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Interstellar Flight: Fanciful Ideas, Realistic Options
Written by Paul Gilster   

It is a great pleasure for me to introduce here in The PI Club the public lecture given by my friend Paul Gilster in the City Hall of Aosta during the Sixth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, Missions to the Outer Solar System and Beyond. In the evening of the 8th July 2009 Paul painted a colourful fresco of one of the boldest human endeavours - the flight to the stars. I feel lucky and honoured that I was allowed  to be there and could listen  to Paul's vivid, wonderful  talk. Enjoy!

Tibor Pacher 

Paul - on the left - at the beginning of his speech. Next to him is Giancarlo Genta, who organized the conference and on the right is Guido Cossard, assessore of cultural affairs for the town of Aosta. Photo Credit: Raffaele Antonio Tavani
The following talk, which I delivered in Aosta, Italy, had as its primary purpose to acquaint an audience of non-scientists with the fact that while interstellar flight is the greatest challenge humanity will ever face, surmounting these distances can be done without breaking any laws of physics. We all hope for breakthroughs that will give us fast access to the stars, but if these do not arise, we can still hope for advances in engineering that may make much slower missions a possibility. Migrating from colossal engineering concepts to smaller, more workable designs continues to occupy interstellar theorists worldwide.
Are there realistic options for going to the stars? The answer is yes if we understand that by ‘realistic,’ we mean methods that violate no laws of physics. The challenge is extreme but surmountable. We have gone to the edge of the Solar System, and in doing so we have moved out about one hundred times the distance between Earth and the Sun. That has taken our Voyager spacecraft over thirty years to achieve. The nearest star, by contrast, is 277,000 times as far as the Earth is from the Sun, a staggering thirty nine trillion nine hundred billion kilometers from home. That star is Alpha Centauri, which is actually not one but three stars whose combined light seems to form a single object as seen from Earth. There is an active hunt for planets there.
Interstellar travel dwarfs our notions of time and distance. It takes light 4.3 years to travel from Alpha Centauri to us – these stars are, in other words, 4.3 light years away. If Voyager 1 were on a course for Alpha Centauri, it would not arrive for 73,000 years. Even so, the concept of a mission to the stars, once considered nonsense, has begun to take on definition. It is important to realize that science builds incrementally, one step at a time, and that by extending our existing methods and creating new techniques that do not violate the laws of physics, we are making progress toward interstellar flight.
We have learned that chemical rockets are not up to this challenge. This is because it takes a lot of fuel to get to the stars. And if we keep adding propellant to push still more propellant, the ratio of fuel to payload simply goes off the chart. The Apollo moon mission used a chemical rocket that weighed 600 times more when fueled than it did when empty. If we wanted to reach Alpha Centauri with a spacecraft the size of the Space Shuttle, we would need an amount of fuel that is larger than the amount of mass in the entire universe! And that is just for a flyby mission, no stopping, that takes fully 900 years to get there.
We must consider what we can do instead of chemical rockets. The first thing that comes to mind is nuclear energy, and here we turn to Project Orion, a spacecraft that would explode nuclear devices behind the vehicle. Developed in the midst of the Cold War, Orion was to use the shock wave from these explosions, which would strike a pusher plate to provide momentum to the vehicle. There are those who believe that Orion remains the only workable concept for interstellar flight in our current bag of tools. As analyzed by physicist Freeman Dyson, who himself worked on the design, an upgraded Orion might reach 10,000 kilometers per second, making for a 120-year journey to Alpha Centauri.
Orion grew out of work at Los Alamos National Laboratory and experiments conducted at the Eniwetok nuclear test facility in the Marshall Islands. Remarkably, the team found that ordinary metals could withstand the high surface temperatures caused by nuclear explosions. This method became known as ‘nuclear pulse propulsion,’ and a miniature Orion was test flown with chemical explosives in late 1959, demonstrating that pulse propulsion could power a vehicle that remained stable while in flight.
