by prof. Carlo Pellacani

People have always talked about aliens: they can be found in ancient myths, in Gothic tales and in the more recent sci-fi literature. To this day, the theory of the ancient extra-terrestrial stars in several TV series. Nevertheless, the latest developments in space exploration by means of space telescopes and the discovery of remote planets in our galaxy have definitely changed the questions we ask ourselves about possible forms of extra-terrestrial life. An approximate calculation of the number of inhabitable planets in the entire observable universe leads to striking estimates. Nevertheless, due to the limits posed by the theory of relativity, the idea of visiting even the planets closest to Earth is unconceivable, at least for the time being, and probably for many decades in the future. The distances are unbelievably great and our expertise in the field of space travel is still too rudimentary. Aliens certainly exist but because of our underdevelopment in the scientific and technological field we haven’t the least possibility of reaching them. Maybe interstellar travel will always be inaccessible for us humans, maybe it is the cyborgs – self-aware robots – that will make it for us.
Aliens have definitively changed statute since the NASA and the European Space Agency have decided to put into orbit an impressive number of space telescopes which, freed from the noise of the Earth’s atmosphere, could observe the Universe and discover for the first time the existence of billions of galaxies made up of billions of stars. Answering to the unfathomable motivations of scientific curiosity or to the even more unfathomable ones of the many governments who have decided to invest astounding resources in space research, both agencies have promoted a common research on the existence of planets, launching more, extremely expensive space telescopes, although focusing only on a narrow portion of our galaxy for the time being. In a little more than 20 years, about 5,000 planets have been discovered and given names which are unintelligible to ordinary people, such as Gliese 667 Cc or Kepler-62f. Cosmologists think that at least ten of these planets are potentially inhabitable, and these observations, as we mentioned, only concern a relatively small part of the Milky Way. Considering that there are billions of galaxies in the entire Universe, we can guess – without exaggerating – that there are at least billions of planets that are potentially inhabitable. The theory of relativity, extensively experimented since 1920, categorically prevents us from physically accessing a high percentage of these planets of the farthest galaxies, allowing us to acknowledge their existence only on a theoretical level. The distances from Earth, even in our galaxy outside the Solar System, are so great that for the time being they prevent us from visiting all the discovered planets because of our lack of technologically suitable equipment: according to the theory of relativity, in fact, the speed of light is an unsurmountable limit for any vehicle travelling through the Universe. The energy needed to make a spaceship move at the speed of light (300,000 kilometres per second) would be limitless; furthermore, even if it was possible to reach that speed, the journey to reach the planets discovered outside the Solar System would take thousands of years! The distance between the Earth and the few planets discovered so far that are relatively close to us and potentially inhabitable are so huge that even at half the speed of light, the journey to reach them would last thousands of years. Moreover, as we are going to explain later, space is expanding and the remotest galaxies are moving away from us faster and faster. The speed at which the Universe expands is not limited by the speed of light because the latter is an unsurmountable limit only for the physical objects moving through space. In spite of these notable relativistic limitations, we can only come to the logical conclusion that somewhere in the Universe there certainly are some alien civilizations. This implies that aliens are no longer the fictional projection of our fears or our fantasies as psychologists used to think, but there is a very high probability that they physically exist in places that are nevertheless so remote that they are most likely out of our reach.
So, when we wonder what these life forms look like, we can no longer answer using only our imagination or our nightmares, but unavoidably even more complex questions arise: as we only know life on our planet,canwe infer that aliens are necessarily similar to us? We are the only possible form of intelligent life in the entire Universe? Could we be visited by alien civilizations or will we be able to visit them one day?
