“The planet is the cradle of reason, but one cannot live in the cradle forever,” wrote Konstantin Tsiolkovsky at the beginning of the 20th century. Today, scientists are increasingly talking about the fact that sooner or later people will have to leave Earth and go in search of a new home. Many are sure that we should look for it from other stars, to which we will get thousands, or even tens of thousands of years. We are talking about one of the attempts to solve this problem - about the hypothetical project of the "ship of generations".
Don't sleep
In science fiction books and films, the crews of interstellar ships are usually immersed in suspended animation during the flight. Convenient: a long way for them flies by like an instant. However, if you measure this situation to reality, inconsistencies immediately arise. What will happen to the spacecraft over the years of flight? Will it be able to repair itself and restore if necessary, will the security systems be able to take into account all the risk factors and bypass obstacles? What if the technologies that ensure the suspended animation of astronauts fail, as in the recent movie "Passengers", whose heroes woke up 90 years ahead of schedule? How much invaluable scientific data will humanity never gain if we abandon flight experiments in favor of sleep?
Perhaps such questions made people think about how to overcome the endless expanses of space without falling asleep. You can apply the "rotational method": for example, every year several cosmonauts wake up and take control of the state of the spacecraft. A year later, they are replaced by the following. But what if, by the time the expedition is sent, humanity has not found a way to safely dive into a long sleep-suspended animation? After all, while these experiments are only at the initial stage.
The result of such discussions was the projects of "ships of generations". It is a vessel for interstellar travel at a speed much slower than the speed of light. Such a ship would have to fly for thousands of years. During this time, the first colonists will grow old and die, their descendants will take their place. This scenario will repeat itself many times before the expedition arrives at its destination.
One of the most famous projects of ships of generations was created on the basis of the Orion. This "explode" (nuclear impulse ship) was developed in the USA in the middle of the twentieth century. He was supposed to move due to a series of nuclear charges, activated at a short distance behind the ship. Part of the explosion products hit the "tail" of the spacecraft, where a massive reflector plate absorbed energy and, with the help of a system of shock absorbers, transferred it to the spacecraft. The scale of the Energy Limited Orion Starship project is amazing: the diameter of the ship was 20 kilometers. According to the calculations of the developers, this ship could reach the nearest star system Alpha Centauri in 1330 years. The dimensions of the ship were enough to accommodate a real ship of generations - in fact, a small space city. However, NASA relied on cheaper projects, and Orion remained a theory.
However, if things had gone differently, could we have sent the first colonists into space today? Unfortunately no. The generational spacecraft concept solves many of the theoretical problems of long space travel - and creates a number of new problems. We will find out what difficulties the ships of generations can face and what needs to be taken into account when going to distant stars.
Where are we flying?
Supporters of space colonization are divided into two groups: someone creates projects for terraforming Mars, and someone is sure that it is possible to find a new Earth only from other stars. Exoplanet researchers confirm that it is possible to find a space body suitable for life outside the solar system, although this is not easy.
For a successful resettlement, it is important that the planet found in many ways resembles the Earth. We need a temperature acceptable for earthly life and water in a liquid state. The star around which the planet revolves should behave as "calmly" as possible - frequent and intense flares cause sharp jumps in temperature. The flow of charged particles from a star can damage the planet's atmosphere, and over time "blow off" almost the entire gas envelope. Perhaps, in the solar system, this happened with Mercury.
The area of space around the star, in which there can be liquid water on the planets, is called the habitable zone. This is a kind of "middle" zone of the planetary system. The planets in it are not too far from the star, they receive enough energy so that the water does not freeze. But at the same time, they should not be too close to the star - the water can evaporate. In English literature, this site is called the "Goldilocks zone" in honor of the tale of a girl who fell into the house of three bears. While the animals are not at home, she decides to sleep a little and alternately lies on three beds: one is too hard, the other is too soft, and the third is just right.
It would seem that we, too, can simply “sort through” all the planets in a certain system and choose the appropriate one. Alas, not all planets in the habitable zone are suitable for us: liquid water is possible on them, but all other conditions on the surface of such a planet may be unbearable for earthlings.
In the summer of 2016, astrophysicists at the European Southern Observatory announced the discovery of the closest exoplanet to Earth. It revolves around Proxima Centauri - the closest star to the Solar System - and is now called Proxima Centauri b. According to scientists, it is located in the habitable zone of its star and may well have liquid water. None of the known climate models contradicts this. But it's too early to call Proxima Centauri b our new home. It is much closer to its star than the Earth is to the Sun, and the effects caused by this proximity can be unpredictable.
