Going Interstellar (21 page)

The big helot who had helped Harrod after the whipping stared at the monitors and the course plots with a frown. “I fear for you, Harrod-Lord: how can you be sure you will escape this Ark in time?”

“All is arranged,” he answered. “Now, you must go—and lead your people wisely. And kindly.”

The square-jawed helot frowned even more mightily, but then nodded and left, the other three trailing behind him.

Harrod rolled the ship slightly to port, bringing up the evacuation tubes so that they would fire at an angle, sending the escape pods into a tight cluster of islands in the mid-northern hemisphere. He had just finished calculating the pods’ collective entry angle when the lead helot’s voice boomed from the command suite’s speaker. “We are ready, Harrod-Lord.”

“Very good. Now seal up.”

“And you will be coming down, too?” The voice was worried.

“Yes. I am. I’m coming down, too.” And with that Harrod cut the commlink.

Three minutes later, Harrod discharged the escape pods. Spat free of their keel-lining launch tubes, the pods began their glittering, and ultimately, red-hot arcs down toward Senrefer Tertius Seven. And once the last of them was away, and he saw that the four hundred glowing dots had survived their entry and were now well within the atmosphere, Harrod sul-Mellis angled the great, crippled Ark into a more acute transequatorial trajectory. He checked the sensors: House Shaddock’s away-craft had landed. House Mellis’s bridge module was almost upon them. And as the tattered remains of those two embittered Houses commenced their planetside struggle for dominion, they would certainly not think to look over the shoulder. After all, no threat was expected from that direction.

Consequently, given the opportunity to surprise them both, Harrod pushed the
Photrek Courser
into a steeper descent, watching the blue margin of the atmosphere rise up to meet him as he set his course guidon directly atop the icons denoting the survivors of both Houses.

As he rode the Ark down toward their conjoint landing ground that was, by now, also a killing field, Harrod wondered if this outcome was, in fact, not the best of all possible occurences. With the
Courser
crippled and now plunging to her own death, later generations from this worldlet would have no starship with which to send away yet another wave of bitter, defeated Exiles. This time, descendants of the helots—who were even now emerging from their surf-caught escape pods—would have to learn to settle their differences, find ways to understand and even embrace their enemies, rather than exterminate and banish them.

Or maybe not: he couldn’t know. Harrod could only give those future generations—and the forces of hope and fate—a chance to create a better society than the one they had come from.

Atmospheric buffeting made the
Courser
’s bow begin to buck. A bit of downward thrust steadied the nose, which eased into the smooth arc of a fast descent. He checked the ship’s projected impact point and smiled: for an Intendant, a lesser being, he was doing a most admirable job.

Most admirable indeed.

FUSION STARSHIPS

Dr. Gregory Matloff

When the history of humanity’s expansion into the galaxy is written in the capital city of Tau Ceti Three, the entry for Gregory Matloff may well read, “He was one of the pioneers in the field of interstellar travel. His theoretical analyses of the technologies that might enable the human species to travel between the stars inspired generations of scientists and engineers, and are the basis of the starships that enabled settlement of this part of the galaxy.”

This is the second of his essays for
Going Interstellar
, and in it he describes a propulsion system that many believe will be the first to take us to the stars.

***

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 = mc
²

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.