Interstellar space travel. How can we do it?
With the recent advancements in space travel and upcoming missions to both Moon and Mars we are very likely to see considerable progress in spacefaring technologies. Coupled with that, we will also get the fusion engines in the near future that with all likelihood will be coming out in their compact form rather sooner than later (my article on the subject: https://astara-store.com/blogs/default-blog/scalability-of-fusion-reactors-smaller-means-sooner). This can set us on a course for something even more fascinating than colonizing Mars and Moon. And by that I mean the interstellar space travel. Even though it might sound distant right now, we are on the course of obtaining all of the necessary technologies needed for it. With the biggest point of interest being the creation of the engines that are faster than the current propulsion systems. With fusion engines being in the closest to becoming the sustainable future of space travel (one of those being mentioned in my previous article) and other more exotic forms of propulsion coming along after it. Including solar sails, anti-matter drives, Ion drives, and even blackhole drives.
First of all, let us look into the fusion engines. In May last year, United States Congress has approved a 125 million USD funding in the development of both fission and fusion rockets. With a nuclear thermal propulsion demonstration scheduled for the upcoming years. With the Russian space agency – Roscosmos working on a plasma rocket engine in collaboration with the Kurchatov institute – a project that was made possible by advances made in the study of plasma fusion processes. While the Advanced Concept Team at the European Space Agency (ESA), in collaboration with European universities is conducting a study on the feasibility of open magnetic confinement fusion propulsion. China intending to develop a whole ‘fleet’ of nuclear carrier rockets by the mid-2040s.
The core principle of movement in those rockets will be done through the simple reaction of heated exhaust gas that is expelled at high speed through a nozzle and in a reaction where the thrust force is exerted on the vessel. With the conventional rockets being propelled by chemical combustion, requiring considerable amounts of fuel – liquid hydrogen and liquid oxygen, or kerosene-like propellant all of which are ignited at different stages of ascent into space.
Expelling the hot gas is not the only way to provide thrust. Robert Goddard has suggested that in the 1990s rockets could use electricity to eject electrons or charged ions with a velocity in the range of 10 kilometers per second.
Fusion is currently considered in its non-explosive application. With the reactors potentially serving as a heat source that would bring propellant to extremely high temperature (with high-velocity exhaust) or expel ultra-hot plasma to provide the thrust. The speed of such a rocket would range from 150 to 350 kilometers per second. Being able to reach Mars in 90 days or less. While the conventional rockets being able to reach it in eight months. Moreover, it would shorten all of the travel distance across the solar system in general.
And if we are talking about large interstellar travel, we would likely need scalable fusion technology, definitely something smaller than ITER, which would be easier to put on the orbit. A NASA-funded joint venture between the University of Washington in Seattle and a small company named MSNW LLC is focused on the development of advanced space propulsion systems and have developed a small field-reversed pulsed-fusion device. That can be turned into a “fusion drive engine” with a particular aim on the deep space travel. While Princeton Plasma Physics Laboratory (PPPL) is in a collaboration with a company called Princeton Satellite Systems, working on a fusion drive engine for space exploration. Two years ago, NASA awarded a half-a-million-dollar grant to the venture that was distinguished by a US Federal Laboratory Consortium award in October, last year. For those reactors, the deuterium/helium-3 reactions are planned with the direct fusion drive engine having neither of these limitations and with both elements being stable and so the reaction products, such as hydrogen and helium. With the reaction being “aneutronic”, requiring no heavy shielding for protection.
The biggest catch is that the temperature required to fuse deuterium and helium-3 nuclei is ten times higher than that required for DT fusion and no device has yet achieved this level of energy. With the present record being held by the Japanese tokamak JT-60U, which reached an ion temperature of half-a-billion degrees. The developers of the direct fusion drive claiming that their engine could be operational as early as 2028. With fusion energy being the main source of energy for traveling inside and beyond the solar system.
Solar sails are another option for the interstellar flights and missions inside our solar system. However, even though the technology is theoretically possible it is still far off. The core idea of it is to use the photons of light with a large-enough sail to propel the spacecraft forward. This is achieved by photons hitting the solar sail and transferring the momentum to it, giving a small push and another small push when they bounce off the sail. Both are very slight, but in the vacuum of space where there is nothing to slow down the sail, each push changes the sail’s speed. Such sail would allow the ship to accelerate constantly, potentially reaching greater speeds than can be achieved by the conventional, chemical rockets.