The key here is temperature. A chemical rocket ignites fuel inside the vehicle, so that temperature places limits on how powerful the engine can be. Orion reaches incredible temperatures – up to 66,000 degrees Celsius -- but the temperatures are immediately dissipated because they occur outside the spacecraft. The gases from the explosion are only in contact with the pusher plate for a fraction of a second. The beauty of Orion was that the bigger the model, the more efficient the system. Orion promised vehicles carrying crews of a hundred, launched directly from Earth. The Orion team even conceived a mission to Saturn, with the expectation that it could fly as early as 1968.
Needless to say, that mission never happened because of the Test Ban Treaty in 1963 and continuing concerns about nuclear proliferation. Today, much of this work remains classified. But the design is fascinating when pushed to its limits. One proposal called for a colony ship, an interstellar craft that would use a copper pusher plate twenty kilometers in diameter. Fifteen billion kilograms of deuterium (a form of hydrogen) would go into the production of thirty million nuclear bombs, each of which would explode 120 kilometers behind the vehicle at intervals of 1,000 seconds. The total acceleration time would be five hundred years, with the same for deceleration. Super-Orion would carry 20,000 Earth settlers to Alpha Centauri on a flight lasting 1800 years. This is a ‘generation ship,’ in which the descendants of the original crew are the ones who actually arrive at the destination.
Orion is a propulsion system that could be built without violating any laws of physics. But other projects brought forth ideas that, although they strain our engineering, also violate no physical laws and offer solutions for coming generations. One of these was the Project Daedalus starship, designed by members of the British Interplanetary Society in a less than intimidating setting – London’s Mason’s Arms pub -- in the 1970s.
While the venue was unlikely, the working team was serious, and Daedalus remains the only complete study of an interstellar spacecraft ever compiled. The mammoth report, published in the Society’s journal, examines a fifty-year mission to the red dwarf Barnard’s Star some 5.9 light years from the Sun. At the time, there were thought to be planets there. Daedalus was to be a flyby mission that would make a swift pass through the destination system, reaching a velocity of 36,000 kilometers per second in its cruise to the star. For propulsion, Daedalus would use fusion, the process that powers the Sun as hydrogen is converted to helium and huge energies are released.
The Daedalus designers planned to use deuterium and a form of helium called helium-3 for their fuel pellets. Each of these would be bombarded by electron beams in a combustion chamber to create a series of micro-explosions. Helium–3 is a form of helium with a nucleus of two protons and one neutron. While only hundreds of pounds of helium-3 can be found on Earth, the Daedalus designers knew they would need huge amounts of this material, for theirs would be a two-stage mission – think of rockets separating as they go into orbit, one stage dropping away as another begins firing. With Daedalus, the first stage would burn for two entire years, the second for almost as long, before shutting down to coast to destination. The plan for fuel acquisition was to mine the atmosphere of Jupiter, where helium-3 can be found in abundance. As you can see, building the Daedalus starship assumes that we have built a space-based infrastructure that is capable of such feats.
Daedalus comes from the era of mammoth starship thinking. The probe’s earliest designs called for a 450 ton probe with a reaction chamber some 100 meters in diameter; even with scaling back to the multi-stage design, the entire mission demanded fifty billion fuel pellets, requiring 30,000 tons of helium-3 and 20,000 tons of deuterium. Daedalus was well beyond our means and obsolete before ever being built. But it has energized further study, including a soon to begin update called Project Icarus that will re-examine its basic technologies.
But perhaps there are more exotic technologies for getting us to the stars. Both in Europe and the United States, researchers have actively studied the so-called ‘solar sail’ concept, one that would use the momentum imparted by light from the Sun to drive the vessel. While photons have no mass, they do impart momentum. Their push is so slight that at Earth’s distance from the Sun, their pressure is ten thousand times weaker than the force of the wind at Earth’s surface. But because they hit the sail continuously, a constant pressure is exerted and thus a constant acceleration on the spacecraft. Each tiny push mounts up. A solar sail could be attached to a payload and carry its cargo outward from the Sun or, tacking like a sailboat, move back and forth between the planets.