These questions are extremely difficult to answer at the moment in realistic and scientifically grounded terms. Nevertheless, if we trust the writers and journalists who promote the theory of the ancient visitors, as they love to call it, we can find answers without committing to a series of enormously complex and expensive researches. They think that the aliens have always been on our planet, since the time the first civilizations appeared around 5,000 years ago, or even before. According to this strange theory, they did not seem to be much different from us from a biological perspective, although they were provided with outstanding technological resources. Unfortunately, we can’t understand much about the biological nature of these hypothetical aliens: the theory of the ancient visitors only provides us with extremely vague information and we do not have scientific data about the possibility of relevant alternatives to the development of intelligent life, based on physical and chemical mechanisms that are different from the ones we know. According to the reconstruction made by the supporters of this theory, we know that the aliens of the time effectively mated with human beings begetting a healthy offspring, but we know that the conditions for fertility on Earth are met only within the same species, whereas babies are rarely born from relations between genetically related species and these babies are sterile anyway. Nevertheless, in both cases a strong genetic similarity is needed, therefore these ancient aliens, provided that they really existed, would have to be extraordinarily similar to us from a genetic perspective. This may lead us to suppose that, if life exists on other planets, it may present the same biological structures that we observe on Earth. This conclusion seems to be highly unlikely and therefore it seems to contradict the theory of theancient visitors. We should also consider that ancient aliens would have landed on Earth after travelling distances which are remarkably great, even in galactic terms. This means that, at least as far as space travel is concerned, their technological expertise was already highly superior to the one we have today. Furthermore, it can’t be excluded that their knowledge in the field of biology was so sophisticated that they could go beyond the impasse of coupling between different species still observable today on our planet.
We still have to deal with some big problems which are linked to our current ignorance: we still have to understand how life could have appeared on Earth from a spontaneous transformation of lifeless matter into living matter, in certain environmental conditions dating back to remote ages; furthermore, analysing the history of life on Earth, we have discovered that chance has played a crucial role in the evolution of species until the appearance of the so-called Homosapiens. Essentially, it has been extensively demonstrated that the positive interaction between living creatures and environment should necessarily be taken into account in order to explain the appearance of the human species as we know it today. It is also true that the chemical analysis of meteorites has shown that they carry biological matter, and it cannot be excluded that life on Earth has precisely developed from the biological matter carried through the Universe by meteorites; in the field of science, there is a considerable number of supporters of this hypothesis which is called PANSPERMIA. If we accept this hypothesis, the genetic similarity between ancient aliens and the human race can seem less surprising, at least to some extent, but the high level of technological expertise of the ancient aliens is an unavoidable fact. Nowadays, the human race is not able to organize space travels that cover long distances because our current technological expertise doesn’t allow us to equip spaceships with appropriate engines. Space travel poses further problems because of the radiations in the interstellar space: our species could develop because the different layers of the Earth’s atmosphere have protected life from these radiations, whereas our spaceships are not equipped with screens suited for ensuring the same protection to astronauts in space. Ancient aliens would have necessarily belonged to a much more advanced civilization than our present one. After all, if the Universe – according to our current knowledge – is 14 billion years old, it is absolutely plausible that an alien civilization could reach – a long time ago, somewhere in our galaxy – such an advanced level in the technological field. Therefore, if we accept the PANSPERMIA theory to justify the surprising fecundative ability of ancient aliens, and if we take into account the age of the Universe, we will be likely to consider the theory of the ancient visitors not completely impossible but only extremely improbable. The members of a civilization like that, according to Susan Schneider from the University of Pennsylvania, would most certainly have a form of super-intelligence, probably due to an intensive use of calculators which are incredibly more powerful than the ones we have on Earth, and the main representatives from this field of scientific research agree with her. If we were or have been visited by aliens provided with such powers, Schneider concludes, their civilization would have to be unimaginably more advanced than ours and it would have appeared at least one billion years before the human species.
The first to formulate the theory of the ancient visitors was Erich von Däniken, many years ago, in the well-known book Twilight of the Gods, where the myths of the most ancient civilizations are analysed once again in order to show that those that were believed to be deities were more likely aliens equipped with technological resources unconceivable for those primordial terrestrial civilizations. Many ancient deities appeared in the sky upon strange objects that Däniken interprets as possible alien spaceships, and they often lived in terrestrial dwellings without appearing in public. The theory of the ancient visitors can be surely situated in a legendary context instead of in a psychological one, but as we have explained at the beginning, the fact that aliens are real and tangible beings, unknown – and not completely theoretical – inhabitants of an endless number of planets scattered in the cosmos, refers to different branches of physical and biological sciences to suggest hypothesis and theories outside both legends and unconscious psychology. From entities produced by our fears or our legends, here they are turned into unknown elements of the reality of our Universe.
Can there be life forms completely different from those we observe on Earth? The subject is extraordinarily interesting and a new branch of scientific knowledge that is still taking its first steps, astrobiology, is being developed these years in order to find an answer to this question.
