If controlled nuclear fusion cannot be achieved, interstellar travel is impossible? In fact, these are two different things.

Some people believe that nuclear fusion is a hurdle to upgrading human civilization. If controlled nuclear fusion is successfully developed, humans will be able to embark on interstellar travel, and civilization will be elevated to a higher level.

This statement is reasonable, but it is also a bit specious. This is because it confuses the boundaries of two different issues and combines two things into one. In fact, specifically speaking, controlled nuclear fusion is one thing, and human beings going into deep space is another.

Comparative advantages between controlled nuclear fusion and traditional energy Since humans have emerged from animals and become intelligent species, the progress of civilization has been accompanied by changes in the way energy is used. Along the way, humans have experienced the use of firewood energy, fossil fuels, electricity, and nuclear energy. Human civilization has also developed from the primitive age to the agricultural age, the machine industry age, the automation age, and the information age.

All these used energy sources cannot compare with the advantages of nuclear fusion. According to Einstein's mass-energy equation theory, the mass and energy of matter are equivalent. If the mass can be completely converted into energy, the energy will be very huge. However, the conditions for converting mass into energy are very harsh. Nuclear energy is the energy obtained through mass-energy conversion.

The expression of the mass-energy equation is: E=MC^2. Here E represents energy, M represents the mass of any substance, and C represents the speed of light in vacuum.

According to this equation, every 1kg (kilogram or kilogram) of matter converted into energy can obtain 9*10^16J (joule) of energy, which is equivalent to 25 billion kWh of electricity. If the energy obtained from burning traditional energy sources: firewood is about 1.2×10^7J/kg, raw coal is about 2.1*10^7J/kg, and crude oil is about 4.2*10^7J/kg, according to the mass-energy equation, the mass-energy conversion rate is about: firewood 0.000000013%, raw coal 0.000000023%, crude oil 0.000000047%.

The mass-energy conversion rate of nuclear fission is about 0.1%, which is about 7.7 million times that of firewood, 4.35 million times that of raw coal, and 2.13 million times that of oil. The mass-energy conversion rate of nuclear fusion is about 0.7%, which is 7 times that of nuclear fission. The efficiency of the same 1 kg of raw material is 7 times higher than that of nuclear fission. In this way, the mass-energy conversion rate is 53.9 million times that of firewood, 30.45 million times that of raw coal, and 14.91 million times that of oil.

What is more advantageous is that the raw materials for nuclear fusion are inexhaustible on Earth for a long time. The main raw material is hydrogen (mainly deuterium or tritium, the isotopes of hydrogen), and water in nature is a hydrogen hydroxide compound. As long as the deuterium and tritium in the water are separated, it can be used as nuclear fusion fuel.

According to calculations, each liter of seawater contains 0.03 grams of deuterium, which can be extracted and used as nuclear fusion fuel to generate electricity, equivalent to 300 liters of gasoline. The total water reserves in the world are 136 trillion tons, and the deuterium reserves are as much as 40 trillion tons, which is equivalent to 120 trillion tons of oil reserves.

According to some organizations, the world's oil reserves are still around 900 billion tons. If 5 billion tons are mined each year, the reserves can be used for another 180 years. But the problem is that oil pollution, mainly carbon emissions, are increasingly affecting the environment. The greenhouse effect is becoming increasingly apparent. If it is not changed and reversed, humans may not survive until the end of this century.

The nuclear fusion fuel reserves contained in water are equivalent to 44,000 times the world's oil reserves. More importantly, nuclear fusion has another advantage, which is that compared with chemical fuels and nuclear fission energy, it is a completely pollution-free clean energy that does not produce radioactive pollution or greenhouse gases such as carbon dioxide.

Therefore, achieving controlled nuclear fusion is the best option for the development of human civilization at present.

The huge advantage of nuclear fusion in the field of aerospace: controlled nuclear fusion can produce huge amounts of energy with very little fuel, so it is a major advancement in human energy utilization. If mature controlled nuclear fusion technology is available, it will certainly be a boon to humankind's journey into deep space as a power source for future deep space voyages.

The fuels needed for spacecrafts are mainly chemical propellants, and the amount required for launch is very large. For example, the Saturn V of the last century had a takeoff mass of 3,040 tons, but the Apollo spacecraft sent to the Earth-Moon transfer orbit had a mass of only 45 tons, and the payload was only about 1.5% of the launch mass, while the fuel consumed was more than 2,700 tons.

