Going Interstellar (collection)

FUSION STARSHIPS Dr. Gregory Matloff

Okay, you want to go to the stars! If you are not in too much of a hurry, if you have lots of money and if you’ve got access to solar-system resources, there is a way. If we had to, we could probably manage all this in the not-too-distant future.

We’re talking about nuclear-fusion-propelled starships. A common physics joke goes something like this: “fusion is the energy source of the future and always will be!” But it may be that our first crude terrestrial fusion-power pilot plants will soon be ready. And space applications will inevitably follow.

Fusion will not provide Star-Trek style spacecraft. But it could propel and power robotic probes requiring a century or so to cross the interstellar gulf. Human-occupied ships requiring generations to cross between stars may also be fusion powered.

Although this type of experimental reactor (more properly called “thermonuclear fusion”) is still not on line, the physical basis for it has been around a long time. Humanity’s understanding of thermonuclear fusion (and other nuclear processes) can in fact be traced to Albert Einstein’s Miracle Year of 1905.

Early Fusion History

Few of his contemporaries would have guessed that Albert Einstein would change the world. Working as a Swiss patent clerk, this young German Jew had not distinguished himself in college. Without the help of his wife (also a physicist), Albert might not have completed the studies leading to his bachelor’s degree.

Hardly a man of action, young Albert was a dreamer. After work he would travel by tram to enjoy dinner with friends in local cafes and restaurants. He loved this mode of travel. One day, he daydreamed that the tram was a light beam upon which he was a passenger, looking back at the Earth. Suddenly, in a flash of inspiration, he had it! This was the secret of Special Relativity. For better or for worse, the Atomic Age was born.

For decades, physicists had grappled unsuccessfully with the observationally confirmed fact that the speed of light in vacuum was a constant 186,300 miles per second (300,000 kilometers per second). Even if you observed a laser projected from a starship passing at near-light speed, the velocity of the photons in the beam would still be measured as traveling at 186,300 miles per second.

As a consequence of this inconvenient truth, physicists had to accept the strange aspects of the Lorentz-Fitzgerald Contraction. As you observe a speeding starship fly past, it will be foreshortened or contracted. As its velocity approaches that of light, the Earth-bound observer will see the ship’s mass increase. Even less comprehensible, time on the ship will slow down. It sounds almost like Alice falling into the rabbit hole, or a Timothy Leary-style acid trip!

Today, the Lorentz-Fitzgerald Contraction is a verified aspect of the real world. But in the early twentieth century, it was still a theoretical novelty. And physicists such as Einstein struggled to fit it into their concepts of reality.

Another problem was magnetism. Since James Clerk Maxwell had derived his famous equations around 1870, physicists knew that electricity and magnetism were connected. Although they accepted the fact that electric charges in motion produced the force called magnetism, they wondered how this could be.

From the vantage point of his speeding trolley car, Einstein would form the framework for the solution to both problems. He proposed that time was a fourth dimension like the three familiar dimensions of height, length, and width. Combining the four-dimensional space-time geometry with a constant value for light speed in a vacuum, Einstein theoretically justified both the Lorentz-Fitzgerald Contraction and the existence of magnetism.

The explanation of magnetism was brilliant. Imagine an infinite line of electric charges, each separated from its neighbor by a constant distance. Any electric-field detector will measure a field strength depending on the device’s sensitivity and distance from the nearest charge. Now accelerate the charges up to a fraction of light speed. By the Lorentz-Fitzgerald Contraction, the separation between adjacent charges will decrease. More charges will be within the detector’s range and the measured field strength will increase.

Brilliant as this insight was, it was not enough to ensure Einstein’s future. So he labored to integrate gravity into relativity theory. The resulting theory, dubbed General Relativity, perceives the mass of a gravitating object (such as the Sun) as locally warping the four-dimensional fabric of space-time. Observations of stars near the solar limb during a post-World-War-One solar eclipse confirmed the predictions of general relativity. Einstein would go on to win a Nobel Prize and become a name equated by the general public with genius.