A fresh discovery from early 2017 - seven exoplanets near the cool red dwarf TRAPPIST-1 in the constellation Aquarius. All planets are similar in size to the Earth. Hypothetically, all seven planets may have liquid water, but it is most likely found on the planets TRAPPIST-1e, f, and g. Astrophysicists speculate that new telescopes - such as the European Extremely Large Telescope, which began building in Chile in 2014 - will be able to show for sure whether there is water on these planets.
The main thing is that even the exoplanet closest to Earth is still at a great distance from us. It is 4, 24 light years before it - to travel this path, existing spacecraft, even without taking into account the time for acceleration and deceleration, will take tens of thousands of years. By comparison, the planets around TRAPPIST-1 are about 40 light years away. Technology advances, but distances in space still seem endless. This makes us think again and again about projects such as a ship of generations.
Engines of the future
But maybe there is still a way to travel these distances faster? The capabilities of existing spacecraft are clearly not enough, but new developments are ongoing. One of the most impressive projects is the solar (photonic) sail. It uses the pressure of light on a mirrored surface. In the solar system, a sail can be powered by sunlight, and this technology already exists. In 2010, the Japanese spacecraft IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) went into space. It is equipped with a square sail with a side of 14 meters, consisting of four "petals". Solar panels are attached to them. The task of IKAROS was to successfully open the solar sail and move with its help, and the Japanese device coped with this to the fullest. However, the pressure of sunlight is relatively small, so other sources will have to be used to go beyond our system. There are projects for overclocking such a device using a laser. The solar sail has undeniable advantages: it does not require fuel and can be relatively light in itself. However, for the time being, mankind does not have enough resources to launch an interstellar sailing ship. Very powerful high-precision laser systems or a fundamentally new solution to this problem will be required.
Another promising engine that already exists is the ionic one. Its working fluid is ionized inert gas (argon, xenon) or mercury. The ionized substance is accelerated in an electrostatic field to very high speeds. The system for extracting positive ions "pulls" them out of the substance and throws them into space, providing movement. Ion engines were used in the Hayabusa (in 2010, delivered soil samples of the asteroid Itokawa to Earth) and Dawn (launched in 2007 to study Vesta and Ceres).
This engine achieves high specific impulse and low fuel consumption. The disadvantage of modern ion engines is extremely low thrust, so such a ship will not be able to launch from Earth, it will have to be built outside the planet.
Another interesting concept is the Bassard interstellar ramjet engine. A ship equipped with such an engine captures the material of the interstellar medium (including hydrogen) using a "funnel" of a powerful electromagnetic field. The diameter of the funnel should be thousands, if not tens of thousands of kilometers. The collected hydrogen is used in the ship's thermonuclear rocket engine. This ensures the fuel autonomy of the vessel.
Alas, this engine also has many technical limitations. Its speed is not that high, because when capturing each hydrogen atom, the ship loses a certain momentum, and this can be compensated for by thrust only at a relatively low speed. To overcome this limitation, it is necessary to find ways to make the fullest possible use of the trapped atoms.
Society on board
How many people can go on an interstellar expedition? Experts' assessments differ significantly. This is despite the fact that most of them are optimistic about the duration of the flight in hundreds, not thousands of years. In 2002, anthropologist John Moore of the University of Florida suggested that a small village population of about 160 would suffice to create a stable population for a 200-year flight. At the same time, cruel "social engineering", as in dystopias, will not be required, the family familiar to us will become the basis of the space colony. Each will have about a dozen suitable marriage partners. Even today - with a seemingly endless choice - most people do not exceed this number of partners in terms of long-term relationships.
However, in such small populations, there is a danger of reduced genetic diversity. It can decrease both gradually and unexpectedly - for example, in the event of a dangerous infection, the expedition will face the "bottleneck effect", in which the population size drops sharply and then gradually recovers. The gene pool is getting poorer, and this is reflected in the descendants of those who survived the disaster. In the animal kingdom, this effect influenced the genetic diversity of cheetahs - it is assumed that at one time only a few individuals were able to survive. The species was on the verge of extinction, now only about 7000 cheetahs live in the wild around the world. Due to the long closely related crossbreeding, they do not differ in resistance to diseases, and in the wild, most of the cubs do not live up to a year.
Another threat to colonists is the founder effect. It occurs when a small number of representatives of a certain species inhabit a new territory. They do not preserve the entire gene pool of the original population, therefore, they may also face the problem of a gradual reduction in genetic diversity.
Anthropologist Cameron Smith of Portland State University calculated in 2013 that tens of thousands of people are needed to deal with these threats over 150 years of flight. According to his estimates, a stable population needs about 40,000 people, of which at least 23,500 are of childbearing age. However, the colony can be smaller if it has a large enough embryo bank at its disposal.
