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Starships

 

It was only a few centuries ago that the human race realized those bright lights in the night sky were suns, like our Sun. We then realized that those other suns probably had worlds orbiting around them, some possibly like our world. Since that time, one of the dreams of the human race has been to visit those other worlds in ships that travel between the stars. But as we began to realize the immensity of the vast distances that separate our star from the other stars, we began to despair of ever building a starship using the puny technology that the human race controls. Science fiction writers, in an attempt get their storybook heroes to the stars before the readers got bored, evoked starships with faster-than-light drives, space warps, and other forms of future technology that was indistinguishable from magic. At the same time, the general public evoked fantasy starships in the form of flying saucers flown to the Earth from other stars. These starships were propelled by antigravity or magnetism, and were piloted by benevolent little green men that would save the world from its folly.

If little green men can cross the great gulf, can we?

Yes. It is difficult to go to the stars, but it is not impossible. The stars are far away, and the speed of light limits us to a slow crawl along the starlanes. To travel to the stars will take years of time, gigawatts of power, kilograms of energy, and billions (if not trillions) of dollars. Yet it can be done—if we wish to. And if we decide to go, what kind of starships can we build?

It turns out there are many types of starships possible, each using a different technology. There are some starships that we can build now. For these technologies we know the basic physical principles and have demonstrated the ability to achieve the desired reactions on a laboratory scale. All that is needed for the design, engineering, and construction of the starship is the application of large amounts of money, material, and manpower. There are also some promising starship designs that use future technology that is barely distinguishable from magic. Here we know the basic physical principles, but we have not yet controlled the future technology in the laboratory. Once we have turned that future technology from magic into reality, we can then proceed with starship designs based on those technologies.

It is not easy to comprehend the distances involved in interstellar travel. Of the billions of people living today on this globe, many have never travelled more than 40 kilometers from their place of birth. Of these billions, a few dozen have traveled to the Moon, which at a distance of almost 400,000 kilometers, is ten thousand times 40 kilometers away. Our interplanetary space probes have even gone past Neptune, ten thousand times further out at 4,000,000,000 (4 billion) kilometers. However, the nearest star at 40,000,000,000,000 (40 trillion) kilometers is ten thousand times further than that.

The spacing between stars is so large that there is no standard unit of measurement that gives any sense of the immensity of those distances. Even in terms of the distance between the Earth and the Sun, one astronomical unit or AU, the nearest star is 270,000 AU away. To cut interstellar distances down to size, we use the unit of distance which has the name of a unit of time in it, the lightyear.

A lightyear is the distance that light, traveling at 300,000 kilometers per second or 186,000 miles per second, travels in one year (which is 365.2422 days, 8766 hours, 525,949 minutes, or 31,556,926 seconds), thus, one lightyear equals 9,460,530,000,000 (9.5 trillion) kilometers. It takes light 1.3 seconds to reach the moon, 8.3 minutes to reach the Sun, 4.2 hours to reach Neptune, and 4.3 years to reach the nearest star system.

The nearest star system is called Alpha Centauri. Also known as "Rigil Kent", it is the brightest star in the southern constellation, Centarus, and the third brightest star in the sky after Sirius and Canopus. Alpha Centauri is not a single star, but a collection of three stars. The nearest of those stars is a small red dwarf called Proxima Centauri. The other two stars are a tenth of a lightyear further away and are called Alpha Centauri A and B. Alpha Centauri A is similar to our Sun, while B is slightly redder. These two stars orbit around each other every eighty years, while Proxima circles the pair with a period of millions of years.

To carry out a one-way robotic probe mission to this nearest star system in the lifetime of the humans that launched the probe will require a minimum speed of ten percent of the speed of light. At that speed, it will take 43 years to get to Proxima Centauri and 4.3 years for the radio information to get back to Earth to tell us what the probe found as it zoomed through the three star system at a sizable fraction of light speed.

Much further away in the heavens are some single star systems with stars that are also similar to our Sun. These are felt to be our best candidates for finding an Earth-like planet. They are Epsilon Eridani at eleven lightyears and Tau Ceti at twelve lightyears. To reach these stars in a reasonable time will require starship velocities of thirty percent of the speed of light. At this velocity it will take nearly forty years for the starship to get there, plus another eleven or twelve years for the information to return to Earth.

Yet, although we need to exceed ten percent of light speed to get to any star in a reasonable time, if we can attain a cruise velocity of thirty percent of the speed of light, then there are seventeen star systems with twenty-five visible stars and hundreds of planets within twelve lightyears. This many stars and planets that are reachable with starships limited to less than thirty percent of the speed of light should keep us busy exploring while our future engineers are working on even faster starship designs.

No matter how fast we can make a starship go, we must resign ourselves to the fact that interstellar travel will always take a long time. Even if we had a starship that traveled at the speed of light, it would take over 4.3 years to travel to the nearest star system, then another 4.3 years before a message (or the starship) returns. We don't have speed-of-light starships yet, and won't for a long time. Although time will pass quickly for the crews on relativistic starships, decades and centuries will pass before the stay-at-homes learn what the explorers have found.