The currently available sails would be made of lightweight materials, such as Mylar or polyamide, coated with a reflective surface. Lightsail 2 uses 4 triangular Mylar sails that are only 4.5 microns (1/5000th of an inch) thick. Those will unfold using 4 cobalt booms that unwind like tape measures. The sails shall have a combined area of 32 square meters (344 square feet). There is no minimum size for a solar sail, but for the same size spacecraft, bigger sails will capture more sunlight and accelerate the spacecraft more quickly. The greater the size of a spacecraft that we are talking about, the larger the sail size would be. Therefore, it might remain a doubtful option for the large interstellar spacecraft.
Another very likely possibility would be the ion thrusters. The ion propulsion system’s efficient use of fuel and electrical power may enable modern spacecraft to travel farther, faster and cheaper than any other propulsion technology currently available. The maximum speed that a spacecraft can reach with such thrusters can go up to 90 000 meters per second (over 200000 mph), compared to the Space Shuttle that was able to reach the speeds of only around 18000 mph. with the fuel efficiency up to 90 percent. Compared to 35 percent of chemical fuels. The downside of such a propulsion method is the low thrust, thus they are not usable for escaping the Earth’s gravity and launching the spaceships into space. On top of that, they should operate in a vacuum. However, it is perfectly suited for the long space travel, where even tiny amounts of constant thrust add up to much shorter travel time and much less fuel consumption.
As their names say, ion thrusters eject ions instead of combustion gases to create thrust and the applied force makes the spacecraft move forward. The ion is an atom or molecule that has an electrical charge because it has lost (positive ion) or gained (negative ion) an electron. In the case of ion propulsion, they have lost electrons, thus they are positively charged. Plasma is an electrically neutral gas in which all positive and negative charges from neutral atoms, negatively charged electrons and positively charged ions add up to zero. It is the fourth state of matter and exists everywhere in nature. It has some of the properties of a gas but is affected by electric and magnetic fields, making it a good conductor for electricity. In ion thrusters, plasma is made of positive ions and equal amounts of electrons.
“NASA's conventional method of producing ions is called electron bombardment. The propellant is injected into the ionization chamber from the downstream end of the thruster and flows toward the upstream end. This injection method is preferred because it increases the time that the propellant remains in the chamber.
In such ion thrusters, electrons are generated by a hollow cathode, called the discharge cathode, located at the center of the thruster on the upstream end. The electrons flow out of the discharge cathode and are attracted (like hot socks pulled out of a dryer on a cold day) to the discharge chamber walls, which are charged highly positive by the thruster's power supply. When a high-energy electron (negative charge) from the discharge cathode hits, or bombards, a propellant atom (neutral charge), a second electron is released, yielding two negative electrons and one positively charged ion. High-strength magnets are placed along the discharge chamber walls so that as electrons approach the walls, they are redirected into the discharge chamber by the magnetic fields. Maximizing the length of time that electrons and propellant atoms remain in the discharge chamber, increases the chances that the atoms will be ionized.”
NASA is also researching the electron cyclotron resonance to create ions. With the method using high-frequency radiation (usually microwaves) coupled with a high magnetic field to add energy to the electrons in the propellant atoms. Causing the electrons to break free of the propellant atoms and create plasma. With ions being able to be extracted from this plasma.
“In a gridded ion thruster, ions are accelerated by electrostatic forces. The electric fields used for this acceleration are generated by two electrodes, called ion optics or grids, at the downstream end of the thruster. The greater the voltage difference between the two grids, the faster the positive ions move toward the negative charge. Each grid has thousands of coaxial apertures (or tiny holes). The two grids are spaced close together (but not touching), and the apertures are exactly aligned with each other. Each set of apertures (opposite holes) acts like a lens to electrically focus ions through the optics.
NASA's ion thrusters use a two-electrode system, where the upstream electrode (called the screen grid) is charged highly positive, and the downstream electrode (called the accelerator grid) is charged highly negative. Since the ions are generated in a region that is highly positive and the accelerator grid's potential is negative, the ions are attracted toward the accelerator grid and are focused out of the discharge chamber through the apertures, creating thousands of ion jets. The stream of all the ion jets together is called the ion beam. The thrust is the force that exists between the upstream ions and the accelerator grid. The exhaust velocity of the ions in the beam is based on the voltage applied to the optics. Whereas a chemical rocket's top speed is limited by the heat-producing capability of the rocket nozzle, the ion thruster's top speed is limited by the voltage that is applied to the ion optics, which is theoretically unlimited.