Notice the beauty of this concept. To get around the problems of chemical rockets, we simply leave the fuel at home and fly a spacecraft using a natural propellant. Clearly, to operate efficiently, a solar sail must collect as much sunlight as possible. Thus we need a large sail, and we also need a lightweight one that can more readily respond to the momentum imparted to it by the photons. Sails must also be durable enough to stand up to temperature changes, micrometeorite hits and other hazards of space. Experimental sails under study have been made of thin, metal-coated plastics called mylar and kapton.
Considerable work on solar sails and related technology occurred here in Italy during the 1980s and 1990s, beginning with research at Alenia Spazio in Turin, which was interested in developing an inflatable radio telescope called Quasat. Regular meetings in the following decade spun off sail variants like SETIsail and ASTROsail, mission concepts that would listen for extraterrestrial radio signals or make astrophysical studies in deep space. The latter two merged in a concept called FOCAL, which would use sail technology to reach the Sun’s gravitational focus. This is the place at which light from objects directly behind the Sun is bent by the Sun’s gravity in such a way as to become highly magnified, allowing extraordinary opportunities for studying distant objects. This ‘gravity lens’ is available to us if we can push about 550 times as far from the Sun as the Earth. Unlike an optical lens, the gravity lens persists beyond that distance, so that a probe will be able to observe its target on the other side of the Sun for a lengthy period of time. The complex optical demands of the FOCAL mission are under active investigation by physicist Claudio Maccone.
A 1996 meeting of the International Academy of Astronautics in Turin presented the Aurora Project, a probe to the edge of the Solar System. This mission would use a 150 kg probe with a solar sail, boosted by a close approach to the Sun to reach a speed roughly three times that of Voyager 1. Giancarlo Genta, who organized the ongoing space conference here in Aosta and who kindly translates my remarks this evening, did an impressive structural analysis of this sail. In Europe and the United States, only funding holds us back. We are close to launching our first sails, and Italy can be proud of its contribution to this work.
We can envision solar sails operating in the inner Solar System, but after we go beyond the orbit of Jupiter, the force imparted to the sail has dropped drastically. But there is a way to push a sail to interstellar space by using a so-called ‘Sundiver’ maneuver. Here the sail is furled and hidden behind a small asteroid that is used as a heat shield. The spacecraft/asteroid combination swings to within less than 800,000 kilometers from the surface of the Sun, at which point of closest approach the sail is brought from behind the asteroid, unfurled, and allowed to receive the full brunt of the Sun’s abundant photons. Accelerations would be huge, but solar sail expert Gregory Matloff has shown that designs like these could reach Alpha Centauri in a thousand years. That is sufficient for a so-called ‘worldship’ design, but far longer than the fifty years we’d like to see as the upper limit on such missions.
But what other ways can sails get us to the stars? With the right kind of equipment, the loss of the Sun’s photons need not be a problem. In the future, we may use a laser beam to drive a sail that would be hundreds of kilometers in diameter. The laser beam would be focused by a thousand-kilometer lens -- a lens made out of concentric rings of ultra-thin plastic film alternating with empty rings, built in the outer Solar System between the orbits of Uranus and Neptune. These mission designs, conceived by a physicist named Robert Forward, dwarf our present-day engineering. But Forward discovered that if we could build them, a powerful laser beam could be focused so tightly that it would retain its shape over interstellar distances.
So tight is this beam that at the distance of Alpha Centauri, it is still converging. Such a laser system could be powered by stations in orbit near the Sun, which would fire their beam to the focusing lens in the outer Solar System. The scale of this project is enormous, but we cannot rule out the future use of nanotechnology to build large structures by using legions of tiny ‘assembler’ robots that could mine asteroids for raw materials. Such nanotechnology may be available within two decades.
