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The artistic imagination of the Guangfan spaceship shows that the sails of the spaceship are curved by lasers from the earth. Photo Source: in Masumi Shibata, courtesy of Breakthrough Initiatives science fiction, we generally refer to large spaceship formations as space fleets. The propulsion of the space fleet can also refer to the sails of the ancient navy-but in space, it is no longer the sea breeze that hits the sails, but the laser beam. With laser sails, scientists have found that we may actually be able to accelerate spaceships to 20% of the speed of light and reach Proxima within 20 years.

The universe is so vast that even science fiction films have given birth to a kind of "space opera", in which a person's life often seems insignificant. The naked and cruel contrast between the long travel time and the short life span of human beings is an important reason for this grand epic feeling.

At present, human beings can not directly increase the speed of the spacecraft, but can only pursue to use as little fuel as possible, change the orbit of the spacecraft as efficiently as possible, and ultimately rely on the orbit determined by gravity to carry out interstellar navigation. The corresponding price is that the time it takes to reach a celestial body is often positively related to the orbital period of the celestial body. For example, it may take about three days to reach the moon with an orbital period of one month, but it may take more than 200 days to fly to Mars with an orbital period of 23 months (687 days).

Even if this is the case with travel within the solar system, interstellar travel is even harder to imagine. New Horizon, which is destined to fly out of the solar system, was launched in 2006 and arrived at Pluto in 2015. The variables brought about by too much time are so great that it was launched on a mission to explore Pluto, the ninth planet in the solar system at that time. But by the time it reached its destination, Pluto was no longer a planet.

If interstellar travel takes too long, you may even find it "less important" when you get to your target. Photo: Voyager 1, launched in Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute1977, took 35 years to fly out of the solar system. Today, Voyager 1, which has been flying for nearly 45 years, is the farthest man-made object from Earth at 21.5 light-hours (21.5 hours of light). But Proxima, the closest star to Earth, is also 4.22 light-years away. Liu Cixin's Trisolaran is located in that galaxy, but even water droplets made by trisomy take 200 years to reach the solar system-apparently longer than anyone.

The root cause of all the new propulsion methods is the limitation of self-contained fuel. In the case of interstellar travel without external input, if you want to accelerate, you have to throw something out according to the conservation of momentum. Conventional chemical rockets can emit gases from burning fuel behind them at a speed of several kilometers per second. But it is almost impossible for us to continue to increase the gas ejection speed of chemical rockets. after all, there is an upper limit on the energy of chemical bonds, and the maximum ejection speed of chemical rockets can only reach about 10 kilometers per second. To continue to improve the efficiency of fuel use, engineers can also use the principle of particle accelerators to accelerate fuel particles backward at extremely high speeds. The Hall engine in the core module of the Tiangong space station, which is responsible for adjusting attitude and maintaining orbit, uses such technology.

But as long as the spaceship brings its own fuel, some of the fuel will inevitably be used to accelerate the remaining fuel. Can we provide extra fuel for the spacecraft?

Solar sail is such a spacecraft. We all know that light has the nature of particles and photons have momentum. Therefore, wherever the sun can reach the solar system, it exerts a push. If we open a large reflective film, we can "whack" the momentum of solar photons to accelerate the spacecraft. In fact, some people have already done so. The Falcon heavy rocket was launched on June 25, 2019, aboard LightSail 2, and the spacecraft launched its own solar sails on July 23 of that year. Although LightSail 2 rolled randomly for a long time due to equality after deployment, the team finally got it to work as planned. In the end, they succeeded in measuring evidence that LightSail 2 was driven by sunlight.

LightSail 2 schematic diagram, which can get the acceleration of 0.058mm / s ²from sunlight. Photo Source: The Planetary Society, however, the power of sunlight is still too weak. Such a weak force, not to mention being used for interstellar navigation, can't even overcome the air resistance at an altitude of 700 kilometers. According to the research team's own published data, LightSail 2 will not be able to maintain its orbital altitude for a long time, and as the orbit decreases slightly, it will probably fall into the Earth's atmosphere.

Pictures of LightSail 2 coming back from Earth's orbit. Image source: The Planetary Society uses lasers instead of sunlight after all, the sun just radiates energy indiscriminately in all directions, and we may be able to use stronger lasers to power the sails. And that's what the Igor Bargatin team, an associate professor at the University of Pennsylvania, thinks. Professor Bagatine said bluntly: "if we want to reach another star in our lifetime, we need a relativistic speed close to the speed of light."