But in the publicity and excitement accompanying Einstein’s meteoric rise, a seemingly minor aspect of special relativity was generally ignored by non-physicists. From the imaginary vantage point of his light-speed trolley car, Einstein considered the total energy of a stationary object on Earth’s surface. Since the object was not moving, it had no kinetic energy (or energy of motion). Since it was at the same level as the Earth-surface reference frame, it had no potential energy (or energy of position). But it did posses “rest energy.” The quantity of rest energy is dependent upon the speed of light in vacuum (c) and the object’s mass (m). Rest energy is defined in that awesome expression:

Rest Energy = mc²

Appearing in a footnote in one of Einstein’s special relativity papers, this definition of rest energy indicated that mass could be converted into energy and energy could be converted into mass. Physicists could no longer talk about the conservation of mass or the conservation of energy, but nature would now conserve “mass-energy.”

Specialists in the 1920s began to utilize mass-energy conversion and conservation in their research. Physical chemists such as Marie and Pierre Curie had pondered the question of how decay particles in radioactive processes obtained their energy. The obvious answer was that a small fraction of the mass of the decaying nucleus was converted into a particle’s kinetic energy.

Astrophysicists such as Sir Arthur Eddington had wondered how the Sun and other stars could maintain stability for the immense durations required by the fossil record. Once again, the answer required mass-energy conversion in the stellar interior.

But could humans ever tame this process or derive benefit from it? The answer came as war clouds were gathering once again in Europe. Fortunately for all of us, the censors in Nazi Germany were not well trained in nuclear physics or appreciative of its potential. As the Second World War approached, a group of German physicists solved the problem of tapping nuclear fission energy—and published their results in the open literature!

In 1938, it was known that one particular isotope of uranium—Uranium 235—was radioactive. When it decays by nuclear fission (splitting), this massive nucleus splits spontaneously into several less massive (daughter) nuclei and fast-moving (thermal) neutrons. It was also known that the fission of this nucleus could be induced by bombarding it with thermal neutrons. In their epochal paper, Otto Hahn, Lisa Meitner and Fritz Strassmann calculated the density of uranium required to trap emitted neutrons within the U-235 sample. The rapid reaction of uranium in the sample would produce enormous energy. It became known as the chain reaction.

Few realized it at the time, but this simple calculation would provide the basis of both the atomic bomb and the fission reactor. One who recognized the potential immediately was our old friend Albert Einstein.

If we could go back in time a few decades to observe any historical event, one choice might be Einstein in his office at the Institute of Advanced Studies opening the German physics journal containing the epochal paper. Perhaps he was wearing his baggy sweater and smoking his pipe as he opened the journal and read the paper. Perhaps he did a few calculations to check the result.

Einstein knew what the Nazis planned. He had been fortunate to escape Europe and had worked to save family members and colleagues. As a non-native English speaker with a good knowledge of German and Yiddish, he may first have dropped the pencil on his desk and removed his glasses. Then he may have muttered “Oy Mein Gott,” as the terrible reality sank in.

An ordinary mortal may have visited a Princeton pub and drunk himself into oblivion. But Einstein was far from ordinary. He crafted a letter describing his concerns and posted it to President Roosevelt.

If one of us writes a concerned letter to the President of the United States (or any other world leader) we might expect a response from a low-level intern. But Roosevelt realized that Einstein was no ordinary mortal. And he knew that war clouds were thickening. He responded by convening a conclave of the best American nuclear experts to check the validity of Einstein’s concerns and the German team’s calculations. The Manhattan Project, which would result in the atomic bombs dropped on Japan in the final days of World War II, had started!

Even Einstein was amazed (and saddened) by the power of his mass-energy footnote. When he was interviewed after the Hiroshima bombing, he implied that perhaps he should have been a plumber!

After the war, nuclear experts in both the US and USSR realized that the atomic bomb—which works by the fission, or splitting, of heavy atomic nuclei—was not the final answer to humanity’s destructive quest. Work would be devoted to the more powerful thermonuclear bomb—which operates by fusing or combining light atomic nuclei in a manner analogous to the Sun.