Astrobiologists have defined a program in several steps to establish if one or more of the discovered extrasolar planets are capable of supporting some life form, perhaps completely different from our own. Neglecting the less friendly planets like those exclusively consisting of gas, and focusing on the solid ones with a rigid surface similar to the terrestrial one, it is necessary:
1) to verify the probable presence of liquid water (water has chemical and physical properties that are in any case needed for the development of intelligent beings) and the possible existence of other volatile gases in the planet’s atmosphere, together with the presence of organic compounds;
2) to develop astronomical techniques that can answer point (1);
3) to preliminarily develop surveys about the possible presence of basic forms of life on planet Mars;
4) to develop researches and identify the technologies needed to verify the presence of any life form on some moons of the Solar System such as Europe, Ganymede, Callisto and Titan;
5) to develop our knowledge about the sources of organic and inorganic matter on the potentially inhabitable planets and moons of the Solar System.
The list prepared by astrobiologists is actually much longer but it would be boring for the readers, therefore I will omit the rest. In summary, astrobiologists ask to collect all data which is useful to identify the possibility of life forms essentially different from those existing on our planet. The real problem is somehow purely scientific: to identify the possibility of essentially unpredictable and alternative life forms within our Solar System, and then refine the technologies needed to ascertain, with observative instruments, their possible presence on the already discovered planets outside the Solar System. The expected costs to obtain the necessary experimental information are considerably huge and imply long and demanding investments by the governments of the world, as well as an unavoidably close cooperation between the institutions involved in this kind of research.
On a spring day in 1950, Enrico Fermi was talking to the Hungarian scientist Edward Teller at the restaurant in the Los Alamos Centre, the laboratory where they had fervently worked to produce the first prototype of the nuclear bomb ten years before. In 1938, Fermi had won the Nobel prize for his researches in the field of physics, and both of them had left their homeland because of the racist laws imposed by the Nazis. But that day, at the Los Alamos restaurant, they were discussing something else: flying saucers. They were two famous scientists, not likely to accept the hypothesis of saucers flown by mysterious aliens (during and after World War II, many airplane prototypes were built that could be easily confused with flying saucers) and, in the middle of the conversation, Fermi asked Teller whether he believed that someday in the future it would be possible to travel at a speed close to the speed of light. Teller answered that the odds were “one in a million”. The theory of relativity had already been known for some decades and Teller had calculated how much energy would be consumed by a spaceship travelling at a speed a little lower than the speed of light. Fermi kept quiet and thought about it, then he came up with a question that has become quite famous. “Where is everybody?”, he asked, referring to aliens. For a moment, he had considered their actual existence and he had obviously deduced that there should necessarily be some aliens around the Universe, but the true problem was that – besides the questionable sightings of flying saucers which he, as a scientist, scarcely trusted – there was no sure evidence of their existence.
This apparently weird remark uncorked a long series of scientific articles about the reasons of this lack of evidence. A detailed and pleasant analysis of what was later called the Fermi paradox can be found in the book Where is everybody? by Stephen Webb. Our knowledge about the Universe is much more detailed today than in 1950. We do not know which probabilistic estimates had crossed Fermi’s mind at the Los Alamos restaurant but, knowing how great he was in quickly carrying out statistic estimates, we are allowed to project them on our current scientific knowledge. The Universe is probably limitless but the part that we can observe today has a radius of about 14 billion light years (we know that a light year corresponds to 9,460,730,472,581 kilometres) and is made up of many billions of galaxies, each of them containing about 100 billion stars. Scientists’ estimates currently suggest that around 30% of the stars have planets orbiting around them. If each star had just one planet, we could conclude that the number of planets orbiting in the small portion of the Universe that we can study is about 3 followed by 21 zeros. Obviously not all planets can sustain – at least, in theory – life as we know it. According to a reasonable estimate elaborated by scientists, only 1% of the planets have suitable conditions for life, but the estimated number of planets possibly suitable for life in the observable Universe is still impressive: 3 followed by 19 zeros. Nevertheless, as the astrobiologists of the Australian National University observed about the history of our planet, the evolution that leads to the birth of an intelligent species is very fragile; evolution rarely manages to make it through the climate changes that affect almost every planet. To produce an inhabitable planet, the life forms themselves need to somehow regulate the geophysical and meteorological parameters of the planet. Planets such as Venus and Mars were originally suited for developing at least a microbial life stage, but these primordial beings have not been capable of regulating the parameters of those planets and life died out. According to scientists, primordial life forms have almost always been unable to go beyond the so-called Gaian bottleneck – in other words, to determine the planetary changes that could give them access to the next evolution phase, and create the conditions for the development of more complex life forms. Most of the planets where life could potentially develop have presumably failed that test: if we could visit them, we would probably find out that they just host fossils left by extinct microbial species. To our current knowledge, oxygen appeared in the Earth’s atmosphere during a period of time that scientists call the