Because the ion thruster ejects a large amount of positive ions, an equal amount of negative charge must be ejected to keep the total charge of the exhaust beam neutral. Otherwise, the spacecraft itself would attract the ions. A second hollow cathode called the neutralizer is located on the downstream perimeter of the thruster and pushes out the needed electrons.”
The matter-antimatter engine is probably the most appealing option for interstellar space travel. It will be capable of reaching tremendous speed and it looks more likely to become a future possibility now that we have produced antimatter. By pairing together positrons and antiprotons, scientists at CERN created the first anti-atom. As of 1998, they’ve been steadily increasing the number of such atoms that they produce. The core reaction in these engines is about antimatter coming into contact with the normal matter. Through this collisions an explosion emitting pure radiation is produced, which then travels out at the speed of light. The entire mass of the collided objects transfers into extremely powerful energy. The reaction are 1000 times more powerful than the nuclear fission and 300 times more powerful than nuclear fusion. Therefore, they can potentially take us farther with less fuel.
The problem, however, is in the lack of antimatter existing in the universe. Thus, we have to create our own, which is very costly in terms of both energy and resources. This is possible through the “atom smashers”, such as the one at CERN, where the particles are smashed into each other by acceleration achieved through powerful super magnets that propels them to near-light speeds. Some of the particles from this reaction are antiparticles that are separated by the magnetic field. However, only two pictograms of antiprotons are produced each year, a trillionth of a gram. And we would need a lot more than that to have enough fuel for reaching the interstellar destinations.
In October 2000 NASA has announced their early designs for an antimatter engine that can be fueled by small amounts of antimatter. With a trip to Mars taking as little as a millionth of gram. NASA Institute of Advanced Concepts (NIAC) is also funding a team of researchers who are working on a new design for the antimatter-powered spaceship that can potentially have no negative effects from the gamma rays produced in the reaction. The gamma rays being are more harmful than X-rays. They penetrate matter and break apart the molecules in cells. They can also make the engines radioactive by fragmenting atoms of the engine material. This new design will use positrons that make gamma rays with much lower energy.
"The most significant advantage is more safety," said Dr. Gerald Smith of Positronics Research, LLC, in Santa Fe, New Mexico. The current Reference Mission calls for a nuclear reactor to propel the spaceship to Mars. This is desirable because nuclear propulsion reduces travel time to Mars, increasing safety for the crew by reducing their exposure to cosmic rays. Also, a chemically-powered spacecraft weighs much more and costs a lot more to launch. The reactor also provides ample power for the three-year mission. But nuclear reactors are complex, so more things could potentially go wrong during the mission. "However, the positron reactor offers the same advantages but is relatively simple," said Smith, lead researcher for the NIAC study. Such spacecraft would take the astronauts to Mars in about 180 days; and with some additional developments it will be reduced to just 45 days.
On top of that, there is another US venture that is focused on producing antimatter in larger quantities, compared to what is being currently obtained. Its name is Hbar Technologies and it has received the initial funding in 2002 for its early research into antimatter through NASA’s Advanced Innovative Concepts program. Previously the US Fermilab was able to produce a nanogram (billionth of a gram) of antimatter per year until the production line was closed in 2011. However, Hbar Technologies are going to take a broader approach. Instead of capturing a smaller number of particles solely for the high-energy experiments they will capture more generic particles, effectively increasing the production to a microgram (millionth of a gram) per year. Their project would require a research of a magnetic field for the purpose of antimatter storage for the spacecraft that would use such fuel. That on its own would cost them few million dollars and take five years of research. While going as far as producing a milligram (a thousandth of a gram) of antimatter per year would require a humongous investment of billions of dollars. And if we are talking about interstellar missions they would require as much as two grams per year for a mission to be launched every ten years.
