Even the most sophisticated of spacecraft can barely reach the edge of the solar ‎system. Could we one day break this boundary and send fellow humans to distant ‎solar systems?‎

Almost six decades since the beginning of the space age, humanity is still like a person stuck indoors at home and just can’t get out. Several hundreds of astronauts emerged onto the porch, in low Earth orbit, and only about 30 were able to go out in the garden - the moon. Of which only 12 actually walked on its face.

A few unmanned spacecraft were able to visit the neighboring houses, such as Mars, Venus, Jupiter and Saturn, and only three unmanned spacecraft traveled distances further down the road: two Voyager spacecraft launched in the late 70s and are at the edge of the solar system, and the New Horizons spacecraft passed Pluto last year, hoping to reach the edge of the solar system at the end of the next decade.

But our solar system is only one of very many. Our galaxy, the Milky Way, contains about 200 billion suns, and we already know many of them probably have planets. Our own Milky Way galaxy is just one of very many: conservative estimates speak of one hundred billion galaxies across the universe. These magnitudes are very difficult to comprehend. The diameter of the Milky Way is about one hundred thousand light years. In other words, if we move at light speed it would take 100 thousand years to cross it from end to end.

The closest sun, Proxima Centauri, is 4.25 light years away from us; if traveling at the speed of light it would take more than four years to get there. Recently a planet was discovered orbiting this sun that is considered “Earth-like”. But even our fastest spacecraft is still far away from traveling at the speed of light. If the New Horizons spacecraft, which is traveling over a million kilometers a day, were trying to reach Proxima Centauri, the journey would take about 70 thousand years!

Can humankind ever get close to traveling at the speed of light? Can we visit other solar systems? The answer to these questions may not only satisfy our curiosity but also save us. After all, Earth will end its life in about five billion years, when the sun swells and swallows it. But long before that it may be in danger of destruction anyway - a collision with a large asteroid or a man-made disaster - might require humankind to find a new home.

In recent decades several technologies have been discussed that may allow us to push the boundaries of space that is closer to us. Some of them are still quite unlikely, while others approach the more practical end of the scale, even if their implementation will require many years of research and development.

## The world’s most expensive material

In 1928, British physicist Paul Dirac predicted the existence of a strange particle, an anti-electron. He developed the equations that describe the behavior of electrons and concluded that they allow the existence of an identical particle, but with a positive rather than negative charge. A few years later it became clear these particles are not only a strange result of a mathematical calculation, but do in fact exist in reality. In fact, there are not only anti-electrons (or positrons), but also anti-protons, and they can assemble complete atoms of antimatter. When an antiparticle meets its corresponding particle, they annihilate each other and make a tremendous amount of energy.

The tremendous energy released by this encounter of matter and antimatter can supposedly propel rockets and spacecraft at incredible speed. Physicists think that just four milligrams of positrons is enough to allow a spacecraft to reach the planet Mars in a few weeks, instead of the current nine months. But achieving such quantities of anti-matter is not a simple matter. Physicists from large particle accelerators such as CERN are capable of producing tens or hundreds of atoms of anti-hydrogen, but this is far from a sufficient quantity for practical applications.

Anti-matter is now considered the world's most expensive product with the price of one gram estimated in the trillions of dollars, although no one can produce a quantity even close to this. The tremendous energy required to produce antimatter has downplayed interest even more. Antimatter engines can become an important tool for powering spacecraft, but only if an effective way to produce it will be found or alternatively, a means to collect antimatter in space.

An enormous amount of energy, but at the cost of trillions of dollars. Simulation of an antimatter rocket | Source: NASA

## Nuclear explosion in space

Another possible way for spacecraft to move vast distances is using nuclear energy. Today, satellites and spacecraft are assisted by nuclear generators to produce electricity. Such devices are activated by radioactive material, mainly plutonium, which creates electricity from the heat generated by its decay. Nuclear generators provide power to operate the spacecraft instruments, particularly in long-term missions and far distances from the sun where there is not enough light for the production of solar energy.