To date, hydrogen bombs (which can yield thousands of times more energy than the Hiroshima blast) must have a fission trigger. The atomic-bomb trigger is first ignited to raise temperature, pressure and density in the fusion material to levels at which thermonuclear reactions can occur. Although the details of these devices are closely guarded military secrets, it is safe to assume that explosive-fusion reaction schemes involve heavy isotopes of hydrogen, light isotopes of helium, and perhaps lithium and boron.

Before the end of the Cold War (during which thousands of fission and fusion devices were produced) futurists realized that human civilization would ultimately exhaust its fossil-fuel reserves. Perhaps some form of controlled thermonuclear fusion might be the answer to our growing energy needs.

Two basic types of electricity-producing fusion reactors have been proposed and are being researched. One approach uses powerful electric and magnetic fields to confine the plasma (ionized gas) of thermonuclear material. Another major difficulty in achieving controlled thermonuclear fusion is the multi-million-degree temperature at which the reactants must be maintained. Although cleaner (in terms of radioactivity) than the less-powerful fission reactors now in use, currently feasible fusion reactors will also produce some radioactivity.

Confined-fusion technologists use two benchmarks to define their progress. Achievement of “scientific breakeven” would mean that an experimental fusion reactor would produce as much output energy as was used to create the fusion reaction to begin with. “Technological breakeven” means that the energy produced is at least ten times greater than the energy input. At present, experimental confined-fusion reactors operate at about 50% of scientific breakeven. Achievement of technological breakeven will require more time—and money.

Although confined-fusion reactors have promise for terrestrial energy production, inertial fusion might be more useful for in-space propulsion. Inertial fusion reactors operate using small pellets of fusion reactants. These are pelted with electron beams or lasers to raise pellet temperature and density to levels at which thermonuclear reactions can occur. Essentially, an inertial-fusion reactor is a small hydrogen bomb with the fission trigger replaced by electron or laser beams.

An inertial-fusion reactor used to produce terrestrial energy would require considerable shielding to trap the high-energy products of the thermonuclear reactions. But this is less of a problem in space. Since these reaction products largely consist of high-energy electrically charged particles, engineers quickly figured out that they could simply squirt them out the back of the spacecraft as rocket exhaust. Even before Apollo 11 reached the Moon, some scientists realized that inertial-fusion ships might some day reach the stars!

Project Orion—Birth of the Interstellar Dream

Freeman Dyson distrusted bureaucracies. During the Second World War, he worked on crew safety for the British Royal Air Force Bomber Command. Early in the war, he realized that the escape hatches on many British bombers were too small for crewmembers to depart a stricken aircraft while wearing their parachutes. Dyson wrote memo after memo to correct this defect without positive response until late in the war. Embittered, he realized that thousands of brave British airmen must have needlessly perished. He swore that never again would he trust a large bureaucracy to do the right thing. More than anything else, Dyson’s response to his wartime experience helped produce the realization that the stars are not beyond reach.

After the war, when Dyson had moved to the Princeton University Institute of Advanced Study, he mentored Theodore Taylor in his Ph.D. studies. Working on the US atomic bomb project, Taylor had become disillusioned with the effort that went into creating fake cities and nuking them. To him, this was a waste of taxpayer money since the A-bomb, after all, had been “tested” on two very real Japanese cities. Taylor, instead of concentrating on the construction of objects to be destroyed by atomic blasts, asked himself if anything could survive in the hellish vicinity near ground zero.

He designed a pumpkin-sized steel sphere, coated it with graphite, and installed it at the Eniwetok nuclear test site in the Pacific near a 20-kiloton nuclear device. To everyone’s surprise (but perhaps not Taylor’s), the metal sphere rode out the blast with minimal damage. Apparently, the graphite layer had ablated — evaporating at high speed — and carried off much of the incident energy produced by the explosion.

Dyson, Taylor and others saw a possible application for this process. As the Space Age dawned, US defense analysts recognized that there was no known defense against orbital Soviet nuclear warheads. But perhaps a spacecraft propelled by external nuclear explosions might do the trick.

