materials can be burned by the atmosphere more easily, without large pieces of debris, which poses a threat to the ground.

In addition, Nikkei Asia also mentioned a benefit of "wood satellites", which is conducive to communications. Because the wood itself does not block the signal like metal, it can directly let electromagnetic waves pass through. In this way, the satellite antenna can be placed inside, and the wood itself is also lighter, and the satellite can be designed more compactly. More compact.

According to the official statement of Sumitomo Forestry Company, the project is now in the research phase, and will only progress to the satellite design part later. Among them, the Sumitomo Forestry Association is mainly responsible for the application of wood materials in space, and at the same time develops wood materials that are resistant to high temperatures.

Research report by Takao Doi : In a vacuum environment, the mechanical properties of different woods will change over time

At the same time, Kyoto University, a partner of Sumitomo Forestry, will lead the testing session to verify whether the wood can withstand the tests in various extreme scenarios. For example, in a vacuum environment, the mechanical properties of wood, and the growth of trees under low gravity and low pressure.

Incidentally, here is a Japanese Sumitomo Forestry Company. It is not an ordinary building material developer, but a century-old factory in Japan. It established a special research institute in 1991, with wood materials as the core, to study its application feasibility in various fields.

▲ Sumitomo Forestry's w350 plan to build a 70-story wooden skyscraper

In addition to thinking of using wood to build satellites, we also reported earlier that Sumitomo Forestry plans to build a 350-meter tall, 70-story " wooden skyscraper " by 2041, which can be said to be another bold idea.

If the satellite plan is successfully completed, the results will also help Sumitomo Forestry's building projects.

Make wood as hard as metal

Wood is one of the earliest materials that humans come into contact with, and it is also one of the few resources that we can obtain from nature and can be continuously regenerated. Further tapping its value and potential will benefit far more than traditional categories such as architecture and furniture.

In 2018, researchers at the University of Maryland published a study that successfully transformed massive natural wood into a high-performance structural material, basically as hard as metal.

▲ Super wood with hardness comparable to traditional metal materials. Picture from: nzgeo

According to a report in "Nature" magazine at the time, researchers first poured wood into concentrated hydrogen peroxide liquid, then boiled it to remove part of the lignin and cellulose in the wood material, and then performed ultra-high temperature mechanical heat pressing. , The wood is densified , its density has reached 3 times the original, but the hardness has increased by 10 times.

▲ Comparison of untreated wood and densified wood

After testing, the treated "super wood" has the strength equivalent to most structural metals and alloys. The research team also tried to shoot the wood with a high-speed steel column, in which the untreated wood was completely penetrated; the super wood effectively slowed the rate of fire; as for the super wood with multiple layers, it further jammed the column. internal.

This also proves that through the improvement of structural components, we can indeed endow wood with richer properties and even develop new characteristics.

Another advantage of super wood is cost. According to scientific researchers, compared with Kevlar fiber boards of the same thickness, the protection ability of super wood will be slightly weaker, but the cost is only about 5% of Kevlar. If it is used in construction, transportation and other fields, the cost will undoubtedly be significantly increased. reduce.

The same is true for the new wooden satellites. Changing the shell from aerospace materials to new types of wood can also greatly reduce manufacturing costs, reduce weight, and reduce volume.

But for satellites to be launched into outer space, the hardness is only up to the standard, and may not be able to meet its requirements. What we most often say is resistance to high and low temperature, corrosion resistance, and longer service life, which still require actual testing to verify.

How serious is the space junk now

As more and more satellites are launched into low-Earth orbit, space junk has become an increasingly serious problem.

Some people like to say that the world's largest garbage dump is not on the earth, but in low earth orbit.

This is also related to the satellite launch process. The rocket is launched from the launch center, and then boosted step by step. The first and second boosters that use only fuel will separate from the rocket body and fall back to the surface of the earth, and some are directly burned in the dense atmosphere.

However, a considerable part of the rocket wreckage, or fairing, was left in outer space, unable to be disposed of, and became what we call "space junk."

So, can the wooden satellite envisioned by Japan this time alleviate the problem of "space junk"? Some space enthusiasts still hold a negative attitude.

In their view, the advantages of wood satellites are that they are low-cost, easy to burn, and do not leave harmful elements. However, the latter two advantages can only be manifested when they enter the atmosphere after deorbiting.

But if the satellite itself cannot actively deorbit, the wooden satellite will still become part of the space junk.

Imagine a wood with the hardness comparable to aviation materials, flying at orbital speed, its impact force is actually no different from aluminum alloy, which is the same as an airplane hitting a bird.

How to make satellites actively and passively de-orbit and save orbit resources is still the main method for dealing with space junk. There are also rocket recovery technologies like SpaceX's development to avoid more space junk.

Last year, China’s Chang'e 5 had a "controlled off-orbit landing" process, which allowed the ascender that returned to the moon to crash on the surface of the moon, preventing it from occupying orbit and becoming space junk, affecting subsequent human lunar exploration programs .

And those wrecks that can no longer move can only be solved by external forces. In 2018, a RemoveDEBRIS project led by the European Space Agency attempted to use "space fishing nets" to capture debris in orbit and make it out of orbit into the atmosphere.

In 2016, Japan's "Crane" garbage collector also tried to use a metal chain to absorb the debris and slow it down.

Generally speaking, there are quite a lot of space junk clean-up programs, but not many have achieved significant results.

After all, even with the right technology, such projects often face another problem, and that is cost. At this stage, most space projects are more willing to spend money on scientific research and exploration than processing garbage. It represents a symbol of national strength and a new opportunity for business ventures.

According to statistics, as of today, there are more than 100 million pieces of space junk debris in low-Earth orbit, most of which are only centimeters wide and cannot be tracked at all.

But even if the debris is only 1 cm long, when its orbital speed reaches tens of thousands of kilometers per hour, it will still bring terrible destructive power to the space shuttle or the satellite in operation. Therefore, many aerospace vehicles now design impact-resistant shields to protect the equipment itself.

In the future, if wooden satellites want to operate successfully, the issue of shielding design must also be considered.

On the other hand, more and more satellites will be launched on the surface of the earth. The MIT report shows that it is estimated that by 2025, there will be 1,100 satellites sent from the earth to space every year, and in 2018, the number of newly launched satellites will be only about 360.

In 1978, astrophysicist Donald Kessler (Donald Kessler) mentioned a hypothesis. He believes that if there are too many flying objects in low-Earth orbit, they will eventually cause a series of impacts due to one impact, and countless debris will form a "garbage zone" around the earth, making it impossible for humans to launch spacecraft to In space.

Nowadays, this hypothesis is also known as the "Kessler effect." If we don't want space junk to become a "cage" for the earth, sooner or later humans will have to face these issues.

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