Radioactive generators are designed for prolonged use, but do not produce a thrust that can accelerate a spacecraft. Such power will possibly end in a real nuclear explosion, like that of an atomic bomb. “We could build a spacecraft with a large radiation shield in the back, through which we can throw the bombs. Radiation from the blast would thrust the spacecraft forward, and with numerous such bombs a decent speed can be reached,” says aerospace engineer, Yoav Landsman, and owner of the blog Critical Mass and the chief system engineer for SpaceIL, the organization attempting to land an unmanned Israeli spacecraft on the moon. “This is not a bad alternative use for atomic bombs, and even theoretically possible, but probably not in near future applications, mainly due to safety issues.”

Calculations made in the 50’s and 60’s showed that such nuclear spacecraft can reach one-tenth the speed of light; this speed would allow us to reach Pluto and back within a year. But even at this great speed, it would still take several decades to reach a nearby solar system, and a manned spacecraft would still encounter radiation danger. Initial plans for such power sources ultimately remain on paper mainly because of safety concerns and the ban on nuclear testing.

"Not a bad use for an atomic bomb, but probably not applicable for the time being”. Yoav Landsman | Photograph courtesy of him

## Nuclear fusion reactor and a funnel

The reverse process of nuclear fission, or an atomic explosion, is called nuclear fusion. Instead of using the nuclear breakdown of a big atom like uranium, small atoms like hydrogen are fused to make a slightly bigger atomic nucleus, like helium. This process occurs regularly in the sun, and produces a lot of energy. This process is much safer compared to the fission process because it does not emit radioactive material.

The biggest drawback of nuclear fusion is how much energy is needed to start the process. Hydrogen bombs, which produce one hundred times more nuclear fusion energy than normal atomic bombs, are started by the nuclear fission process, which in turn provides the energy needed for fusion. Scientists dream of cold fusion, which does not require the tremendous heat of the sun or a nuclear bomb, but as such currently remains just a dream.

A nuclear fusion reactor can solve the problem of flight speed in space because it provides even more energy than atomic bombs and with no safety issues. Moreover, space is not entirely empty with quite a few hydrogen atoms floating in it. If we had a way to collect them so as to feed and power the fusion, we would have seemingly endless energy for a very fast journey across space.

Intellectual thinker Robert Bussard calculated that such an engine, known as the Ramjet, with a weight of one thousand tons, will be able to maintain a constant acceleration of 1g – the normal acceleration we feel when the Earth's gravity pulls us and that our bodies are used to. A yearlong flight at such acceleration will bring spacecraft to 77 percent of the speed of light, making interstellar travel possible.

Furthermore, according to Einstein's Theory of Relativity, time moves more slowly as the speed of movement approaches the speed of light. Such rapid travel by spacecraft can theoretically cross the entire visible universe during the lifetime of the crewmembers,  although outside the spacecraft billions of years would have passed.

American physicist Michio Kaku wrote in his book “Physics of the Impossible” (published by Aryeh Nir) that in order to collect enough hydrogen atoms to operate such a spacecraft, the engine would need some sort of funnel with a diameter of 160 km. Theoretically it is possible to assemble such a device in space, where there is zero gravity, in order to produce an engine in space that can take humanity to the edges of the universe without the need for additional fuel. The problem as yet is that we cannot build an effective enough reactor to produce hydrogen fusion without expending more energy than it produces. Without it, energy issues will continue to trouble us on Earth, and flights of enormous distances in huge spacecraft will remain yet a distant dream.

A giant funnel for collecting hydrogen and a nuclear fusion reactor. Simulation of the ramjet | Source: NASA

## Anchor aweigh, hoist the sail

Solar radiation is currently widely used to generate electricity for spacecraft and satellites, but one of the more interesting uses is for rocket thrust in space. Sunlight exerts a small but constant pressure that can apparently be used for the journey through space. This can be done by using some sort of sail, which will reflect the radiation, causing it to be pushed from the sun and onward. The sail should be very light and very long relative to the spacecraft. Also, an effective method to open it in space needs to be found, which is far less trivial than opening it on Earth, in the absence of air.

After several failed attempts, a Japanese spacecraft called IKAROS (short for Interplanetary Kite-craft Accelerated by Radiation Of the Sun) was successfully launched in 2010, the vehicle’s sails were accelerated by solar radiation. The name of course refers to the son of Daedalus in Greek mythology, who built wings to escape from prison, but in this story Icarus was killed when he approached the sun because the wax linking his feathers melted. The spacecraft is powered by sails of 196 square meters (14x14 meters). Some sections change color with the help of a thin layer of LCD, which makes it possible to change the amount of radiation returned, and therefore the path of the spacecraft.