There are concepts that are even farther away in terms of possibility and potential time and resources that they would require. Those are black holes drives, Alcubierre warp-drive and traversing the space-time through wormholes. They would provide the fastest travel speed for interstellar flights and it is possible to at least theorize about them. In 1955 John Wheeler has first introduced the term ‘Kugelblitz’ that translates to ‘ball of lightning’. In his theory, if enough pure energy is focused into a region of space a microscopic black hole would be formed. Applying the research of Stephen Hawking into the black hole’s event horizon such a small black hole would have greater radiated power and shorter lifetime. It will be usable as a power source for space travel if its small enough to produce the right amount of energy, light enough to be reasonably accelerated, but big enough to have a sufficiently long lifespan. Being smaller than a proton it will have a power output of approximately 129 petawatts and the weight of more than two Empire State Buildings.
Potentially a sufficient amount of energy can be gathered directly from our sun. According to Freeman Dyson and his prediction for civilizational advancement a sufficiently advanced one would be able to surround a star with a spherical shell of 1 Astronomical Unit (the average distance between the Earth and the Sun) in radius. Providing a humongous, limitless supply of energy. Its far smaller alternative can possibly be used to capture enough energy for the Schwarzschild Kugelblitz. However, it would require the total energy capture from such black hole. The great option for that is being surrounded by a small Dyson Shell. With the absorbed particle energy being fed to a heat engine, propelling the starship. “When all of the available energy from a typical SK is fed into a 100-percent efficient engine, the starship will reach 72 percent of light speed in the five-year lifetime of the SK. This formidable subluminal speed would allow a starship to reach, within a human lifetime, a number of stars in the solar neighborhood.” The only existing instrument capable of producing Schwarzschild Kugeblitz is a gamma-ray laser. However, its output frequency would need to exceed the current technology by more than a billion times. With its pulse duration being a hundred billion times shorter than any of the current lasers. Being equal to 1/10 second of the sun’s energy output in power of a single pulse.
Another interesting concept is the Alcubierre warp drive. It is most interesting since theoretically it can allow faster than the speed of light travel, becoming a solution to Einstein’s field equations. It would involve stretching the fabric of space-time in a wave, causing the space ahead of an object to contract while the space behind it would expand. With the spaceship then being able to ride the created region of ‘warp bubble’. It derives from the ‘Alcubierre Metric’ that allows a warp bubble to appear in previously flat region of spacetime and for it to move at the speeds exceeding the speed of light. While traveling in such manner the time dilation and other conventional relativistic effects would not apply and the rules of space-time and the laws of relativity would not be broken. This because the ship itself would not be moving within the bubble, but is carried along as it moves.
In 2011 there was a 100-year Starship Symposium during which Harold “Sonny” White made a presentation titled “Warp Field Mechanics 101” at which he shared updated calculations of the Alcubierre Metric. From that time, he and his colleagues have begun testing the theory in their Eagleworks Lab. There have been a number of recent developments as well that have contributed to the subject. Naturally occurring gravitational waves (GWSs) is one such example, being discovered by LIGO scientists in 2016. It has both confirmed the prediction made by Einstein a century ago and proved that the basis for the warp drive exists in nature. "In the past 5-10 years or so, there has been a lot of excellent progress along the lines of predicting the anticipated effects of the drive, determining how one might bring it into existence, reinforcing fundamental assumptions and concepts, and, my personal favorite, ways to test the theory in a laboratory. The LIGO discovery a few years back was, in my opinion, a huge leap forward in science, since it proved, experimentally, that spacetime can 'warp' and bend in the presence of enormous gravitational fields, and this is propagated out across the Universe in a way that we can measure. Before, there was an understanding that this was likely the case, thanks to Einstein, but we know for certain now." Joseph Agnew (an undergraduate engineer research assistant from the University of Alabama in Huntsville’s Propulsion Research Center (PRC)) says. The discovery has demonstrated that some of the effects occur naturally. However, there would have to be considerable advancements in technologies and theoretical framework. "In essence, what is needed for a warp drive is a way to expand and contract spacetime at will, and in a local manner, such as around a small object or ship. We know for certain that very high energy densities, in the form of EM fields or mass, for example, can cause curvature in spacetime. It takes enormous amounts to do so, however, with our current analysis of the problem. On the flip side, the technical areas should try to refine the equipment and process as much as possible, making these high energy densities more plausible. I believe there is a chance that once the effect can be duplicated on a lab-scale, it will lead to a much deeper understanding of how gravity works and may open the door to some as-yet-undiscovered theories or loopholes. I suppose to summarize, the biggest hurdle is the energy, and with that comes technological hurdles, needing bigger EM fields, more sensitive equipment, etc."