Space in the basement, space in the desert
Of course, all these important questions will remain only theoretical for a long time. Today's technology is not able to send a person to neighboring stars, and we will not be able to do this for a long time. But research, in the long term, capable of bringing the space future closer, including ships of generations, has been going on for several decades.
One of the most famous types of such experiments is the creation of closed ecosystems. Passengers of the ship of generations will live in it for thousands of years, so the colony must be completely self-sufficient: there is nowhere to wait for help. This experience will be useful in the development of a new planet. Projects to create closed systems began in the 1970s, shortly after the landing of man on the moon.
In the USSR, in 1968-1972, "BIOS-3" was built. Scientists from the Krasnoyarsk Academgorodok have created a sealed room with a size of 14 × 9 × 2.5 m and a volume of about 315 m³ in the basement of the Institute of Biophysics, consisting of four compartments. "Crew cabins" and equipment occupied only one of them, in the rest there were cameras-phytotrons for growing plants and cultivators of microalgae. Special varieties were used: for example, specially bred dwarf wheat with a shortened stem. 10 experiments were carried out in BIOS-3, the longest one lasted 180 days. The participants managed to create a completely closed system of gas and water consumption. They provided themselves with food by 80%.
In the early 1990s, perhaps the most famous experiment to create a closed system, Biosphere-2, took place. A complex of several buildings and greenhouses on an area of about 1.5 hectares was erected in Arizona. Inside, several natural areas were modeled: tropical thickets, savannah, mangrove forests and even the ocean. About 3000 species of plants and animals lived in "Biosphere-2". The project team consisted of eight people - equally men and women. They supported the work of water and air circulation technology, engaged in subsistence farming and carried out various experiments.
The first stage of the experiment lasted two years. Within a year, the "colonists" were able to organize the production of food: in the first months, people were constantly hungry. Later, they adapted to the new diet, and many of the participants' health indicators improved as a result of the experiment, for example, decreased blood pressure. The biggest problem was the drop in oxygen levels. Project participant Jane Poynter recalls: “When you lose a lot of oxygen - and our level has dropped significantly, it dropped from 21% to 14.2% - you feel terrible. You wake up gasping for air because the composition of your blood changes. In a dream, you stop breathing, then finally you inhale and wake up. This is terribly annoying. Outside, everyone was convinced that we were dying."
It is believed that the oxygen level began to fall because the microorganisms of Biosphere-2 multiplied more actively than expected. The same thing happened with insects. It was forbidden to destroy them with the help of pesticides: this could upset the balance of the artificial biosphere. As a result, the organizers of the project had to go to the falsification of data: the missing oxygen was pumped into the system. When this became known, criticism fell upon the participants in the experiment. But the oxygen level continued to fall, even with gas supplies from outside, and exactly two years after the start, the first phase of the project was terminated. On the whole, the experiment was found to be unsuccessful. But do not belittle the importance of such experiments. First, they show a lot of pitfalls in the calculations and help to create more realistic models. Secondly, these projects remind: for the colonization of space, not only powerful engines are needed. To someday get to other planets, humanity will need a wide variety of knowledge and skills.
A riot on a ship?
Many difficulties await the participants of millennial expeditions. Some of the problems are related to the environment: for example, the destructive effects of space radiation. It can contribute to the development of cancer, damage to the bone marrow, and disorders in the immune system. Therefore, going into space, you need to properly protect yourself. Radiation prediction systems that take into account many parameters will be needed. The main task is to determine the degree of harm to health and constantly maintain a balance. Colonists will inevitably have to take risks, and ship designers will have to find a way to fit protective elements on a ship without sacrificing payload.
No less dangerous are, oddly enough, moral and ethical difficulties. People who are sincerely dedicated to their work, who believe in the need to conquer other planets, will go into space. But will their descendants be able to preserve this faith and will they want to? What if the representatives of the "intermediate" generations one day feel trapped in a high-tech space prison? Ethics must find an answer to these questions, otherwise problems cannot be avoided.
The consequences are unpredictable: from crew pessimism and apathy to open conflicts. In the confined space of the ship, misunderstanding of fathers and children or ideological disputes will become catastrophic. This is confirmed by the history of the same "Biosphere-2". When it became clear that oxygen levels were falling inexorably, the experimenters split into two groups. Some wanted to immediately leave the "Biosphere", others - by all means bring the project to the end. It is said that the conflict has flared up to such an extent that many of the former participants in the experiment still do not speak to each other. But they spent only two years in a closed system!
So, while humanity is just beginning the path to the stars. Much more research will be required to create viable designs for a self-sustaining space colony and a reliable interstellar craft.