Oh! I have slipped the surly bonds of earth
And danced the skies on laughter-silvered wings;
Sunward I've climbed, and joined the tumbling mirth
Of sun-split clouds—and done a hundred things
You have not dreamed of—wheeled and soared and swung
High in the sunlit silence. Hov'ring there
I've chased the shouting wind along, and flung
My eager craft through footless halls of air.
Up, up the long delirious, burning blue,
I've topped the windswept heights with easy grace
Where never lark, or even eagle flew—
And, while with silent lifting mind I've trod
The high untrespassed sanctity of space,
Put out my hand and touched the face of God—John Gillespie Magee
The chief engineer pushed himself up the accessway and floated into the fusion tube's control room, tacked on to the bottom of the spin platform's landing deck. Gravity at that level was just a dozen centimeters per second squared, enough to give a sense of down and not much more. You could sit, but you couldn't really walk. He launched himself on a long, graceful parabola to the main console, where the squeeze-field controller was monitoring the startup of the drive.
"How are we looking?" The chief engineer leaned over the controller's shoulder to see the readouts himself.
"The field is stable at fifty percent and we've held it there for an hour. We're ready to ramp it up whenever you like."
"Good." The chief engineer turned to look out the panoramic windows. The mirror cone had been taken down for the fusion tube startup and the vast cylinder of Ark's interior world surrounded him, the land far below sharply divided into sections of light and shadow by sunlight no longer softened by reflecting off the cone. The land around the construction shack was carved into a neat patchwork of farm fields; farther away the fields merged into the uniform green of forest and grassland that stretched back to the distant blue ring of the ocean. Overhead he could see the entire length of fusion tube, a fat, silver cylinder that stretched the great ship's central axis to a vanishing point against the distant aftwall. In the other direction the foredome curved down to join the forewall, visible only where the sunlight glinted against its transparency. A sliver crescent of Earth was visible where they met, and he could just make out the gossamer rails of the Cable, dropping away from the spin platform to the planet far below.
He took a deep breath and turned his gaze back to the instrument panels. The view was hypnotically beautiful, but it delivered three kinds of vertigo at once and there was work to be done. Today I need to concentrate. Today is success or failure, there will be no in-between.
"Start the boundary vortex," he ordered. "Bring the field up a little more."
"Field now at fifty-five percent," the field controller reported.
The chief engineer nodded. "Fuel control, start the methane feed, just give me a few grams."
"Da, starting now." A second later the fuel technician added, "Fuel line pressure is ramping. Accelerator voltages are good, we're feeding."
"Chamber pressure coming up," added the squeeze-field technician. "Plasma temperature is spiking."
"Boundary vortex flow is good," put in the cooling-system tech.
"Field instabilities?" asked the chief engineer.
The field controller shook his head. "Everything's flat, it looks good."
"Let's just take our time, bring everything online slowly." The techs tapped their consoles and the readouts responded, bearing silent witness to the titanic forces now at play in the core of the fusion tube. Reflexively the chief engineer looked up at the long, silver cylinder again, but it showed no outward change. Which is a good thing. The only change he might see would be the explosive rupture of the superconducting nanotube magnet windings if they failed to contain the thousand-tesla magnetic field that confined the fusion reaction. That would end the startup, his career and possibly humanity's entire dream of interstellar flight. And perhaps my life too. They were only two hundred meters from the tube, and a bad rupture could easily destroy the control room. Somehow that seems the least of my concerns.
"Plasma loading is complete." The fuel tech paused, checking his readouts. "It looks like we have fusion."
"That's confirmed," added the field controller.
The chief engineer nodded and breathed out. "Good, let's throttle up." We've passed the first test. In the fuel injector, methane molecules were being broken and stripped of their electrons so the naked carbon and hydrogen nuclei could be accelerated into the center of the field, where the inexorable magnetic pressure there compressed them to twice the density of the sun's core.
"Field strength is seventy percent. Reaction temperature is twenty million kelvins and rising."
"Good, let's have some throttle."
The field controller tapped his console again, dialing down the field strength on the magnetic nozzle at the far end of the thirty-kilometer drive tube. The most energetic nuclei began to leak through the weakened barrier, moving at a significant fraction of the speed of light.
"Tube fields are good at seventy percent. Bringing them up to eighty percent."
The chief engineer put a finger to his lips, watching the process, his mind jumping ahead to anticipate what might yet go wrong. "More throttle, and more fuel."
Slowly, inexorably the readouts inched higher. The chief engineer watched in silence now, as his acolytes progressively awakened the furies that would drive Ark to another star. The magnetic-field strength peaked at its full rated maximum, and slowly they brought the fuel flow up to match it. At the far end of the drive tube the particle exhaust reached ten percent of lightspeed, and a camera link showed it as a faint blue-violet line, reaching out laser-straight until it faded into the inky blackness surrounding it.
"We are running stable," the fuel technician reported. "Reactor flow is constant at eight kilograms per second."
The chief engineer smiled. And so, we are under way. There was no sensation of acceleration, the tremendous thrust the drive provided served to accelerate Ark's massive bulk at only a millionth of a gravity. It would be seventeen years before they even broke free of Earth orbit but the thrust would continue, decade by decade, century after century, until Ark was moving a significant fraction of lightspeed herself. He looked up to the fusion tube again and saw the first tangible hint of their success. It had begun to glow a faint, dull red. The tremendous energy produced in the tube was trapped in a thin layer of xenon gas, spun in a high-speed vortex to protect the inner surface of the fusion tube while leaving its center in vacuum to keep the fusion plasma from quenching. The xenon circulated over and through a graphite sponge and before being siphoned off through microchannels cut into the hydrogen-cooled tungsten inner heat sink that formed the inner wall of the fusion tube before being recirculated into the vortex. A second system cooled the inner heat sink, channeling its heat past the field coils in their liquid-nitrogen bath and radiating it through the silica-clad tungsten of the fusion tube's outer sheath. As he watched, the tube's color rose slowly up the spectrum, cherry red to hot orange to brilliant yellow-white. The tube section immediately above the control room was different, thicker and with cooling sufficient to allow water vapor to condense against it, but even so the rest of the tube was hot enough that he could feel its heat. The readouts were stable. He breathed out, recognizing the tension he'd been carrying only when it was released. We are under way.
"Disconnect the Cable," he ordered, and was surprised by the sudden applause that rose in the control room. And I should have seen that coming. They had cut their link to Earth, and from that instant forward the long dream was realized. Their fate was their own. He went on with the startup checklist, verifying that every system in the drive was running smoothly. There were no moving parts in it anywhere, just magnetic pumps and thermosiphons. No other approach would serve for a system that would have to run, hands-off and undisturbed, for ten thousand years or more. On the ground below, the sharp sun shadows softened and faded under the fusion tube's light. We don't need the sun any longer. We have our own now, to light us through the long, dark night of our journey. On the other side of the foredome the stars were waiting patiently.