Their paper was published in Nano KuaiBao. According to the description of the paper, they conceived a kind of light sail. The sail is about 3 meters in diameter, but it is only 100 nanometers thick and weighs only 1 gram. Scientists can fire lasers at it from the earth to provide power. Just like a sailboat sailing in the sea, the strong light pressure will make the sails arc, which can better disperse the tension and reduce the risk of being torn. The materials of Guangfan are alumina and molybdenum disulfide, which are relatively easy to obtain.

According to the Bajatin team's simulation, the light sail, which is about 3 meters in diameter, can carry a chip-sized detector and accelerate to about 20% of the speed of light. Although the load is only one chip, it sounds even more shabby than the "ladder plan" of "brain-only" in "three-body". However, it seems not difficult to accept the thought that the speed can reach 0.2 times the speed of light, which is faster than the water droplets of a trisomy.

The simulation of the acceleration distance and temperature of the light sail when the laser intensity is different. Photo source: Nano Lett. The sci-fi scene of 2022, 22, 1, 90-96 is the goal of a sub-project of the international investor Yuri Milner Breakthrough Initiative (Breakthrough Initiatives) called Breakthrough Starshot (Breakthrough Starshot). Stephen Hawking and Yuri Milner announced the official launch of the project in 2016. Their goal is to build a probe that can reach Proxima within 20 years, because it is the closest star to us.

However, apart from the distance, Proxima is indeed in a very interesting system. As written in three-body, Proxima is in a three-star system, and it has a planet called Proxima b. Proxima b is very close to Proxima, and the star activity of Proxima is very intense. Proxima b the whole planet is easily scorched by the flares of Proxima. The Australian Parks Telescope even claimed to have received mysterious radio signals from Proxima during the Breakthrough listening (Breakthrough listen) program, but this turned out to be interference.

It is not difficult to ensure that heat dissipation provides a large amount of energy to an object simply and rudely, so that it can reach a very high speed. As early as August 1957, when the United States conducted an underground nuclear test during Operation Plumbbob, a steel manhole cover was blown up by a nuclear bomb. Scientists estimate that the speed of the manhole cover is more than 66 kilometers per second, and some even suspect that it was the first human object to be sent into space before the Soviet Sputnik 1 (which went into space on October 4 of that year). However, later analysis showed that the manhole cover was probably overheated and vaporized before it flew out of the earth's atmosphere.

Of course, such laser sails bear millions of times more energy than solar sails. If the heat dissipation of the sail is not done, the laser source that powers the sail will properly become a laser weapon, and the sail will be destroyed instantly, like the manhole cover bombed by a nuclear bomb.

That's what the Aaswath Raman team, an associate professor at the University of California, Los Angeles (UCLA), is considering. In a paper published in Nano KuaiBao, the research team proposed a scheme of "photonic crystal design". They believe that the light sail must be made up of many smaller fabrics, which must be arranged to match the wavelengths of a wide range of thermal radiation. For single fabric, the holes matching the incident laser wavelength must be evenly distributed on it. In this way, the light sail can not only gain enough momentum, but also fully dissipate heat and avoid melting and disintegration. Team members say they will manufacture such structures on a small scale in the future and use high-power lasers to verify the feasibility of the design.

However, everyone says it may take decades to achieve a practical light sail that can reach 20% of the speed of light and transport a chip. But at least, it has been proved that laser sail is feasible in principle. Professor Bagatine said: "A few years ago, even doing some theoretical work was considered irrelevant. Now, not only do we have a design, but the materials we use are actually available in the laboratory."

In fact, there is no need to be very pessimistic about the development of interstellar travel technology. After all, in 1903, you asked the Wright brothers, who had just achieved power-controlled flight, to imagine the future. I'm afraid they would never have imagined that only 60 years later, Armstrong took a big step on the moon. In the more than 50 years since the moon landing, although mankind's progress in space technology does not seem as crazy as it was during the Cold War, on the whole, science has never stopped its progress.

From this point of view, interstellar travel, laser sails, 20% speed of light, 20 years to reach Proxima, these sci-fi terms, do not seem so far away.

Reference link:

Https://voyager.jpl.nasa.gov

Https://www.planetary.org/sci-tech/lightsail

Https://secure.planetary.org/site/SPageNavigator/mission_control.html

Https://www.eurekalert.org/news-releases/943728

Https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03272

Https://en.wikipedia.org/wiki/Operation_Plumbbob

Https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03273

Http://www.xinhuanet.com/techpro/2021-04/30/c_1127394313.htm

This article comes from the official account of Wechat: global Science (ID:huanqiukexue). | Wang Yu revised | Bai Defan

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