proterozoic era, which probably started 2,500 million years ago. Atmospheric oxygen was produced by the photosynthesis process carried out by microscopic unicellular organisms; on our planet, oxygen combined with groundwater and started to oxidise carbon producing carbon dioxide, thus causing the atmosphere as we know it to start forming. The species that later appeared did so in an atmosphere rich with the oxygen produced by the first unicellular organisms. Plants need oxygen compounds to live, but at the same time they release a substantial amount of them, increasing the quantity of oxygen in the atmosphere. Anyway, despite the problems posed by the Gaia theory – essentially regarding the role of chance in the evolution of life on a planet similar to Earth – and considering the immensity of the universe and the number of planets, we come to an obvious conclusion: aliens inevitably exist somewhere in the observable universe. Our conclusions can’t be much different from the ones Fermi drew in the Los Alamos restaurant and we can understand why he asked “Where are all of them?” and also why he asked Teller his predictions about the possibility of building – in a more or less distant future – spaceships that could travel at a speed lower but significantly in the order of the speed of light. But let’s go back for a moment to the previously mentioned PANSPERMIA theory… The astronomic conditions in our galaxy seem to allow the diffusion of organic material from one planetary system to the other. The original idea was formulated in 1871 by the renowned scientist Lord Kelvin in a note to the British Association for the Advancement of Science. Another famous scientist, Svante Arrhenius, published a book on this topic in 1908 called Worlds in the Making. If a comet hits a planet where life has developed, it can cause the emission from its surface of microscopic particles of material that can in turn contain microscopic unicellular organisms, viruses, or parts of organic material such as DNA fragments. Almost two hundred years later, this hypothesis was confirmed by the discovery of organic material in cosmic dust fragments. Accurate calculations – carried out up until recent times – confirmed that the electromagnetic radiation of the stars can accelerate these cosmic dust grains up to a speed that can oscillate between 45 and 60 kilometres per second for microscopic fragments of the planetary surface. Bigger fragments can hardly travel at such a speed; therefore, they can’t be generally considered as vehicles of the galactic panspermia. However, both electromagnetic radiation and the cosmic rays’ particles travelling through galactic space can destroy these organic traces. It could take a grain of galactic dust from 10,000 to 1 million years to cover the average distance between two stars, but recent progress in molecular biology has revealed the incredible survival ability of microorganisms, thanks to spontaneous autorepairing mechanisms and the possibility to reactivate themselves in favourable conditions and travel in spore form, a configuration that is more resistant to radioactive damage. We must consider once again the limitations posed by the theory of relativity; however, although 300,000 kilometres per second can seem an extremely high speed, if we compare it to the dimensions of the explorable universe we realize that it would take a ray of light hundreds of millions of years to travel from one side of the explorable universe to the other – an unsurmountable limit for a spaceship, be it equipped with the most advanced – and to this day still unknown – technology.
Up until here we have focused on the limitations posed by the incredible dimensions of our universe, but we also have to consider the ones regarding the temporal dimension. Earth is about 4.5 billion years old, but the human species as we know it is only 200,000 years old (Homo sapiens appearance) – that is to say, only a moment compared to the age of the universe. According to scientists’ estimates, there are many planets that could potentially host life, but if their distribution in the observable universe is reasonably uniform, the average distance between them measures many million light years, even if we don’t take into account relativistic corrections. The first galaxies, with their stars and planets, formed billions of years ago; any intelligent species could have appeared on those planets billions of years ago, and the probability that they have now become extinct for some reason is rather high. Even if they travelled through space, what are the odds of them ending up on our planet in the extremely short period of time of 200,000 years – that is to say, since the appearance of Homo sapiens? The presence on Earth of an intelligent life form is, compared to the age of the universe, an infinitely small fraction. According to the data supplied by scientists, the Milky Way – the galaxy that contains the Earth and the Sun and the one on which we will focus to avoid the vastness of numbers – contains 100 to 200 billion stars, an extremely high percentage of which has planets orbiting around it. The estimated number of habitable planets is also very high; moreover, the Milky Way’s radius measures 946 millions of billions of kilometres and its average thickness 9 millions of billions of kilometres, so it would take about 100 years for light to cover the distance between the two most distant points. As I’ve already said perhaps too many times, the speed of light can’t be reached by any spaceship, however technologically advanced it may be. Assuming that a distant civilization was able to build spaceships that could travel at a tenth of the speed of light (30,000 kilometres per second), it would take about 1000 years for them to cross the Milky Way. Around the Galactic Centre, stars rotate at high speeds – around 200 kilometres per second – but the rotation speed is not the same for all stars (it decreases as we move away from the centre of the galaxy) and this leads us to conclude that the distance between two stars in the Milky Way slowly changes in time. We also have to consider the wide portions of our galaxy where interstellar gases cause the continuous formation of new stars. So, the different rotation speeds around the central nucleus and the life and death cycle of stars mean that, at a galactic level, the “geography” of the stars close to our Sun slowly changes on the time scale of the galactic rotation.