After seven months of flight, the spacecraft passed close to the planet Venus and completed the first inter-planetary flight using a solar sail. Then it went into orbit around the sun and continues to today. Currently NASA and other agencies are experimenting with this technology, especially in small spaceships.

“The transition of solar-powered sails from a small spacecraft to a big manned spacecraft should take into account the mass of the spacecraft. The sail should grow according to how big it is; larger spacecraft would require a huge sail on a scale of a few kilometers, even hundreds of square kilometers. It is a huge challenge,” says Landsman. “On the other hand, the advantage of such a spacecraft is that it does not consume fuel. However, another problem is how to slow such a spacecraft down after it has picked up so much speed, and of course, bring it back home. This is why it may need to harness the radiation of other suns it will visit.”

The spacecraft is steered using the changes in color. Simulation of IKAROS in space | Illustration: Andrzej Mirecki. Source: Wikipedia

## A laser that still does not exist

If radiation can accelerate a spacecraft sail, why settle for solar radiation? An exciting new venture that made headlines this year has launched some relatively small spacecraft by propelling them with a powerful laser beam. “Spaceships the size of a smartphone, with a sail of a few meters and a strong laser beam with a diameter of several meters that focuses on just their own sail, will be able to reach one-fifth the speed of light, and therefore reach the solar system of Proxima Centauri within 20 years,” explains Professor Avi Loeb, Head of the Department of Astronomy at Harvard University and Head of the Advisory Committee for Breakthrough Starshot, a new venture financed by Russian tycoon Yuri Milner, along with partners like Professor Stephen Hawking and Facebook founder, Mark Zuckerberg.

“Currently there is no laser available like this, that produces energy levels similar to those required for launching a spacecraft. This has been made possible thanks to the discovery in recent years of the ability to connect several small rays to one powerful reserve, and thanks to the advancement of miniaturization of electronics,” said Loeb. “Currently chemically-driven spacecraft are launched at a speed of 15-20 kilometers per second, and it has hardly progressed since the beginning of the space age. We want to be a thousand times faster, using a laser thrust on each spacecraft for only a few minutes. This will be a huge leap forward - but is not impossible.”

Milner, the space enthusiast, named the product after the first cosmonaut, Yuri Gagarin, and the first phase has seen an investment of $100 million for feasibility studies that should take several years, with the total cost of the project expected to reach$10 billion. This testing phase includes the examination of the effectiveness of such a system in a vacuum chamber on Earth to see whether the spacecraft can reach the desired speed.

A speed a thousand times greater than with ordinary chemical propulsion. Simulation of the focused laser beams | Source: Starshot Breakthrough

Loeb and Milner, both 54, hope to see their spaceships make it to the Centauri solar system in their lifetimes, despite the twenty-years flight time with the addition of four years for the data signals of the Centauri spacecraft to be sent back through to us. Another limitation of the spacecraft is its inability to return, stop or even to slow down when it reaches its destination. “It just passes next to the solar system and continues on its way,” said Loeb. “But the information that will be sent will open a new window for information about the solar system closest to us.”

Loeb is convinced that small sail spacecraft are the key to studying the universe, and even in our search for a home for humanity in the distant future. “You can explore all sorts of things with telescopes, but there is no substitute for visiting new places, and it can be done with a space sail, just as Columbus did in a sailing ship in discovering the New World. The spacecraft will better enable us not only to reach other solar systems, but also to better explore our solar system. We can send spacecraft to the moons of Jupiter within less than a day and Pluto in three days - today such tasks require many years of flight.”

Maybe sail spacecraft will open up the universe, but the possibility that people will fly them and leave the solar system is even further still. “To launch people on these flights of many years duration, a spacecraft of thousands of tons is required, as well as a propulsion based on nuclear fusion,” admitted Loeb. “One of the advantages of our system is the tiny size of the vessels, and will be at least fifty or a hundred years before we know if it is possible to develop such technology for manned flights. I assume that people will be able to leave the solar system in the distant future, but it may not be necessary. It might be that technology will allow us to combine human life with robotic components, and instead of sending people to distant places we could our minds over there.”