Why should we bother going to the stars if it is so difficult? There is one reason that should be obvious to us all. It is built into our genes. We cannot resist it. But it is so selfish, so animal . . . that we often try to ignore it.

We must go to the stars to spawn.

For survival of the human race it is necessary that the human race leave the Earth. Homo sapiens has survived quite nicely on the Earth for tens of thousands and perhaps millions of years. So did the dinosaurs. But the dinosaurs are now extinct. If the human race is to survive, some small portion of it must leave this big blue egg and travel somewhere else to start a new branch of the human race.

For a while, we can escape the death of the Earth from ice ages, meteorite strikes, or other catastrophes by having some of our population in space stations, or by colonizing the other planets around our Sun. But to escape the ultimate death of the Sun, it will be necessary for the human race to establish viable colonies on planets that are around other, younger suns.

Another major reason for interstellar travel would be to find other intelligent lifeforms. Some argue that if the lifeforms on planets around other stars were intelligent, we could communicate with them by radio signals. Not all intelligent lifeforms will have radio, however. It is easy to dream up alien civilizations which are intelligent, have information and technology which would be of value to us, and yet, because of their environment, they can not and will not have radio technology. For example, life could evolve on an ocean-covered world in the form of intelligent whale-like or octopus-like creatures. These beings could be highly advanced in music, mathematics, philosophy, hydrodynamics, acoustics, and biology, but they would have no technology based on fire or electricity.

If there are beings with radio out there, and if they are willing to transmit gigawatts of radio power instead of just listening like we are, and if we listen in the right direction at the right time at the right frequency with the right bandwidth and the right detection scheme, then a radio search for intelligent extraterrestrials will make a significant contribution to our knowledge. However, interstellar exploration with automated probes, although still decades in the future, is definitely more certain to produce a contribution of equivalent value. In my opinion, interstellar exploration with automated probes is complementary to a program to search for extraterrestrial radio signals, rather than competitive with it.

As Arthur Clarke said in his book, The Promise of Space:

"This proxy exploration of the Universe is certainly one way in which it would be possible to gain knowledge of star systems which lack garrulous, radio-equipped inhabitants."

Also, a lifeform does not have to be intelligent to be important. All life on Earth is made of the same stuff: carbon, hydrogen, oxygen, nitrogen, and trace elements organized into specific chemical compounds such as amino acids, sugars, proteins, enzymes, and other standard building blocks of life-as-we-know-it. The discovery of a different form of life will be extremely important.

A different form of life could have a drastically different chemistry, such as compounds based on the element silicon instead of carbon. This would be useful on planets where temperatures are too high for carbon-based lifeforms.

A different form of life might use a different kind of replicating mechanism, such as a tri-string form of DNA that uses two-out-of-three voting when making the new tri-string to block the effects of mutation. This would probably involve three sexes for procreation and would be useful on planets with high radiation environments.

A different form of life might have almost the same biology as Earth life, except one or two of the amino acids used in the DNA genetic code would be different. Or it might be that the compounds are almost exactly the same as those used in Earth lifeforms except that all the compounds are left-handed. (All the organic compounds produced by living organisms on Earth rotate the polarization of light passing through them to the right.)

It could be that the life found on other planets is exactly like ours in chemical structure, indicating a common origin. (This would not mean the alien animals would look anything like Earth animals. A mosquito, sequoia, human, octopus, whale, and duck-billed platypus are quite different in form, yet we all use the same genetic code.) Our search would then turn to find that common origin. The common origin could be due to the inexorable laws of biology, which only allows one type of life to form. The common origin could be due to a version of the panspermia theory, where life travels between stars as spores pushed by light pressure. Or the common origin could even be due to the "garbage" theory, that life on Earth formed from picnic garbage left by long-ago alien visitors to our barren planet. Once we have found a different form of life, our biologists and medical researchers will have their understanding of "life" stretched. They will then, probably, see new ways to understand, control, maintain, and repair our form of life.

The first travelers to the stars will be our robotic probes. They will be small and won't require all the amenities like food, air, and water that humans seem to find necessary. The power levels to send the first squadron of robotic probes out to the stars are within the present reach of the human race. If we wanted to, we could have the first interstellar probe on the way to the nearest star system early in the next millennium.

The design of the first interstellar probe is the critical driving item in any program for interstellar exploration. The rigors and length of a journey involving high accelerations with high energy density engines, the years of bombardment against interstellar matter at high velocities, and the decades of operation with no means for repair, or even diagnostic help from Earth, means that advanced designs for a self-diagnostic, self-repairing probe must be developed. Ultimately, the computer in the interstellar probe will have to exhibit semi-intelligent behavior when presented with new and unforeseen circumstances.

The requirement of multiple planetary exploration at each stellar system will limit the number and weight of the lander probes available and will put a premium on long range sensor capabilities to gather the same data from orbit. Yet, despite these needs for sensor performance, the energy requirements for achieving flight velocities of a few tenths of the speed of light are so large that the weight of the interstellar probe should be kept to a minimum.