The peripheral location of our Sun in our galaxy puts it in an area where this phenomenon of a very complex cyclic transformation from gas into stars and vice versa is particularly active, so that it is impossible to know the exact “geography” around the Sun – and consequently around our planet – in a remote past. The oldest and heaviest stars located in the central swelling rotate around a gigantic black hole that exerts an enormous gravitational attraction, so it seems unlikely for them to be at the centre of a planetary system with planets inhabited by intelligent beings. The attraction force of our galaxy also keeps attracting new stars from the surrounding area, called “halo”.
What conclusions can we draw about the Fermi paradox?
After the launch of the Hubble telescope, the number of observable galaxies rose dramatically and it is now believed to be between 200 and 2,000 billions. Unfortunately, most of them is moving away from us at a speed which increases with the distance from our galaxy; this speed, which is due to the expansion of the universe, can be greater than the speed of light. In other words, we won’t see them again in the future and – again, because of the limitations posed by the theory of relativity to all physical objects – it will be impossible to reach them; and vice versa, any intelligent being living there won’t ever be able to reach us. The most distant galaxies will disappear forever, at least for us, not because they are moving faster, but because the distance between us is increasing too rapidly. In 1929, contrary to what Einstein believed, Edwin Hubble discovered that the other galaxies were moving away from the Milky Way. Because of the extremely high but finite value of the speed of light, when we observe the sky we see the universe as it was in the past: the light we see from distant stars and galaxies was emitted years, centuries or even millennia ago. The speed at which the universe expands has no limit: imagine that the Adriatic Sea was expanding at a speed higher than the one at which the most powerful motorboats can travel and that airplanes were not invented yet: it would be impossible to reach the opposite coast, and the same would apply to the people living in Albania. Fermi knew this very well in the spring of 1950. Today we also know that the speed at which the universe expands can be higher than the speed of light. We can imagine the universe as a loaf cake stuffed with raisins rather than galaxies: if the loaf cake expands maintaining its shape, the distance between the raisins will increase, and the same applies to galaxies. The speed of light limit only applies to the objects moving inside the cake, not for the speed at which the cake itself expands. It is difficult to believe that the space itself expands and that the loaf cake metaphor can effectively describe the reality of things, but they do. As the perception of space in our everyday life is limited to a microscopic portion of the entire universe, infinitesimal compared to the distance between galaxies, all we can do is trust the astrophysics or read Eyes on the Universe by Isaac Asimov, which is still an excellent educational book even if it was written years ago. So, taking into account the speed of light limit, we can assert with certainty that the probability of being visited by aliens equipped with extremely advanced technology decreases in time, although on a universal temporal scale (at least millions of years). The absurd brevity of the intelligent life presence on Earth compared to the age of the universe – as well as the enormity of the latter – suggests that the probability of an alien spaceship passing close to us is in fact infinitely small. However, we must add two considerations: firstly, the numbers we considered have only been available for a few decades, and consequently our knowledge of the universe has accumulated too rapidly to lead us to definitively verified theories; and secondly, the development of technological instruments is virtually unpredictable. As we have just noted, interstellar travel still represents an unsurmountable obstacle for us humans, and for various reasons. Let’s try to compare the distances between the astronomic objects observable at naked eye in the night sky, and then determine the problems that will have to be dealt with to responsibly organize an intergalactic journey. The Moon is 384,000 kilometres far from Earth, the Sun 150 million kilometres, Mars 225 million kilometres, Pluto 5,750 million kilometres. And beyond the solar system, the Orion Nebula is 1,300 light years far, the centre of the Milky Way 25,000 light years and the Andromeda Galaxy, the closest to us, 2 million light years. Imagine that the destination of our journey is a planet in the Andromeda Galaxy, and that our spaceship can travel at a third of the speed of light: it would take us millions of years to reach it, a period of time far longer than the average life of a human being. For the sake of the argument, let’s also assume that our spaceship has all the necessary characteristics to reach that speed and protect the astronauts from cosmic radiations; the men and women embarking on such a journey still couldn’t hope to reach their destination, and neither would their children and grandchildren. Many generations would pass before the destination is reached. Assuming that everything goes well, the right environment, an