The colossal amount of positive and negative energy required for the warp bubble creation is the main hurdle associated with Alcubierre’s concept. With the only possible current method being the exotic matter. With the total energy required for it being the same as the mass of Jupiter. This is an improvement, however, since the last estimate was equivalent to the mass of the Universe. Thus, it is possible to anticipate a farther reduction in the energy requirements until it reaches a more realistic scale. This can only be achieved through farther advancements in quantum physics, quantum mechanics and metamaterials. In addition to all of that, further progress will have to be made in the creation of superconductors, interferometers, and magnetic generators. Coupled with large requirements for the funding of such a project. However, the current theoretical material definitely proves that it can be achieved and that it will be worthwhile.
The last and probably the most futuristic option for traversing the space through wormholes. Which is hypothetically the shortest route that we can take not only between solar systems but between two points in the universe. And as with Alcubierre’s warp drive, we will be able to bypass the speed of light. However, according to the authors of the study on the subject, Daniel Jafferis, Ping Gao and Aron Wall, it would take longer to traverse through the wormholes than go from one point to another directly. However, they have made considerable progress in theorizing the possibility of traveling through such wormholes. First, they have dwelt into a connection between two black holes that would be entangled on a quantum level, which would be shorter than the wormhole's connection. "From the outside perspective, travel through the wormhole is equivalent to quantum teleportation using entangled black holes," Jafferis said. They have also managed to bypass the major stumbling block for the wormhole travel – negative energy, by using quantum field theory tools and calculating quantum effects similar to the Casimir effect.
It can be concluded that from all the future space travel methods fusion engines and ion thrusters are the closest and everything that approaches quantum physics and bending space-time continuum is the farthest. However, if a dedicated funding is allocated to space programs they would gradually become a reality. However, this is just a part of the interstellar space travel and we should discuss the most likely forms of space ships that would help us achieve it.
The most likely and most holistic way to approach spending years upon years drifting from our solar system to other stars would be to create large space ships with artificial gravity that would be able to support the same level of comfort and life that we enjoy here on Earth. With all likelihood some of you are already familiar with Gerard K. O’Neill’s cylinders. Those were demonstrated in the movie – Interstellar and are the concept that has been around for some time now. The curious thing is that we are rapidly approaching the stage when we will be able to get the lunar materials for such construction. On top of that, we should have expanding lunar space stations that could potentially serve as space docks where such humongous construction project may be fulfilled. The cylinders would potentially provide space for several million people and be of 500 square miles (1,295 km). There are several remarkable things about them that should be clarified. First is that they would provide a constant artificial gravity through rotation. Second, they will be big enough to become a portable piece of our own Earth. Third, they should provide enough space for interstellar travelers to thrive in and enjoy a full, comfortable life. Other options for such space stations would be the Stanford Torus, capable of housing 10000 people and Bernal Sphere “Island One”, a spherical habitat capable of housing 10000 people as well. We would have to keep in mind that entire generations would spend their lifetimes in such cylinders and would have to design them with that in mind. So that they would be extremely livable and not just vehicles for traveling from one star to another. Moreover, we would definitely need to consider terraforming capabilities before we get to constructing interstellar space stations. Thus, we would likely see some of the related technologies being applied on the Moon and Mars before we start taking large steps in interstellar travel. However, it is only beneficial to start looking into it early on.
One of the supporters of such space stations is Jeff Bezos. The Blue Origin programme is just the beginning and an initial step of a larger project. One of his key points during the Blue Origin presentation was that within a couple of centuries we are likely to outstrip any reasonable sources of energy here on Earth. And moving into space colonies would be a solution for it. “These are very large structures, miles on end, and they hold a million people or more each,” he explained. He envisions millions of such colonies housing trillions of people, sustained by continuous sunshine and the vast resources available on the moon, asteroids and other parts of the solar system.” Whether Bezos will be the one to create the O'Neill's cylinders remains to be seen. NASA is already investing considerable resources and scientific knowledge into reaching the planets in our solar system and beyond and only time will show the exactness of the actual interstellar programme.