What is desired in an interstellar probe is a large physical size (to give the transmitting and receiving apertures desired), and high power (for active sounders and data transmission), all combined with light weight. The design of an interstellar robot probe is a challenge, but an important one. For the mass of the interstellar probe determines the size and power of the propulsion system.

The most advanced form of flight-tested propulsion system we have today is electric propulsion. In this type of propulsion system, some source of energy is used to produce electricity and the electricity is used to expel the reaction mass at high speed to provide thrust. In most electric propulsion systems, the electrical energy is obtained from solar cells that convert sunlight into electricity. Unfortunately for interstellar missions, the light from the Sun rapidly becomes weaker as the spacecraft leaves the solar system, so that solar electric propulsion will not get us to the stars.

One possible method for overcoming the problem of the spreading of sunlight with distance is to consider laser electric propulsion. In this propulsion system, the incoherent sunlight is turned into coherent beams of laser light by using the sunlight to pump the laser. The laser energy is then used to illuminate specially designed solar cells that are optimized to absorb that particular color of laser light with high efficiency. The size of the collector arrays could then be made smaller, resulting in a compact, lightweight, efficient energy power conversion system.

Comparative studies were carried out by engineers of the NASA Jet Propulsion Laboratory on the various forms of electric propulsion that might be used for extrasolar space missions to be launched around the year 2000. These studies indicated that a nuclear fission reactor was a better choice than a laser beam collector for the energy source for an electric propulsion system. A nuclear electric propulsion system would be capable of reaching an escape velocity of 150 kilometers per second or 1/2000th of the speed of light in twelve years, after using up all the fuel in the nuclear reactor. Such a spacecraft would be useful for exploring extrasolar space to search for trans-Plutonian planets and nearby "brown dwarf" stars. However, as the JPL engineers pointed out, at this speed it would take the nuclear fission powered electric propulsion system 10,000 years to reach the nearer stars. Long before it got there, it probably would be passed by a vehicle using a faster propulsion system. Thus, although fission reactors are a presently available source of controlled nuclear energy, because of the large masses needed in the fission reactor and the heat-to-electricity converters, nuclear fission powered electric propulsion systems do not seem to be an adequate form of interstellar propulsion.

A very old design for a nuclear powered interstellar vehicle is one that is propelled by nuclear bombs. Called the "Orion" spacecraft, it was invented in the late fifties at the Los Alamos National Laboratory by an inventor of the hydrogen bomb, Stanislaw Ulam. The original goal of the Orion Project was to send manned spacecraft to Mars and Venus by 1968. Because nuclear fuel is so much more powerful than chemical fuel, an Orion mission to Mars would have cost only a small fraction of the Apollo project, which only took us to the moon.

The Orion vehicle works by ejecting a small nuclear bomb out the rear where the bomblet explodes. The hot plasma from the nuclear explosion strikes a "pusher plate", which absorbs the impulse from the explosion and transfers it through large "shock absorbers" to the main spacecraft. Although it seems amazing that anything could survive a few dozen meters away from a nuclear explosion, a properly designed pusher plate with an ablative surface can stand not one, but many thousands of such nuclear explosions.

Freeman Dyson took these well-engineered ideas for an interplanetary spacecraft and extrapolated them to an interstellar spacecraft. The ship would necessarily be large, with a payload of some 20,000 metric tons (enough to support a small town of many hundred crew members). The total mass would be 400,000 tons, including a fuel supply of 300,000 nuclear bombs weighing about one ton each. (This is approximately the world's supply of nuclear bombs. What an excellent way of disposing of them!) The bombs would be exploded once every three seconds, accelerating the spacecraft at one Earth gravity for ten days to reach a velocity of 1/30th of the speed of light. At this speed, the Orion spacecraft would reach Alpha Centauri in 140 years. To give this ship a deceleration capability at the target star, it would need to be redesigned to have two stages. Although the Orion spacecraft has minimal performance for a starship, it is one form of interstellar transport that could have been built and sent on its way in the last decade.

In addition to nuclear bomb propelled starships, we can envision future nuclear technology which could make possible new kinds of nuclear powered starships. This future technology is called controlled fusion. Although our scientists are sure of the basic physical laws behind controlled fusion, our technologists have yet to demonstrate it in the laboratory. This is one form of future technology that is almost certain to turn from magic into reality within a few decades, since the Department of Energy is spending a large portion of its budget on a number of techniques for achieving a controlled fusion reaction on a scale suitable for use in power plants.

One technique, called magnetic containment fusion, involves the use of magnetic "bottles" to confine a high temperature plasma of deuterium (a hydrogen atom with an extra neutron in the nucleus) and tritium (a hydrogen atom with two extra neutrons in the nucleus) until the two forms of heavy hydrogen fuse together to make a helium nucleus and a high energy neutron.