“A giant leap – but a possible one”. Prof. Avi Loeb | Photograph: Kris Snibbe, Harvard Gazette

## Flying cities

Even if the new technologies are materialized, obviously manned flights covering the expanse of the universe will require huge space ships - a kind of “flying city” where people can conduct a full life for many years. “I don’t think anyone would agree to travel for such a long period, even within a structure like the International Space Station,” Landsman says. “It will take something completely new - giant space ships, which according to our existing technologies, will have to be sent into space in segments and assembled in space. And there may be other methods - such as freezing or anesthetizing of astronauts for most of the flight.”

Whether the passengers will be awake or asleep - spacecraft designers will also have to take into account the effects of space travel on humans, which are not simple. A prolonged stay in conditions of no gravity causes the destruction of muscle, bone and other tissues, even if it is only a few months in space. A solution to a flight spanning years will have to be discovered, for example possibly by creating artificial gravity within the spacecraft. Other problems that may make such a journey into space so difficult include strong radiation that is dangerous both to humans and ship equipment, and meteor impacts or other celestial objects. The solution to these two dangers includes shielding the spacecraft, requiring them to be even more heavy, cumbersome and expensive.

## Continual innovation

Alongside the relatively conventional theories presented here, popping up regularly are new ideas for using energy efficient long-range flights. One of them is electromagnetic propulsion (EM Drive) – this engine requires no fuel and does not emit anything, and yet generates thrust. The idea is based on waves trapped in a closed system that are moving between mirrors without losing energy. Sometimes they will create an interference with photons (particles of light) that come from the outside, which generates a thrust without losing anything in the system.

The idea sounds fanciful because it allegedly violates the law of energy and material conservation, but discoverer Roger Shawyer argues passionately that it does work without violating laws. A few days ago, the American Institute of Aeronautics and Astronautics (AIAA) approved the peer-reviewed research paper on the subject and it should be released at the end of December 2016 in his journal.

Another idea published in recent days came from Canadian entrepreneur Charles Bombardier. He said that because most of the energy on space flight is wasted on acceleration and deceleration, it is possible to build a long train-like spaceship that will fly steadily between Earth and Mars. After launching it will receive an initial acceleration and use the force of gravity coming from the planets it will orbit, reaching a speed of about one percent of the speed of light (about 3,000 kilometers per second), which would enable it to pass the distance between Earth and Mars in two days. It will continue to move rapidly thanks to orbit energy, picking up and dropping off spacecraft and cargo without changing its speed.

Unfortunately, although the idea sounds tempting, Bombardier does not explain how such a space train is controlled. It is also unclear how spacecraft would catch up to the train at such great speeds to ‘hitch a ride’. Without answers to these questions, a space train will remain a fantasy.

## Obeying the laws of physics

If we want to transform the idea of traveling to the distant cosmos to a trivial matter, we must find a way to go faster than the speed of light, which is deemed impossible. Such a concept is based on the idea of distorting space-time; stretching the universe in one place and shrinking it another, or a “wormhole” – a shortcut in space-time that would allow us to pass through to somewhere else in the universe, or even a different universe.

“The idea that such things are possible is only speculation based on Einstein's equations, and not clear if actually possible in practicality, and even more so if it can actually be planned,” said Loeb. “In the meantime, there is no substitute for the movement between two points in a straight line at a speed not exceeding that of light. In principle we can reach the speed of light only by accelerating a spacecraft at 1g for a year. The problem is that current technology requires more fuel than exists in the entire Milky Way.”

Even if we wanted to conduct a less ambitious journey and settle for sending humans to our closest solar system within decades, it is clear that it requires a huge financial investment. “I want to see something like this happen, but I’m not holding out for it,” concludes Landsman. “Maybe it will happen if the small sail spacecraft discover a planet in Proxima Centauri that indicates there is something worth us getting out there personally, to drive the process of such a huge investment.”

So if someone in the Centauri system or anywhere else in the vastness of the universe reads this article, we want to come. Really! We just need a little help with our technology. Or at least a little financial assistance.