The stars above us are so beautiful that people even built whole mythologies based on them. They are truly spectacular, and now that we have made it to the Moon and will soon get to Mars, our natural urge will be the path to the stars.
This journey has formed the basis of countless science fiction and film stories. Many people already think that traveling from star to star is easy - you just need to press the trigger, but not everything is so simple. Several major issues remain to be addressed.
Faster than light
Many space travel in science fiction is based on travel faster than the speed of light. In reality, physics prevents such a possibility. And there is no way around this fundamental limitation. Even travel close to light speed faces all sorts of interesting relativistic problems related to mass and energy. Our only option is to use portals - wormholes.
The wormhole needs to be closely monitored, which is currently beyond our reach, and we need to somehow create a second wormhole at its destination. The need to send someone to the other end to create a wormhole is not the best reason for the first interstellar journey. In addition, physical effects when traveling through a permanent or temporary wormhole can lead to the destruction of any matter. You can easily reach your destination in plasma form.
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Teleportation
Classic teleportation involves the presence of a person who activates the device and disappears in order to reappear at the destination. However, in reality, teleportation works much more complicated than shown in the films. Even if we admit the possibility of such a principle, think about it: a person is disassembled into atoms in a teleportation machine, physically transported to a destination and reassembled.
Assembly alone requires incredible machines at the destination, and elementary physical laws will prevent us from manipulating matter with precision at such a gigantic distance - right up to another star. Such teleportation will be possible only to those places where we have already been. Assembly of atoms is not yet available to us, but it is quite possible. You just need to send atoms to another star, and this can be done at the speed of light - clearly faster than sending a body, but it will still take years.
Another option is to collect an exact copy of the person at the other end, and destroy the previous one. But this option is unlikely to suit anyone.
Colony ship
If travel faster than the speed of light is not possible, we can build ships of generations. Light reaches the closest star in four years, but a heavy object will take much longer. Most stars will take at least hundreds of years to fly. On the ships of generations, populations can be born and die until, many years later, they reach their destination. But these ships have a number of problems.
Descendants can simply forget about the original purpose of the mission, since it will turn into a legend for hundreds of years. An intelligent computer system could train people born on a ship to avoid such a failure, but still, it is very difficult to predict what will happen while a generation is replaced by another generation. If something happens to the ship, hardly a generation that has forgotten the intricacies of engineering over the years will be able to do anything.
Mother ship
To eliminate as much uncertainty as possible in generational ships, egg ships can be used. They will carry frozen fertilized human eggs, which will be grown and raised by elaborate machines, which will act as their mothers, parents and educators. Eggs can be turned into humans upon reaching a distant star or planet, and computers will teach future conquerors of space everything there is to know.
Designing such machines may not be possible at the moment, but in the future - why not? In any case, like the ship of a generation, the ship of eggs cannot help a person who wants to go in search of new stars. Farmed people may not like their mission, or they may even be born without a thirst for travel.
Longevity
An alternative to the ship of generations can be the genetic modification of people who can live for hundreds or thousands of years and travel in the course of their lives. All questions of life in space would be settled. Longevity and immortality are carefully studied by science, but the biggest obstacle in these matters is telomeres - the ends of chromosomes that get shorter each time your cells divide.
Eventually, the length of the telomeres will be eaten away, and cells will begin to damage their own viable DNA as they divide. This means that DNA itself contains the number of cell divisions that can occur. Cells divide to replace old or damaged cells like eyelashes, or skin, or parts of the stomach (you know about high acidity in the stomach).
It would seem that the answer is simple: you need to store the length of telomeres. But the fact is that the only adult cells that can do this are carcinogens.
Hibernation
Since longevity and the new generation have not become the answer to an important question, suspended animation can help. In many films and books, people were kept asleep in order to be transported over long distances. In this state, people do not age, or they age very slowly, this is a kind of "sleep mode". Unfortunately, telomeres are a problem here too.
Our bodies always contain a small amount of radioactive elements. They emit small amounts of radiation, which is harmless to us, since new cells are constantly replacing damaged ones. If a person does not age during suspended animation, his telomeres do not shrink and cells do not divide. Any radioactive elements in this state will cause permanent harm to the body, which will ultimately lead to death. Even slow aging will not save you from radiation for long periods of time. It is necessary that the cells divide at the usual rate.
Motion
Even if the human problems of travel to other stars are solved, problems of movement remain. Conventional systems involve burning fuel or jet mass, but getting to another star would take incredible amounts of fuel, which is extremely inefficient. As a solution, you can collect fuel along the way.
In the space between the stars, there are no ordinary asteroids or planets to land on and get fuel. Fortunately, space is far from being a vacuum; it contains many scattered tiny atoms, mostly hydrogen. If you move at high speed, these atoms can be collected and used as fuel in reactions like fusion (if we get to it, of course).
To collect hydrogen, you need a powerful "scoop", according to preliminary calculations, 2000 square kilometers in area. This size will significantly increase the resistance of the ship and reduce the speed to a conventional rocket. Such a system would be highly ineffective and unsustainable. But she was considered.
Damage
The closest star to us is Alpha Centauri. It is located four light years from Earth. It would take 72 million years to reach it in a regular car at 60 km / h. Even if we assume that such a car will be created, during this period all conceivable periods of decay and normal wear and tear will expire, not to mention the almost zero probability of arriving after such a long time.
You need speed, even if it's limited by the speed of light. Because of the tiny atoms scattered throughout space, any ship at high speed will be bombarded with such force that they pierce even the most durable steel.
There are two options: people or cars will constantly patch holes and repair damage, which means you will need a huge amount of repair materials that you will have to carry with you, or the ship will be made of elastic materials that will repair itself. It is these materials that are now being developed at space agencies. The bad news is that scientists don't believe in the possibility of such materials.
Gravity
Our body structure is heavily dependent on gravity. When people do not live in the conditions of ordinary earth's gravity, their organisms begin to suffer. After a few weeks or months, bones become brittle, muscles become weak, and the long-term consequences are generally fatal.
Humans can combat these effects through a variety of exercise and diet, but after a few years or decades in space, the human body will be irreversibly damaged. Even during relatively short flights, vision deteriorates terribly. This is exactly the problem that NASA wants to solve, by the way, before sending people to Mars.
Instead of living in zero gravity, you can create artificial gravity by rotating a space object. Unfortunately, this will require a tremendous amount of energy and fuel, and the spin itself will inevitably cause nausea - in the short term. What will happen in the long term is still unknown, has not been studied.
Food, air and water
People living on a ship for an extended period will need a life support system. They will need to eat, drink, breathe, urinate, defecate, wash, and sleep. Much of this can already be done in space with current technology. But on long journeys, the amount of water and food will become too much to take with you.
The smartest solution would be to take a self-sustaining ecosystem aboard the ship. Plants produce air, are successfully eaten, and consume human waste. Any ecosystem is inefficient enough, but it can extend the life span before arriving at the destination.
The ship's equipment would be severely damaged by the gases that would circulate, however this could be solved by creating smart materials. Algae are being studied closely as they have enormous potential in maintaining ecosystems. But they also have problems - if you eat algae in large quantities, you can seriously poison yourself. And again - genetic modification can solve this issue too.
All that remains is to resolve the previous nine problems.