Another technique, called inertial implosion fusion, attempts to compress tiny pellets of deuterium and tritium by hitting the pellet from all sides with either laser beams, electron beams, ion beams, or beams of high-speed shot. In inertial fusion, the energy and pressure from the incoming beams is supposed to compress the fusion fuel in the pellet, heat it up, and get it to fuse into helium before it has time to expand. It is the "inertia" of the fuel, pellet, and beams that keeps it contained long enough for the reaction to take place.

Once we have achieved controlled fusion in the laboratory, then we can start designing a starship based on those types of fusion techniques that turn out to be feasible. If we achieve controlled fusion by compression and heating of a plasma in a magnetic bottle, then perhaps all we need to do to convert the fusion reactor into a starship rocket engine is to allow the magnetic bottle to "leak" a little bit, and the hot plasma exhaust will produce thrust.

If we achieve controlled fusion by implosion of micropellets with beams of laser light, electrons, ions, or high speed shot, then the same technique can be used to implode the pellets in the throat of a rocket nozzle made of magnetic fields, which will turn the isotropically exploding plasma into directed thrust. Scientists at Lawrence Livermore National Laboratory have already anticipated the success of their laser-imploded fusion program. They have used their fusion plasma computer codes to design a magnetic nozzle for a laser fusion powered rocket for travel in the solar system.

The deuterium-tritium reaction presently being used in both the magnetic containment and inertial implosion fusion research projects involves the use of tritium. Since tritium is a radioactive material with a lifetime of twelve years, any interstellar rocket system using this reaction must have a method of generating the tritium on board. This can be done by capturing the high energy neutrons emitted by the fusion reaction in a blanket of lithium. The neutron causes the lithium to fission into a helium atom and a tritium atom, which can be extracted and used to make more fusion pellets. The weight and efficiency of this auxiliary system, while not serious for an interplanetary spacecraft, can limit the final velocity achievable by a starship.

Alternatively, research on magnetic containment and implosion fusion techniques could produce the higher pressures, temperatures, and densities needed to achieve fusion with other fuel mixtures that don't involve radioactive tritium. These reactors would fuse together fuels such as deuterium and helium-three (a helium nucleus that is missing a neutron), deuterium and deuterium, and protons with protons.

Thus, given some future advances in nuclear technology, we can already envision some propulsion technologies that will get a starship to the stars, although they only travel at one to three percent of the speed of light. At three percent of the speed of light, it would take over 300 years to get to the interesting star systems like Tau Ceti and Epsilon Eridani. These slow travel times for nuclear powered starships are longer than the present lifetime of a human being, so it looks like the travelers on our slow nuclear starships will have to be long-lasting robots rather than ephemeral humans, unless we can come up with some new type of biological technology that will allow a human crew to live longer than our presently allotted three-score-and-ten-year lifetimes. Biologists are presently studying the aging process in cells and multicellular organisms. They are finding that our cells seem to be programmed to stop replicating after a given number of cycles. If they can find the right genetic switch, perhaps they can turn off the aging process and allow us to live the centuries that will be necessary to explore the stars using slow nuclear rockets. With death dead, our only enemies would be accidents and boredom.

Other biologists are studying the process of suspended animation, by either freezing or hibernation. Sperm cells have been kept frozen for decades and are viable enough to produce thoroughbreds and beef cattle when thawed. Even fish and small mammals have been frozen and rethawed. Larger animals, such as people, will be significantly more difficult to freeze because of the need for rapid cooling and thawing to prevent the formation of damaging ice crystals. However, some future technological breakthrough that would now be considered indistinguishable from magic may make possible crews of "corpsicles" who experience the thrilling adventure of exploring one stellar system after another, without having to endure the boring drive through the dark between those adventures.

Scientists studying hibernating animals have found the hormone that initiates hibernation and have used the drug to induce hibernation in other animals. Whether this drug will induce hibernation in humans without causing serious side effects is unknown. Also, it is unknown whether hibernation actually increases lifespan, or just makes living possible when there is insufficient food. Still, there is enough biological research on suspended animation that one of these days we may use that method of keeping a crew alive long enough to carry out century-long exploration missions.

Even if these particular biological technologies do not turn into a real suspended animation capability, there is another method to carry out a slowship mission; let the people die, but allow their children to carry on. A slowship journey to the stars will send a colony of people off in a generation starship. Although only the first generation would be true volunteers, with enough thought and planning we could turn the slow moving starship into a truly acceptable worldship, with all the amenities and few of the problems of living on Earth.

Living on a worldship would be like living in a space colony, except that the colony would have some sort of fusion rocket to push it up to speed and bring it to a halt again at each target star. At each stop, it would refuel its tanks with hydrogen and helium isotopes from a gaseous giant planet like Jupiter. After a few decades of experience in space stations around the Earth, we may have enough confidence in the environmental support systems for a worldship that we could risk sending a colony on an interstellar mission.