This was the birth of the initially top-secret Project Orion. On a future spacecraft, Orion crews would carry with them small nuclear charges. (Okay, they would be small bombs.) The charges would be discharged on command behind a pusher plate coated with ablative material. This pusher plate, which would be impacted by the nuclear blast, would be connected to the rest of the ship by the world’s largest shock absorbers. Bang! Bang! Bang! Explosion after explosion would impulsively propel spacecraft to faster and faster speeds.

Although a full-scale Orion was never constructed, small test models propelled by chemical explosives were successfully filmed careening across the sky. One is on display (near a model of Star Trek’s Starship Enterprise) in the Smithsonian Air and Space Museum in Washington D.C.

As the Project Orion study continued, it became evident that Orion “interceptors” could be capable of velocities in excess of 30 miles (50 kilometers) per second. Some conceptual versions could lift from Earth under their own nuclear drive, unfortunately leaving behind a huge wake of radioactive particles. Variants might ride as the second stage of a Saturn V rocket, exhausting their A-bombs well above Earth’s delicate biosphere.

The high time for Project Orion was in 1961-1963. NASA had been commissioned by President Kennedy to deliver and return humans from the Moon before 1970. Most analysts preferred the Saturn V booster to launch the Moon ships, but this rocket had not yet been tested. So a number of back-ups were suggested. One was Orion.

In this heady period, Dyson, Taylor and their associates investigated the interplanetary potential of Orion. As a Saturn V upper stage, it had the potential of ferrying astronauts to Mars on month-long journeys. Habitats, rovers, greenhouses and livestock could come along as well.

But alas, it was not to be. The Atmospheric Test Ban Treaty dampened the prospects for Orion. And the success of Saturn V doomed it. Before the first Lunar Modules swooped down over the lunar plains, Orion and its extensive documentation seemed headed for storage in some super-secret government depository, perhaps located next to the box containing Indiana Jones’s Ark of the Covenant.

Freeman Dyson was angry. And Freeman Dyson distrusted large government bureaucracies. So he methodically hatched a scheme to save Project Orion from oblivion.

Being a physicist, Dyson planned to publish a paper describing the potential of Orion in a journal. But most physics, astronomy, and astronautics journals have circulations of only a few thousand. He chose to publish in Physics Today, a semi-popular monthly organ of The American Institute of Physics. Many public and university libraries subscribe to this magazine—its monthly readership would therefore be much larger than that of more technical physics journals. Dyson planned a paper that would outline the concept of Orion in visionary terms, and do so in a manner that would not violate his oath of secrecy.

Of course he had to use clever approximations. One was the yield in equivalent megatons of TNT of a deuterium-fueled thermonuclear explosive. Dyson knew that the USSR had just air tested the largest H-bomb ever exploded. The yield of the test was well established and the type of aircraft carrying the device had been announced. Dyson probably could have exactly stated the yield of a fusion explosive—instead, he consulted a standard reference (Jane’s All the World’s Aircraft) and used the payload capacity of the Soviet bomber.

Published in late 1968, Dyson’s paper established him as an early hero of the “Interstellar Movement.” Even with his many approximations, he demonstrated that huge, multi-kilometer fusion-pulse world ships could be constructed that would take up to one thousand years to reach the nearest stars. If the entire US/USSR 1968-vintage thermonuclear arsenals had been devoted to Project Orion, as many as 20,000 people could have been relocated to the Alpha / Proxima Centauri system. What a happy use for the bombs!

Projects Daedalus and Icarus—The BIS follows Up

Now that Dyson and Taylor had opened the “Interstellar Door,” other groups began their own studies. The British Interplanetary Society (BIS), which had studied Moon flight decades before the Apollo Project, was ideally situated to conduct a follow-on study to Orion. British researchers Alan Bond and Anthony Martin directed this study, dubbed Project Daedalus, during the 1970’s. The original Daedalus, a mythological Athenian architect, had escaped imprisonment in Crete with the aid of flapping wings handily crafted from goose feathers.