The Long March 5 Y5 carrier rocket used by my country to launch Chang'e 5 weighed about 870 tons at takeoff. The Chang'e 5 that entered the Earth-Moon transfer orbit weighed only about 8.2 tons, and the payload only reached about 0.94% of the launch mass. Even the most advanced Musk Starship has a takeoff weight of about 5,000 tons, of which 3,400 tons of rocket fuel is required. A fully loaded Starship of 1,420 to 1,470 tons can be sent to the Earth-Mars orbit, and its payload is about 28% of the takeoff mass!

This is already very impressive, but the fuel consumption is still huge, and the actual payload of the starship going to Mars is only 100~150 tons, but the fuel is 1,200 tons.

If nuclear fusion power is adopted, this phenomenon will undergo a subversive change. The mass-energy conversion rate of nuclear fusion can reach 0.7%, so 1kg of nuclear fusion fuel can obtain 6.3*10^11KJ of energy; while the energy released by the complete combustion of 1kg of conventional rocket chemical fuel, such as N2H4 (also known as hydrazine or anhydrous hydrazine) (N2H4+O2=N2+2H2O) is only 19412.5KJ, and the mass-energy ratio is only 1/32.45 million of that of nuclear fusion.

Therefore, if a rocket needs to be launched with 5,000 tons of chemical fuel, only 154 grams of nuclear fusion fuel is needed. In fact, it is far less than this amount, because the launch of a chemical fuel rocket requires so much fuel, and more of the loaded fuel is sent to the sky at the same time and burned in the process of sending it to the sky.

If nuclear fusion fuel is used, it is only necessary to send the payload into space, and there is no need to consume a large amount of fuel in such a vicious cycle of sending fuel. However, the nuclear fusion reactor may occupy a large part of the mass of the spacecraft. But in any case, the mass of the rocket and the spacecraft should be much lower, and the fuel required for launch will be even less.

In this way, as long as you carry a few kilograms of nuclear fusion fuel, you can travel in space for a long time.

In addition, in theory, the specific impulse of nuclear fusion engines is much greater than that of chemical fuels. The specific impulse of chemical fuel rockets is only between 250 and 450, the specific impulse of nuclear fission rockets can reach between 800 and 1000, and the specific impulse of nuclear fusion rockets can reach between 2500 and 20000.

It is extremely difficult to achieve controlled nuclear fusion. In fact, humans have already obtained the energy of nuclear fusion in the last century, which is the explosion of hydrogen bombs. But this energy is only transient and disappears in a "boom". It can only be used for war and deterrence, and is of no use to human life. What humans need is that nuclear fusion can be released slowly and for a long time. This kind of nuclear fusion is called controlled nuclear fusion or "artificial little sun".

The reason why controlled nuclear fusion is called "artificial little sun" is that the sun relies on the continuous nuclear fusion in its core to release huge energy. Controlled nuclear fusion is to copy the nuclear fusion method of the sun to the earth.

However, nuclear fusion in the sun is carried out under an environment of 300 billion atmospheres of pressure and 15 million K of temperature. It is because of the gravity formed by the huge mass of the sun that nuclear fusion is bound to the core and continues. It is impossible to create 300 billion atmospheres of pressure on Earth, so how can we achieve controlled nuclear fusion? Research has found that we can only do something about raising the temperature. To stimulate nuclear fusion and keep it stable and not extinguished, the temperature needs to reach more than 100 million degrees Celsius.

Therefore, how to confine such a high nuclear fusion plasma and how to convert energy into usable energy have become difficult problems that scientists have to overcome. Scientists have struggled for decades and still have not solved it well. This is because there is no material on Earth that can withstand a high temperature of 10,000 degrees, let alone 10,000 10,000 degrees of 10,000 degrees.

Scientific research has found that there are currently three theoretical ways to confine high-temperature plasma, namely: gravity confinement, which is a form of confinement such as the sun; magnetic confinement, which is to create a magnetic well to confine the plasma in the magnetic well to prevent it from hitting any equipment; inertial confinement, which is to use a power source such as a laser or particle beam to impact a miniature spherical target loaded with nuclear fusion material (deuterium or tritium) at high speed, causing the target to form huge inward pressure and stimulate nuclear fusion.

Since it is impossible to form gravitational confinement of hundreds of billions of atmospheres on Earth, the latter two methods can only be used. The most commonly used experimental equipment in various countries is the Tokamak device, which is a magnetic confinement device, the core of which is an artificial magnetic trap. Now, the experiment has achieved the control of high temperatures of more than 100 million degrees in this magnetic trap, but the time is still very short.