The important thing to realize is that our present technology can take us to the stars. To be sure, our first robotic interstellar probes will be slow, consume a lot of power and money, and will return small amounts of data. If we find an interesting planetary system, it will take even more power, and more money, plus biological breakthroughs and crew dedication to take the slowships to the stars. It would be no different in principle than the Mayflower or the ships that colonized the Dutch East Indies. But, no matter how difficult, interstellar travel by slow starships can be done with reasonable extensions of present technology.

There is, however, a fundamental problem with any interstellar mission that travels at speeds less than ten percent the speed of light. For even as a worldship is launched onto its centuries-long journey, propulsion engineers back on Earth will be dreaming about more advanced propulsion systems that can make starships that travel faster than the ship that is leaving. Within twenty or thirty years, those advanced propulsion systems will no longer be a future technology that is indistinguishable from magic, but future reality. After another ten to twenty years, a faster starship will zip past the lumbering worldship, explore the new star system first, then set up a welcoming party for the worldship colonists as they are picked up and brought in by a second wave of fast starships.

Thus, until we run out of ideas for new propulsion systems, it seems to me that no interstellar mission should be launched if it takes more than one hundred years. Instead, the money for the mission should be spent on research to build a faster propulsion system or to find a new propulsion energy source.

In our near future is a magical propulsion energy source that is a thousand times more powerful than nuclear energy. This future source of energy that is nearly indistinguishable from magic is antimatter. As discussed in the chapter "Antimatter", this nearly magical type of matter represents a highly concentrated form of energy with the ability to release "200%" of its mass as energy. When a particle of antimatter, such as an antiproton, is put near a particle of normal matter, such as a proton, the two attract each other and almost instantly annihilate to completely convert all of the mass of both particles into energy. A spacecraft which uses antimatter as its source of propulsion energy could "drive" anywhere in the solar system with mission times ranging from weeks to a few months. A starship using antimatter could travel to the nearest stars in a human lifetime.

Although an antimatter rocket is the ultimate in rockets, it is not necessary to use the rocket principle to build a starship. A rocket consists of a payload, some structure, an energy source, some reaction mass (in most rockets the reaction mass and energy source are combined together into the "fuel"), and an engine that combines the energy with the reaction mass and expels it to provide thrust to the spacecraft. Because a standard rocket has to carry its fuel along with it, its performance is significantly limited. For missions where the final vehicle velocity V is much greater than the exhaust velocity v, the amount of fuel needed rises exponentially as the ratio V/v. Thus, as a practical matter, the final velocity of a rocket is limited to about two or three times the exhaust velocity.

It is possible to conceive of space vehicle designs that do not use the rocket principle and thereby avoid the exponential mass growth implicit in the design of a standard rocket. These are excellent candidates for starships. One example, and a favorite of science fiction writers, is the interstellar ramjet invented in 1960 by Robert Bussard.

The interstellar ramjet starship consists of a payload, a fusion reactor engine, and a large scoop. The interstellar ramjet carries no fuel because it uses the scoop to collect the hydrogen atoms that are known to exist in space. The hydrogen atoms are used as fuel in the fusion reactor, where the fusion energy is released and the energy fed back in some manner into the reaction products (usually helium atoms) which provides the thrust for the vehicle.

Bussard originally estimated that a 1,000 ton vehicle would require a scoop with frontal intake diameter of about one hundred kilometers to achieve a one Earth gravity acceleration through interstellar space with a density of 1000 hydrogen atoms per cubic centimeter. The speed needed for the ramjet to start working is extremely low, so that conventional chemical rockets would suffice to get it started. As the vehicle increases its speed so that it approaches the speed of light, the interstellar fuel flow appears to increase in density due to its relativistic contraction in the space-time reference frame of the vehicle. As a result, the faster the interstellar ramjet goes, the more fuel it collects, so the faster it can go.

If an interstellar ramjet could ever be built, it would have many advantages over other possible starships. Since it never runs out of fuel like fuel-carrying rockets, and never runs away from its source of power like a beamed-power propulsion system, it can accelerate indefinitely. It is the only known starship design that can reach the ultrarelativistic velocities where ship-time becomes orders of magnitude longer than Earth-time. This would allow human crews to travel throughout the galaxy or even between galaxies in a single human lifetime.

A lot of invention and research is needed, however, before the future technology of a Bussard interstellar ramjet becomes distinguishable from magic. We must first achieve controlled fusion. The fusion reactor must not only be light-weight and long-lived, it must be able to fuse protons, not the easier-to-ignite mixture of deuterium and tritium. The reactor must be able to fuse the incoming protons without slowing them down, or the frictional loss of bringing the fuel to a halt, fusing it, and reaccelerating the reaction products will put an undesirable upper limit on the maximum velocity attainable. All of this needed technology is still indistinguishable from magic.

Other versions of the interstellar ramjet concept do not require that the starship carry a fusion reactor. In these concepts, the ramjet scoop is used to scoop up the hydrogen in space. Instead of fusing the hydrogen into helium atoms, however, the hydrogen is heated either by antimatter carried on board, or by antimatter beams or laser beams sent from the solar system.