If we could build such things, the kind of missions that could be flown with them are remarkable. Imagine a thousand-kilogram payload attached to a 3.6 kilometer sail that could attain speeds of one-tenth the speed of light. The probe would require roughly forty years to reach the Centauri system, with 4.3 additional years needed for data return. Another Forward mission involved a staged sail, a 100-kilometer object that would separate so that the outer ring of the sail could reflect light back to the inner ring upon approach, slowing the payload so that it could stop in the destination system. The most exotic of all Forward missions was a manned flight with return capability to Epsilon Eridani, a star 10.5 light years away. The 1000-kilometer sail would reach half the speed of light. The sail would separate for deceleration in the target system, and after a period of exploration, the crew would then use the smallest, inner sail section, letting laser light from Earth (turned on a decade earlier) bounce off the middle sail segment to drive the manned payload back to Earth.
Beaming does not have to involve lasers. Another Forward concept was called ‘Starwisp.’ This was no more than a wire mesh a kilometer in diameter weighing some sixteen grams in total, with microchips planted at each intersection of the mesh. So light that it would be invisible to the naked eye, Starwisp would be powered at 115 times Earth gravity by a 10-billion watt microwave beam, reaching one-fifth of lightspeed within scant days of launch. Twenty-one years later, it would pass through the Centauri system, using microwave power to return images of the encounter. Although later studies showed Starwisp to be untenable (it would burn up upon being lit by the beam), the utility of microwave beaming to drive fast spacecraft is still under active investigation. Laboratory experiments have demonstrated that microwave beaming has the power to drive mesh-like sail materials to high velocity.
What happens when someone pushes our engineering beyond its limits is that other scientists go to work looking for ways to make the concept more feasible. While Forward assumed we would make sails out of aluminum, we now look at exotic materials like niobium or beryllium, and transparent films of silicon carbide. Diaphanous sails made of diamond-like carbon (a material closely resembling diamond), manufactured in orbit with a plastic backing to shape the sail, may prove the most effective of all. Because of its high reflectivity and resistance to heat, such a sail could be accelerated to cruise velocity while still close to the laser source, making smaller lens sizes possible, and reducing the size and cost of the sail.
One of the surprises of my early work on interstellar topics was the realization that concepts like these, interstellar missions that violate no physical law, were under active investigation in a sort of sub-culture of space scientists, most of whom worked independently and met only at conferences like the one now being held here in Aosta. Indeed, interstellar ideas now fly fast and furious. Antimatter, for example, is under serious study. The beauty of antimatter is that it reacts so furiously when it encounters normal matter. A single gram of antimatter would release the energy of a 20 kiloton bomb if it met a gram of normal matter. This energy emerges not only as gamma rays but also as short-lived particles called pions, whose path can be curved by sending them through a magnetic field. Thus there are ways to turn pions into an exhaust stream.
Antimatter is normal matter with a reversed electric charge, and although we’d like to explore its possibilities, we don’t know how to produce it in the quantities needed for propulsion. Right now we can produce no more than a nanogram (one billionth of a gram) of antimatter per year in our particle accelerators. But realistic mission designs can nonetheless grow out of these constraints if we extend our technology forward only a few years. Meeting the lack of antimatter squarely, a sail approach developed by Steven Howe, who is head of an antimatter firm called Hbar Technologies, would require only a tiny amount of antimatter that would be released from the spacecraft to induce nuclear reactions in a sail coated with uranium-235. Howe’s immediate goal is a spacecraft that would top 115 kilometers per second. His long-range goal is a much larger sail to Alpha Centauri.
The antimatter fuel Howe’s sail would use comes in the form of frozen pellets of anti-hydrogen, an antimatter atom consisting of an antiproton orbited by an anti-electron, or positron. Howe is currently studying how much antimatter would be needed for a mission to the outer Solar System. A mission beyond Pluto to the Kuiper Belt, a ring of icy bodies orbiting our system, could be flown, Howe believes, with milligrams of antimatter, whereas a scaled-up Alpha Centauri mission would take grams. New facilities and upgrades to our existing particle accelerators, plus continued work on developing storage technologies, could make this a mission that could be flown in this century.