It was soon determined that the modern Daedalus, although inspired by the Orion conceptual breakthrough, would be a bit different. Several problems were acknowledged with the Orion concept. One was scale—an Orion starship (such as the pulsed thermonuclear rocket shown schematically in Figure 2) would be huge even if its payload were small. This was due to the size of the equipment necessary to deflect the copious particles emitted by even a small thermonuclear blast. Another issue was psychological—how would the crew and passengers of a starship react to a megaton-sized explosion going off every few seconds, at a distance of only a kilometer or so? Finally, it is difficult to conceive of any real-world scenario in which nuclear superpowers would allow use of their arsenals in such a constructive endeavor.

Figure 2. Artist concept of a Project Orion nuclear pulse spacecraft. (Image courtesy of NASA.)

Daedalus evolved as a kid brother to Orion. Instead of using the dramatic thermonuclear-pulse drive, it used a somewhat tamer approach—inertial fusion. Small micropellets of fusion fuel were to be ejected into a combustion chamber equipped with strong magnetic fields. Instead of ignition by a fission trigger, these pellets were to be heated to fusion temperatures and condensed to fusion densities by an array of focused laser or electron beams.

Researchers involved in the effort spent a good deal of time considering fusion fuel cycles. They rejected the deuterium-tritium (D-T) and deuterium-deuterium (D-D) fusion reactions under active consideration for terrestrial energy production. Although cleaner than fission, the copious thermal neutrons produced by these reactions would rapidly irradiate the spacecraft. Instead, they settled on a reaction between a low-mass form of helium (Helium-3) and deuterium. The products of this reaction are electrically charged particles—these are relatively easy to focus and expel with the aid of powerful magnetic fields.

Although the Helium-3/D reaction is the second easiest to ignite after D-T, it has one significant drawback. Helium-3 is very, very rare in the terrestrial environment. Starship designers were faced with four alternatives to obtain the necessary tens of millions of kilograms of this substance.

1. They could pepper the surface of the Earth or Moon with breeder reactors, which produce more nuclear fuel than they consume to produce it.2. Since Helium-3 is a trace component of the solar wind of ions ejected from our Sun, some form of superconducting electromagnetic scoop could mine the solar wind for this isotope—but high temperatures in the inner solar system might render superconducting scoops difficult to build and maintain.3. Tiny amounts of He-3 had been deposited in the upper layers of lunar soil as evidenced by samples returned by Apollo astronauts—but at that time nobody knew how the He-3 concentration varied with depth in lunar soils and how feasible lunar mining might actually be.4. What they opted for was the fourth alternative: He-3 is found in the atmospheres of giant planets. Perhaps a series of robotic helium mines suspended by balloons in the upper atmosphere of Jupiter would be the answer.

Although the Daedalus engine could in concept be used to accelerate and decelerate a “thousand-year ark,” the initial application was expected to be robotic probes that could be accelerated to about 10% the speed of light (0.1c) and then fly through the destination star system. In the 1970s, it was (erroneously) suspected that the second nearest star—a red dwarf called Barnard’s star at a distance of about six light years from the Sun—had Jupiter-sized planets. So Barnard’s Star was selected for the hypothetical star mission.

Project Daedalus resulted in and inspired many papers published in dedicated issues of JBIS (The Journal of the British Interplanetary Society). In 2010, a follow-up BIS study called Project Icarus (after the son of mythological Daedalus who approached the Sun too closely and fell to his death in the Aegean) commenced.

Directed by another British researcher, Kelvin Long, Project Icarus aims to continue and update the Daedalus study. The target star is currently Alpha/Proxima Centauri. Not only is this the nearest star system to our Sun at a distance of about 4.3 light years (roughly 40 trillion miles) but the two central Centauri stars are sun-like and separated enough that multiple terrestrial planets may exist in stable orbits.