The major difficulty with any ramjet starship is the design of the scoop, which must be ultra-large and ultra-light. If the interstellar hydrogen were ionized, then a large, super-strong magnet might be sufficient to scoop up the charged protons. Although some stars have clouds of ionized hydrogen near them, most of the hydrogen near the solar system is neutral. Schemes that use laser beams or electron beams to ionize the hydrogen ahead of the ship have been proposed, but they are not light in weight nor low in power consumption.

The present scientific consensus for the composition of the local interstellar medium is that the solar system is embedded not far from the edge of a warm (10,000 degree) mostly neutral gas cloud with a radius of a few tens of lightyears and a relatively low density of 0.1 atoms per cubic centimeter. This is 10,000 times less dense than the 1000 atoms per cubic centimeter that would be preferred for an interstellar ramjet. That means that a scoop for an interstellar ramjet, instead of being 100 kilometers in diameter, would have to be 10,000 kilometers across to scoop up the same amount of fuel per second.

It gets even worse as we travel further from the Sun, for this warm cloud we are in is surrounded by a larger hot (million degree) ionized plasma with a density of only 0.001 ions per cubic centimeter that extends some 150 lightyears or more in all directions observed. This ubiquitous low density hot gas "bubble" is most likely the result of past supernova events.

Thus, for now, in regions near the solar system, the interstellar ramjet remains in the category of magic. The concept of picking up your fuel along the way as you journey through "empty" space is too valuable to be discarded lightly, however, and I hope that future scientists and engineers will keep working away on the remaining problems until this vision of a future technology, which is presently indistinguishable from magic, turns into a real starship.

There is a another whole class of spacecraft that do not have to carry along any energy source or reaction mass, or even an engine. These spacecraft consist only of payload and structure. They work by beamed power propulsion. In a beamed power propulsion system, the heavy parts of a rocket (the reaction mass, the energy source, and the engine) are all kept at home in the solar system. Here, around the Sun, there is an unlimited amount of energy and reaction mass readily available. In addition, the engine can be maintained and even upgraded as the mission proceeds. Three examples of such beamed power propulsion systems have been published in the engineering literature. All of these versions can be built with "reasonable" extrapolations of present day technology. The examples are pellet-stream-pushed, microwave-beam-pushed, and laser-beam-pushed vehicles.

In the pellet-pushed-probe concept proposed by Clifford Singer, small pellets are used to push a starship. The pellets would be launched by a very long linear electromagnetic mass driver that would stretch across the planetary orbits of the solar system and be accurately aimed at the target star. The accelerator would be powered by energy sources using nuclear or solar power. The high speed pellets would be intercepted by the starship and reflected back in the opposite direction, resulting in an increase in velocity of the starship.

The absolute pointing accuracy of the mass launcher is not a serious limitation. A series of correction stations to adjust the trajectory of each pellet could be located downrange from the launcher along the pellet stream. Each station, for example, would be three times farther downrange and would produce one-third as much velocity adjustment. The coarser adjustments could be made electromagnetically or electrostatically, and the finest adjustments could be made remotely by light pressure from a laser or by interaction with a plasma gun or neutral atom stream. The starship would detect the incoming pellet stream and adjust its position to stay in the stream. One readily feasible method for accomplishing the interception of the high speed pellets at the vehicle is to vaporize them into a plasma with a pulse of photons or particles. The high speed ionized plasma would then be reflected from a magnetic field on the starship in a manner somewhat analogous to the expulsion of plasma from a magnetic "nozzle" in a pulsed fusion rocket system.

Extensions of the pellet stream concept include changing the pellet composition and velocity so that the pellets are made of antimatter or fusion fuel that is captured at a low relative velocity, then used in an engine for acceleration and deceleration. Deceleration could also be accomplished by rebounding the pellets from an expendable unmanned lead ship to decelerate the manned vessel at the target system. Of course, once the "interstellar highway" has been traversed once, then a pellet-stream launcher can be constructed at the other end for relatively easy two-way travel.

Another form of beamed power propulsion uses beams of microwaves to drive the starship. Microwave energy has the great advantage that it can be made and transmitted at extremely high efficiencies, although it is difficult to make narrow beams that extend over long distances. Because of the short transmission range, the starship being pushed by the microwave beam must accelerate at a high rate to reach the high velocities needed for interstellar travel before the starship gets too far from the transmitting system. The accelerations required are larger than a human being can stand, so microwave pushed starships seem to be limited to use by robotic probes. There is one design that looks quite promising. I call it Starwisp, because of its extremely small mass.

Starwisp is a light-weight, high-speed interstellar flyby probe pushed by beamed microwaves. [See Figure 6.] The basic structure of the Starwisp robotic starship is a wire mesh sail with microcircuits at the intersection of the wires. The microwave energy to power the starship is generated by a solar powered station orbiting Earth. The microwaves are formed into a beam by a large fresnel-zone-plate lens made of sparse metal mesh rings and empty rings. Such a lens has very low total mass and is easy to construct.