appropriate food stock, medical treatment, forms of entertainment and personal rooms would have to be guaranteed, and many hundreds of people would have to embark, with an equal proportion of men and women, to make sure that after millions of years their descendants are still alive. It would also be necessary to establish some rules forbidding all couples to have more than two children. Artificial gravity would have to be provided in one section of the spaceship at least, to prevent the bone structure and the muscular system to gradually become weak. The astronauts would have to form a small society that – considered the exceptionality of the situation – would need medical and psychological assistance throughout the entire journey, as well as food and water. This kind of spaceship is appropriately called generation ship. Such a ship would have to be necessarily built in a terrestrial orbit and would require an extraordinary amount of energy and resources, probably too much for a single country or even a small group of countries. As well as having to deal with all the physical and psychological problems of the members of the crew, it would be necessary to provide them with all the instruments and rules required to pass to the new generations the essential skills for the maintenance of the spaceship, even in the event of a social or mechanical crisis. But how can we make sure that a gigantic machine such as a generation ship runs perfectly for millions of years, and that the small astronauts’ society doesn’t take bad decisions in the total isolation in which it will be for millions of years? We must consider that radio waves travel at the same speed of light, and consequently messages from controllers back on Earth would take too much time to reach the ship in the event of an emergency. Let’s suppose, for example, that the ship asks for help when it is only ten light years far from Earth; it would take ten years for the message to reach the control centre on Earth and another ten for the answer to reach the ship; any urgent communication would be technically impossible. Confronted by the countless problems involved in such a journey, futurologists have imagined that – rather than humans – we could send cyborgs into space: intelligent automata somewhat similar to the character played by Arnold Schwarzenegger in the film The Terminator, but with a nicer personality and socially harmless. But the creation of such a robot is a problem perhaps more complex than interstellar travel. There are two possible interpretations of the word cyborg. If we asked experts, they’d tell us that humans have always used new tools to make certain activities – both physical and intellectual – possible and less laborious. If we asked common people, on the other hand, they’d tell us that agriculture and livestock farming were developed in order to avoid the hardships of an uncertain survival based on hunting, and consequently the plough was developed to reduce the amount of work in agriculture, and then they would enumerate a series of instruments external to our body that help the human race in the most laborious and difficult tasks... and in the end they would say that we are all not just ourselves, but also all the physical instruments that have supported us in every activity until the development of modern civilization. They would probably mention state of the art smartphones, or perhaps they would focus on the rapid evolution of computers in the last decades, affirming that our thoughts are nothing more than the result of extremely sophisticated softwares running on that biologic computer that is our brain, and that in a future not so far away it will be possible to reproduce our thoughts in a supercomputer. Historically, many philosophers and artificial intelligence experts have opposed this vision of a computational nature of human thought, although the real problem today is represented by the self-aware nature of our thought. Consequently, the interpretation of the word “cyborg” is very important for the understanding of what futurologists mean when they talk about sending cyborgs to explore the universe. In the first case, we are talking about sending men and women equipped with external physical instruments that make them more intelligent, almost omniscient, and able to make it through an intergalactic journey – something that futurologists advise against, perhaps out of empathy or of a sheer respect for the conventions of our civilization. In the second case, on the other hand, we are talking about sending machines, probably robots that are somewhat self-aware. Apparently, the two solutions seem antipodal, but even if we don’t send real men and women into space, there is still a problem: what are the differences between a human being and a self-aware machine – self-awareness being a necessary condition to make it through an intergalactic journey? Several questions arise: robots won’t need medical assistance, but if one of them breaks up, who would fix it? Perhaps a more self-aware robot? Assuming there is a sort of robot-in-chief, what would it decide to do? Fix the other robot – if it’s possible – or take it apart to build another one? Taking apart a self-aware robot is terribly similar to killing a human being... it is not unlikely that our descendants will share our moral preconceptions and will prefer to sacrifice self-aware machines rather than human beings.