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Space travel does not refer to travel between the planets of the solar system, but to planets outside the solar system (i.e., planets of other stars that are not the sun). Why space travel? This is an issue that must be addressed when discussing space travel. One reason may be that our planet, Earth, will eventually run out of resources.

Another reason: since there are planets and perhaps life beyond the solar system, why shouldn't we look at them? Humanity has always been full of curiosity, as evidenced by the longevity of science fiction.

In order to travel to other star-planetary systems, we have to overcome all kinds of big obstacles: scientific, social, and economic. NASA and the U.S. Department of Defense R&D Agency have so far allocated $500,000 to the sci-fi Centennial Starship project, which aims to enable space travel within 100 years. Although this goal may seem overly optimistic, it also reflects scientists' desire for space travel. In October 2011, the United States held the "Centennial Starship Symposium", which was attended by renowned astronomers and science fiction writers. The purpose of the workshop was to identify the challenges and possible solutions to space travel.

Space travel can be daunting. If the distance between the Earth and the Moon is assumed to be 20 meters, then the distance between the Earth and the nearest star outside the Sun, Alpha Centauri, is the actual distance between the Earth and the Moon - 384,400 kilometers.

Over the course of several thousand years, humans have gone from 4 kilometers per hour to 40,000 kilometers per hour on the Apollo lunar spacecraft. However, to reach Alpha Centauri in a few decades, the Apollo spacecraft would have to be 10,000 times faster, that is, close to the speed of light. In fact, in order to achieve space travel, we need to not only fly faster, but also fly faster faster. Despite all these seemingly insurmountable obstacles, scientists believe that travel to planets beyond our solar system will one day be possible. Let's take a look at the five steps of space travel. Step 1: Build a starship

For space travel, the speed that today's rockets can achieve is nothing short of snail speed. Starships need powerful new ways of propulsion. Thermonuclear rockets

The ship must have fuel to have propulsion, and the increase in the speed of the ship depends on the gradual increase in fuel. To achieve 3 times the velocity of the exhaust nozzle gas, 20 times the weight of the rest of the rocket (the so-called "dry weight") is required. To be honest, the chemical combustion of hydrogen and oxygen is simply too slow. Thermonuclear rockets using fission cores allow large manned spacecraft to travel within the solar system, provided that they can harvest and use resources from other places, such as gas giants (hydrogen is one of the main components of gas giants' atmospheres). Back in the Cold War, the Soviet Union and the United States began to develop thermonuclear rockets. In fact, in order to realize manned travel between the planets of the solar system as soon as possible, thermonuclear rockets are the best delivery vehicle. However, space travel would require the kind of nuclear reaction that drives the sun – fusion. In 1978, a British starship concept project called "Daedalus" (Daedalus, an ancient architect and sculptor who built labyrinths for the kings of Crete) proposed to use "inert fusion" to drive rockets.

That is: the hydrogen isotope pellets are compressed by a laser in all directions until they are compressed into a very small volume; The pressure increases enough to produce a hydrogen nucleus fusion reaction, releasing energy, and the heat is ejected from the exhaust nozzle at ultra-high velocity. Thermonuclear rockets could send us to nearby stars, but the flight time would still be hundreds of years, not to mention the fact that nuclear fusion would have to be achieved on Earth first, which has not been possible so far. In the longer term, we may have to develop matter-antimatter rockets. When antimatter reacts with normal matter, they annihilate each other, producing 300 times more energy than fusion reactions. The problem is that we have so far not been able to develop the technology to make large quantities of antimatter.

In the famous American science fiction film "Star Trek", the warp engine leads humans to travel faster than light speed in the galaxy. Is faster-than-light travel really possible? According to Einstein's theory of general relativity, superluminal speed is not impossible with negative mass. With a negative mass, it is possible to distort the time-space shape, allowing transport between extremely distant locations.

Objects with negative mass, including objects and space, behave in such a way that the gravitational field ceases to exist or has no effect on objects or space. Although none of us have seen anything with negative mass, quantum mechanics allows such things to exist. There has been a debate in theoretical physics about whether negative mass can be used for space travel, because the amount of energy required to distort space-time is so incredible that it far exceeds what can be obtained from a star.