The microwaves in the beam have a wavelength that is much larger than the openings in the wire mesh of the Starwisp starship, so the very lightweight perforated wire mesh looks like a solid sheet of metal to the microwave beam. When the microwave beam strikes the wire mesh, the beam is reflected back in the opposite direction. During the reflection process, the microwave energy gives a push to the wire mesh sail. The amount of push is not large, but if the sail is light and the power in the microwave beam is high, the resultant acceleration of the starship can reach hundreds of times Earth gravity. The high acceleration of the starship by the microwave beam allows Starwisp to reach a coast velocity near that of light while the starship still close to the transmitting lens in the solar system.

 

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Fig. 6 - Starwisp: a microwave-pushed interstellar probe.

 

Prior to the arrival of Starwisp at the target star, the microwave transmitter back in the solar system is turned on again and floods the star system with microwave energy. Using the wires in the mesh as microwave antennas, the microcircuits on Starwisp collect enough energy to power their optical detectors and logic circuits to form images of the planets in the system. The phase of the incoming microwaves is sensed at each point of the mesh and the phase information is used by the microcircuits to form the mesh into a retrodirective phased array microwave antenna that beams a signal back to Earth.

A minimal Starwisp would be a one kilometer mesh sail weighing only sixteen grams and carrying four grams of microcircuits. (The whole spacecraft weighs less than an ounce—you could fold it up and send it through the mail for the cost of first class postage.) This twenty gram starship would be accelerated at 115 times Earth gravity by a ten gigawatt (10,000,000,000 watt) microwave beam, reaching twenty percent of the speed of light in a few days. Upon arrival at Alpha Centauri some twenty years later, Starwisp would collect enough microwave power from the microwave flood beam from the solar system to return a series of high resolution color television pictures during its fly-through of the Alpha Centauri system.

Because of its small mass, the ten gigawatt beamed power level needed to drive a minimal Starwisp is about that planned for the microwave power output of a solar power satellite. Thus, if power satellites are constructed in the next few decades, they could be used to launch a squadron of Starwisp probes to the nearer stars during their "checkout" phase.

Once the Starwisp probes have found interesting planets, then we can use another form of beamed power propulsion to visit those planets, called laser sail propulsion. Although microwave beams can only be used to "push" a robotic spacecraft away from the solar system, if we go to laser wavelengths, then it is possible to design a beamed power propulsion system that can use laser beams from the solar system to push a starship to the nearer stars, and then push the starship and its crew back home again.

Laser sail propulsion is the one method for achieving star travel with human crews that is closest to reality. It will be some time before our engineering capabilities in space will be up to building the laser system needed, but there is no new physics involved, just a large scale engineering extrapolation of known technologies. In laser sail propulsion, light from a powerful laser is bounced off a large reflective sail surrounding the payload. The light pressure from the laser light pushes the sail and payload, providing the needed thrust. The laser sail starship is about as far from a rocket as is possible. The starship consists of nothing but the payload and the lightweight sail structure. The rocket engine of our starship is the laser, powered by an energy source such as the Sun. The reaction mass is the laser light itself.

The sails that the laser craft would use would be advanced versions of the Sun-pushed light sails that have been designed by the NASA Jet Propulsion Laboratory for comet missions and fast trips to the asteroid belt. The lasers would be advanced versions of the high power laser arrays that were studied, built, and tested for the Space Defense Initiative Office of the Department of Defense. The basic physical principles of the lasers and sail are known, all that is left to do is the engineering.

For interplanetary operation and interstellar flight, the lasers would be in near-Earth space and powered by sunlight collected by large reflectors, sending their beams out to push the sails of the interplanetary fleet with the light pressure from their powerful beams. For pushing an interstellar starship, the lasers might work better if they were in orbit around Mercury. There is more sunlight there and the gravity attraction of Mercury would keep them from being "blown" away by the back reaction from their light beams. The lasers would use the abundant sunlight at Mercury's orbit to produce coherent laser light, which would then be combined into a single coherent beam and sent out to a transmitter lens floating between Saturn and Uranus.

The transmitter lens would be a fresnel-zone-plate lens with dimensions tuned to the laser frequency and consisting of wide rings of one-micrometer-thick plastic film alternating with empty rings. The transmitter lens would not be in orbit, but would either be freely falling (very slowly at that distance from the Sun), or "levitated" in place by rockets or by the momentum push from a portion of the laser light passing through it. The lens would be 1000 kilometers in diameter (as big as Texas) and mass about 560,000 tons. A lens this size can send a beam of laser light over forty lightyears before the beam starts to spread.

The first interstellar mission that could be performed with this laser and lens system would be a one-way flyby robotic probe mission to the nearest star system. The robotic probe would have a total mass of one metric ton, about one-third each of payload, support structure, and thin aluminum film reflecting panels. The sail portion of the probe would have a diameter of four kilometers.