It is acknowledged that using fusion rockets to accelerate to and decelerate from 0.1c will require an enormous amount of fuel, but an un-decelerated probe that crosses the interstellar void in 50 years and then flies through the destination star system in just a few hours is not acceptable. It would be difficult to justify the expense and the effort for only a few hours worth of data. So Icarus researchers are considering non-rocket deceleration techniques. Approaches include reflecting the very tenuous interstellar plasma and/or the stellar wind(s) of the destination star(s) and using a light sail directed towards the destination star for terminal deceleration.

To again interject physics humor: the rest is simply a matter of engineering….

The Fusion Ramjet

Robert Bussard contributed to many aspects of fusion research. But when he finally achieved his fifteen minutes of fame in an episode of Star Trek: The Next Generation, his surname was pronounced “Buzzard.” What a pity for a true space visionary!

Bussard’s most famous contribution to the study of thermonuclear propulsion in space is the interstellar ramjet, which he considered in 1960. Although the Bussard interstellar ramjet may never be technologically feasible, it does represent one of the very few physically possible modes of interstellar transport that could be capable of near-light speed velocities.

In its pure form (Figure 3), the interstellar ramjet is both simple and elegant. Ahead of the spacecraft, some form of scoop projects an electromagnetic field with a diameter measured in thousands of kilometers. Interstellar protons and electrons, called a “plasma,” are directed towards the scoop by the specially tailored electromagnetic field. The plasma enters the ship and is directed to a fusion reactor at its core. Inside this reactor, plasma density and temperature are high enough to fuse protons and produce helium and energy. The energized helium exhaust is expelled from the rear of the spacecraft. As with any rocket, the reaction to the exhaust accelerates the spacecraft forward.

Figure 3. The Bussard interstellar ramjet would use interstellar hydrogen scooped from deep space propellant mass. (Image courtesy of NASA.)

The interstellar ramjet requires no on-board fuel. Both energy and reaction mass come from the local interstellar medium. In their epochal and very popular book Intelligent Life in the Universe (Holden-Day, San Francisco, 1966), the American astronomer Carl Sagan and his Russian co-author I. S. Shklovskii demonstrated the awesome potential of an ideal interstellar ramjet by showing how it could accelerate to nearly the speed of light—and cross the universe within the lifetime of the on-board crew (while, with thanks again to Special Relativity, billions of years elapsed on Earth).

One advantage of the ramjet is shielding from interstellar dust. Although micron-sized interstellar dust grains are very rare in the local interstellar medium, dust impacts at near the speed of light would have an effect worse than a stationary ship being impacted by multiple shotgun blasts. Such impacts may limit non-ramjet interstellar cruise velocities to a few percent of the speed of light. But since the flow of collected protons deep within the ship’s electromagnetic scoop field will collide with and atomize the fragile dust grains, ramjets will not be so limited in terms of cruise velocity.

A number of science-fiction authors have featured the interstellar ramjet in their stories. Perhaps most notably, the ramjet appeared more than once in Larry Niven’s Tales of Known Space, and propelled the crew of Leonora Christine on an impossible but entrancing voyage in Poul Anderson’s Tau Zero.

But, alas, rigorous scientific skeptics began to chip away at this most exciting concept. It was found that most electromagnetic scoops are efficient drag brakes, reflecting interstellar ions rather than collecting them. But this was not the most serious problem—by the mid-1970’s few believed that human technology could ever tame the proton-proton thermonuclear reaction. Even the catalytic carbon cycle may forever be beyond our capabilities.

There is a way around this, but it does not seem practical for high-speed flight. As tabulated by Eugene Mallove and Gregory Matloff in The Starflight Handbook (Wiley, NY, 1989), both deuterium and Helium-3 exist in the interstellar medium (and the solar wind) at concentrations of a few parts per hundred thousand. If it is possible generate electromagnetic scoop fields hundreds of thousands of kilometers across, collect the fusion fuel from the hydrogen ions, and fuse the deuterium and Helium-3, some form of ramjet might be possible. But it will be a far cry from the dream ships of Bussard, Sagan and Schlovskii, Anderson, and Niven.

Fortunately, the ramjet idea was too attractive to abandon. So a number of less capable alternatives to the proton-fusing ramjet have been proposed. Some of them might just work.