In 2021, China set a world record of maintaining 120 million°C plasma operation for 101 seconds and 70 million°C plasma operation for 1056 seconds.

Nuclear fusion ignition and maintenance itself requires a lot of energy input. If the energy input is greater than the output, it is worthless. Therefore, how to make the output energy much higher than the input energy is the key to the problem. On February 9 this year, the European Joint Ring Laboratory broke a good news that two types of hydrogen (deuterium and tritium) were pressed together to generate 59 megajoules of power in 5 seconds.

Although this amount of energy is only equivalent to about 16 degrees of electricity, which can only boil a few pots of water, it is of great significance, just like the record of how long China has maintained high temperatures. Nuclear fusion expert Dr. Arthur Turrell commented: "This is an amazing result and a milestone, achieving the maximum energy output of nuclear fusion reactions in history." He believes that although this time is not long, it is a very long time on the nuclear time scale. With this breakthrough, the road ahead will be much easier.

However, many experts believe that it will take at least 30 to 50 years to achieve commercial operation of controlled nuclear fusion. Nuclear fusion power is not the only option for going into deep space.

The nuclear fusion engine is certainly one of the ideal power options for humans to fly out of the solar system and into deep space in the future, but it is not the only option, nor the best option.

In addition to nuclear power, scientists have also proposed many power solutions for deep space travel, such as light sail technology. The famous scientist Hawking launched a "Breakthrough Starshot" plan during his lifetime, which uses lasers to bombard light sails to reach 20% of the speed of light (60,000 kilometers per second) and drag a stamp-sized probe to Proxima Centauri. After more than 20 years, it can transmit photos of Proxima Centauri.

However, this idea is very difficult, and there is no further news on the progress after Hawking's death.

There is also the idea of ​​using solar sails, which is to use the light pressure of stars to propel spacecraft to distant places. Although the light pressure is very small, there is no need to carry fuel. As long as there are stars, it can be pushed forever, and it will get faster and faster, and finally reach a very high speed.

The plasma thruster, which is now widely used in aerospace, is based on the principle of ionizing the gaseous working medium first, accelerating the particles under the action of a strong electric field, and pushing the spacecraft through the reaction force. The power thrust of this method is very small, but the specific impulse can reach 1000~30000, and the efficiency is extremely high.

There are also nuclear pulse rockets, which use small nuclear explosions as propulsion technology. Their specific impulse can reach 10,000 to 1,000,000, and they can fly at speeds of 10 to 12% of the speed of light.

There is also a reactive engine, which is different from the traditional engine that needs a working fluid (fuel or electricity) to do work. This engine can convert any form of energy into mechanical energy. However, this engine is still in the theoretical and experimental stage. Some people think that it violates the law of conservation of momentum, and there are still many controversies.

Of course, antimatter has the highest energy efficiency ratio. When antimatter and matter collide, they will annihilate each other. The annihilation process will explode the entire energy of the mass, which is a 100% perfect mass-energy conversion. When 1kg of antimatter annihilates 1kg of matter (any matter we see in daily life), 2kg of total energy will be generated. This energy is 1.8*10^17J, equivalent to 50 billion kilowatt-hours of electricity, which is 285.7 times the energy of nuclear fusion.

Using antimatter as fuel, the specific impulse can reach 1 million to 10 million.

However, antimatter is very difficult to obtain and preserve, because antimatter annihilates when it encounters matter. With current human technology, if all the world's national income is used to produce antimatter, even if we don't eat or drink for a year, we still can't produce 1 microgram (one thousandth of a gram). Therefore, considering using antimatter as a power source for deep space travel is more fantastic than a dream.

Theoretically, humans can also achieve wormhole travel and warp speed travel in the future. Both of these deep space travel methods can exceed the speed of light many times without violating the speed of light barrier, but they require huge amounts of energy or even negative energy. If these technologies can be realized, human civilization will break through to a new stage.

So far, nuclear fusion is the energy utilization method with the highest mass-energy conversion rate discovered by humans, except for antimatter. However, the mass-energy conversion rate obtained by this method is only 0.7%. There is still a 99.3% gap between the perfect conversion of antimatter and 100%. Will there be a new conversion method to fill this gap in the future? No one knows.

The universe is vast and boundless, and the natural laws that humans currently understand are still few and superficial. There must be many more profound laws waiting for humans to discover and understand. There will definitely be more and better choices for the future of space exploration.

What do you think? Welcome to discuss, thanks for reading.

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