One of the greatest concerns of O’Neill was the increase of population on Earth and the eventual need for humanity to adjust to larger populations. “As population continues to expand, we’ll have to abandon the development of greater individual freedom and accept a much more regulated life with diminished options,” O’Neill said in a 1979 interview with OMNI magazine. In his vision, there would be far fewer people living on Earth and a larger number of people living in space. This would definitely lighten all of the resource consumption burdens and help overcome any issues that have to do with the overpopulation. “O’Neill’s proposed colonies would be mile-wide spheres or cylinders, spinning to create artificial gravity on the inside. They would be constructed from material mined from the moon and delivered into space using enormous electromagnetic catapults. Mirrors would pipe in the sunshine, and solar panels would provide continuous electricity. As the population expands, people would simply build new cylinders to accommodate their needs.” This can be approached step-by-step by starting with the private space stations or hotels and then gradually creating larger space stations until we can reach the necessary size. However, we should consider both Moon and Mars projects and how they will influence it. First of all, we will have reusable rockets that would eventually establish a logistical route between Earth and other planets. Another part would be the necessary establishment of industrial and greenhouse facilities on both Moon and Mars as the colonies there grow and develop. Thus, we would have some basic infrastructure at that point. Coupled with that we will certainly have several space stations at Earth and Moon orbits. Those would be the main points for all space travel in our solar system (you can read more on the subject and on developing sustainable colonies and resource extraction on the Moon in my previous article: https://astara-store.com/blogs/default-blog/moon-settlements-how-and-when-will-we-be-able-to-build-them). On top of all that we will certainly have fusion-powered rockets sooner or later. Therefore as both lunar and Martian projects develop and reach a large enough scale we will have a basic foundation for approaching the interstellar space travel. Moreover, we would have some progress in developing more exotic propulsion systems that would definitely coincide with the need to launch multiple missions to Mars and even farther planets within our solar system. Making the travel time both faster and more efficient. As previously mentioned it would be great to be at the stage of applicable terraforming technologies as well. However, we will definitely have several initial approaches tested on Moon and Mars by that time and will have at least something. Those would be applicable to distant planets in other solar systems as well as for the purpose of recreating the Earth-like environment on the large interstellar space stations themselves.
On top of that, we would have to make a certain launch for exoplanets. The most reasonable approach would be to create several such large starships and send them to several planets. As mentioned previously it would important to equip them with the necessary equipment for terraforming. Which would start with recreating the breathable atmosphere, sufficient water supplies and everything else for the sake of recreating the environment that would resemble the Earth as close as possible. Otherwise, the spaceships should have everything to provide a wholesome life for their citizens while they drift through space in search of new planets. The colonies would use solar power and for electrical energy for growing crops and having the most complete flora and fauna, coupled with transparent walls with mirrors aiming sunlight through its walls and the day-night that we would need in general. The structure consists of two cylinders rotating in opposite directions on a bearing to mitigate the gyroscopic effect. Each cylinder proposed to be 20 miles long and 5 miles in diameter. Each divided into 6 large sections-stripes, 3 for windows and 3 for habitable zones. The industrial and recreational facilities will be placed on the central axis in a zero-gravity environment. As those two giant cylinders rotate around their axis they would bring into action the centripetal force of any object in the inner surface and create artificial gravity. The acceleration equation would be: a=v2/r and it would effectively substitute the acceleration value on Earth. Given that it would rotate roughly 28 times per hour to simulate an appropriate gravitational force. In addition to acceleration, the cylinder is designed with a ratio of gases similar to what can be found on Earth. Though the pressure would be half of that at sea level. This wouldn’t impact our breathing substantially but it would bring down the need for gas and the thick construction walls. The proposed cylinder would also have provisions wherein the habitat would be able to control its own micro-climate through the arrangement of mirrors and by altering the ratio of gases in the cylinder. The cylinder would have to become a huge thermos if we are to survive the cold space between stars. That’s the part where the mirrors that capture the sunlight would play their role, enhancing the amount of sunlight that we would receive.
It can be concluded that there is a humongous potential for space travel and we would definitely see considerable progress in spacefaring technologies the farther current space programs develop. We will quite certainly see the fusion drives in the near future. And with a dedicated amount of resources and scientific thought dedicated to the subject we should be able to start approaching the more extraordinary methods of space transportation. The most interesting point to look forward to after we make the initial colonies on the Moon and Mars would be the establishment of extraterrestrial industrial complexes. Those would be the first step towards interstellar exploration. With all likelihood, we will see some considerable progress in the next 50 years. Coupled with the greenhouses established on other planets and our first attempts at terraforming we should have a more defined vision of how we can achieve it.