By now you may be asking whether we are abandoning the science of the far future, the kind of propulsion systems that let Star Trek’s Enterprise flit all over the galaxy with ease. No, and in fact a recent book called Frontiers of Propulsion Science, written by many of today’s top researchers, goes into great detail on the ideas at the edges of known physics that could change everything. The Alcubierre ‘warp drive,’ for example, grew out of work in the 1990s by a Mexican physicist who realized that while nothing could move faster than the speed of light through space, there was no limitation on the way space itself could move. If it were possible to compress space in front of a starship while expanding it behind, the vehicle would obey known physics but arrive at its destination as if it had traveled faster than light. A number of high-quality scientific papers have continued to examine this concept.
But the point I am making is that while we study such exotic ideas, we must also build on what we know of physics and consider applying our engineering to ideas that might be tested in the near future, by which I mean within a century. And we must always be on the lookout for ideas that on first glance seem to be impossible, but which when considered in a different light, prove to offer new approaches that could work after all. Such an idea is the Bussard ramjet, a starship that gathers energy by using a huge magnetic ‘scoop’ collecting interstellar hydrogen, and sending that hydrogen to an onboard fusion reactor.
We know that interstellar space, far from being empty, contains appreciable amounts of gas that could serve as raw fuel, if we can figure out how to start a hydrogen fusion reaction aboard the vehicle. The beauty of the Bussard concept is that such a spacecraft becomes more efficient the faster it goes, so that speeds close to the speed of light are not out of the question. Einstein’s theories tell us that speeds like this make time slow down for the crew as compared to those left behind. In fact, the crew of such a ship, as Carl Sagan demonstrated in the 1960s, might travel to the center of the galaxy, some 25,000 light years away, within a single human lifetime, although 25,000 years would have passed back home on Earth.
But when serious engineering studies followed up on Robert Bussard’s suggestion, the ramjet idea fell through. It turns out that magnetic scoops on this order, some of them tens of thousands of kilometers in diameter, would actually induce more drag on the vehicle than the thrust they provided. By studying this effect, researchers have been able to show that a magnetic ‘scoop’ of this kind -- a magnetic ‘sail’ created by a loop of superconducting cable --could actually serve as something equally useful, a brake that could slow down a starship as it approached its destination system by reacting with its solar wind, the stream of particles pushing out from Sun-like stars.
Making progress incrementally – one small step at a time -- is how this work proceeds, as scientists around the globe contribute their insights to the question of interstellar propulsion, usually without formal backing by government space agencies, and often working on their own time at universities, companies and research labs. You may be amazed to know, for example, that in the US, NASA has shut down both its Breakthrough Propulsion Physics project at Glenn Research Center in Cleveland as well as its Institute for Advanced Concepts in Atlanta. In the absence of government funding, be aware that a global team of researchers has now gathered under the name of the Tau Zero Foundation to continue the work that has been done in the past. Leading it is Marc Millis, who until several years ago was the head of the Breakthrough Propulsion Physics project for NASA. My Centauri Dreams site serves as the Foundation’s news forum. The idea of the TZF is to target peer-reviewed research that needs funding to proceed, engaging philanthropists to help pay the bills.
There are many reasons to go to the stars. Species survival demands that we recognize the Solar System to be an extremely dangerous place, filled with comets and asteroids that, on occasion, still strike our planet. An asteroid may well have been responsible for the impact that killed the dinosaurs 65 million years ago, while the explosion of a small asteroid in Siberia in 1908 shows what can happen even in today’s relatively sedate Solar System. Once we have attained the stars, we save the species from the existential threat that life on a single planet around a single star poses, and we open ourselves to a future of renewed growth and evolutionary change in the galaxy.
A final reason? We cannot help ourselves. Most sensible people stayed home when sailing ships first set out for the Antipodes back in the 17th and 18th Century, but there was no shortage of business people, writers, adventurers, criminals and dreamers who proved willing to get on those cramped ships to make the journey. In a similar way, calculates physicist Freeman Dyson, rogue elements of our own population will gradually spread out into the Solar System, the same mix of wanderers and dreamers, perhaps, and one that will set its sights not just on the outer planets but the stars that beckon beyond.
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Last Updated ( Wednesday, 22 July 2009 )
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