If enough energy can be captured, a space-time region can be distorted to create a "warp bubble" or "space-time bubble". This "bubble" is only the size of a starship. The space-time in front of the bubble will be compressed, and the space-time behind it will expand, propelling the spacecraft forward. But where does the energy come from? How is the energy generated?

Scientists used to estimate that the energy needed to create a "warp bubble" is equivalent to the mass of a galaxy (Einstein showed us that mass and energy are interchangeable, and that both mass and energy can shape space-time). Now, scientists believe, perhaps Jupiter's mass will be enough. But even then, we still have a long way to go. Be warned: even the largest hydrogen bomb can only convert a few kilograms of matter into energy. Distorting time-space on the scale of a "warp bubble" is not something that is within the reach of 21st-century science and technology, and even practical experiments in this area are far from being able to do so – a concept that remains a theory to this day. For now, at least, warp engines powered by dual lithium crystals, like in Star Trek, are still stuck in the prop stage. Rate of fire energy sails

As early as 1610, after noticing that the comet's tail was blown away from the direction of the sun, the German scientist Kepler came up with the idea of propelling a spacecraft with sails. Today, there are really flying machines powered by solar sails, such as interstellar kites-flying vehicles accelerated by solar radiation. However, it would take thousands of years to reach even the nearest non-solar star to Earth with such a vehicle. What has the potential to truly achieve interstellar flight is a 21st-century spacecraft, the velocity-of-fire energy sail, or sailboat for short. The idea is to use the ability of electromagnetic waves to transmit energy through space to generate power over ultra-long distances. The source of the beam is the projector plus an antenna, which projects a powerful laser or microwave onto an extra-large sail. The sail emits a laser beam or microwave beam, gaining momentum to "push" the spacecraft. Such a spaceship appeared in the second episode of the Hollywood sci-fi blockbuster Star Wars. The projector looks like a satellite receiver dish, but many, many times larger.

The most expensive part of a sailboat is the projector. It will be built in space using materials mined from the moon or asteroids, positioned close to the sun, powered by intense solar energy. The biggest advantage of beam energy is that the heavy projector is left behind, while the light sail is driven far away with the crew and load. The projector can then be repurposed for future missions. Just like the railroad in the 19th century, once the tracks were laid, the cost of the train itself was much smaller. The physics of the sailboat have been proven, but how to build super-giant projectors and space sails is a big problem. The width of the projector can be several thousand meters, and the length of the space sail can be several hundred kilometers. Economic studies have shown that they are too inefficient and expensive, but scientists are still wondering if speed-of-fire energy sails might one day be suitable. Step 2: Deep space navigation

Choosing a route is not difficult, the difficulty is finding a reference location and overcoming interstellar risks. The target star has been chosen, but how can we know where our spacecraft is in the vastness of space? To determine the position of a starship, triangulation can be used to determine the angle between the spacecraft and several known stars, or to locate multiple pulsars. Pulsars are rotating neutron stars that emit regular, intense microwave pulses at intervals as short as a few thousandths of a second. The speed of a starship can be determined by measuring the pulse frequency. As the spacecraft moves, the spacecraft speed will need to be adjusted using Doppler shifts. Also, the space between planets is not empty, and interstellar dust is also a big problem. While a single speck of dust may be only a few millionths of a meter in diameter, a starship traveling 10 light-years away would have to endure 1,000 hits per square millimeter. During the journey to Alpha Centauri, the ship will slowly but surely be hit (or eroded) by interstellar dust, and the hull will be smashed. One way to avoid this erosion is to place a metal foil plate a few meters in front of the spacecraft. The incoming dust particles pass through the foil board, pass through and are ionized (as charged electrons or ions), and then hit an electrostatic shield—some kind of "force field", perhaps a charging grid. This "shield" will protect all ship parts behind it. It only takes a few thousand volts to steer an electron, while it takes 1 million volts to steer an ion.

For the deep space of a vacuum, it is not a problem to generate such an electrostatic shield. Therefore, the remaining risk is the larger particular. Although such particles are rare, we do not know how rare they are and whether they pose a threat. However, ionized laser pulses from the spacecraft should be able to stop them under radar guidance. Deep Space Travel Navigating Itself Scientists recently announced that stars reaching the end of their lives may help spacecraft navigate when traveling deep into space.