The probe would be pushed at an acceleration of three percent of Earth gravity by an array of solar-pumped lasers with a total power of 65,000 megawatts or 65 gigawatts. While this is a great deal of laser power, it is well within our future capabilities. Power levels of this magnitude are generated by the Space Shuttle rocket engines during liftoff, and one of the ways to make a high power laser is to put mirrors across the exhaust of a high power rocket. If the acceleration is maintained for three years, the interstellar probe will reach the velocity of eleven percent of the speed of light at a distance of only one-sixth of a lightyear. At this distance it is still within range of the transmitter lens and all of the laser power is still focused on the sail. The laser is then turned off (or used to launch another robotic probe) and the robotic starship coasts to its target, flying through the Alpha Centauri system forty years after launch.

When I first invented the concept of laser-pushed lightsails back in 1962, I thought it was obvious that since all the laser can do is push the lightsail, it would not be possible to use a solar system laser to stop the lightsail at the target system. The idea seemed to be limited to fly-by precursor robotic probe missions. It wasn't until twenty years later, while trying to find a new way of traveling to the stars for a novel I was writing, I realized that if the lightsail were separated into two parts, then one part could be used as a mirror to reflect the laser light back toward the solar system. That retrodirected light could then be used to decelerate the other portion of the lightsail. When I worked out the equations and put numbers into it, I found that not only was it a good science fiction idea, but it would really work. The concept has since been published as a scientific paper in the Journal of Spacecraft and Rockets, and one of the references to prior work in the scientific paper is my novel, The Flight of the Dragonfly, later reissued by Baen Books in a much expanded version as Rocheworld.

If the reports from the unmanned probes are favorable, then the next phase would be to send a human crew on an interstellar exploration journey. More than just the nearest star system will ultimately need to be explored, so I designed the laser lightsail starship to allow a roundtrip exploration capability out to twelve lightyears, so Tau Ceti or Epsilon Eridani can be visited within a human lifetime. I assumed the diameter of the lightsail at launch to be 1000 kilometers in diameter, the same size as the transmitting lens. The total weight would be 80,000 tons, including 3,000 tons for the crew, their habitat, their supplies, and their exploration vehicles. The lightsail would be built with three stages. There would be a disc-shaped inner "return stage" portion, 100 kilometers in diameter, that would carry the payload and crew, and return them to Earth. This would be surrounded by a ring-shaped "accelerator stage" portion, 320 kilometers in diameter with a 100 kilometer diameter hole. Together, these two sails constitute the "rendezvous stage" that would stop at the target star. This in turn would be surrounded by the "decelerator stage", 1000 kilometers in diameter with a 320 kilometer diameter hole. [See Figure 7.]

All three portions of the lightsail would be accelerated together at thirty percent of Earth gravity by 43,000 terawatts of laser power. At this acceleration, the lightsail would reach a velocity of half the speed of light in 1.6 years. The expedition would reach Epsilon Eridani in twenty years Earth time and seventeen years crew time, and it would be time to stop.

At a half-lightyear from the target star, the 320 kilometer rendezvous stage would be detached from the center of the lightsail and turned to face the large ring-shaped decelerator stage that remains. The laser light coming from the solar system would reflect from the decelerator stage acting as a retro-directive mirror. The reflected light would decelerate the smaller rendezvous sail and bring it to a halt at Epsilon Eridani.

 

067187686407.jpg
Fig. 7 - Roundtrip interstellar travel by laser-pushed lightsails.

 

After the crew explored the system for a few years (using their rendezvous stage lightsail as a solar sail), it would be time to bring them back. To do this, the 100 kilometer diameter return stage would be separated out from the center of the 320 kilometer ring-shaped accelerator stage. The laser light from the solar system would hit the accelerator stage and be reflected back on the return stage. The laser light would then accelerate the return stage and its payload back toward the solar system. As the return stage approached the solar system twenty Earth-years later, it would be brought to a halt by a final burst of laser power. The members of the crew would have been away 51 years (including five years of exploring), have aged 46 years, and would be ready to retire and write their memoirs.

 

It is difficult to go to the stars. But it is not impossible. There are not one, but many, many future technologies, all under intensive development for other purposes, that, if suitably modified and redirected, can give the human race a magic starship that will take us to the stars.

And go we will.

 

Recommended Reading

Robert W. Bussard, "Galactic Matter and Interstellar Flight," Astronautica Acta, Vol. 6, pp. 179 ff (1960).
 

Freeman J. Dyson, "Interstellar Transport," Physics Today, Vol. 21, pp. 41 ff (October 1968).
 

Robert L. Forward, "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails," Journal of Spacecraft and Rockets, Vol. 21, pp. 187-195 (March-April 1984).
 

Robert L. Forward, "Starwisp: An Ultra-Light Interstellar Probe," Journal of Spacecraft and Rockets, Vol. 22, No. 3, pp. 345-350 (1985).
 

Leonard D. Jaffee, et al., "An Interstellar Precursor Mission," Journal of British Interplanetary Society, Vol. 33, pp. 3 ff (1980).
 

Clifford E. Singer, "Interstellar Propulsion Using a Pellet Stream for Momentum Transfer," Journal of British Interplanetary Society, Vol. 33, pp. 107 ff (1980).

 

 

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