The engine was the size of a cathedral.

Silas Noah knew this because he had walked its base perimeter — three hundred and forty-seven steps, each one measured by the pacing he had developed in the coal mines of Northumberland, where he had learned that distance is not measured in meters but in the number of paces it takes to get from the shaft to the surface and back without stopping. Three hundred and forty-seven steps. And that was just the base. The engine rose above him like a mountain of brass and iron and riveted steel, its fluted surface venting steam in great white plumes that turned the winter sky grey.

It was November 1893. The temperature was minus eighteen degrees Celsius. The Thames had frozen solid three weeks ago, and the ice was thick enough to support horse-drawn carriages crossing from London Bridge to Southwark. But the cold was not what made Silas shiver. It was the engine.

In forty-eight hours, it would ignite. And when it did, it would change everything.

"Chief Engineer Noah?"

Silas turned. Isabella Crawford stood behind him, wrapped in a woolen greatcoat, her hair tucked beneath a scarf, her notebook clutched against her chest like a shield. She was thirty-two, slight-constructed, with eyes the color of the sea before a storm and a mind that could calculate orbital mechanics in her head while coughing blood into a handkerchief.

She was also the finest meteorologist in the Royal Observatory, and the only person on Earth who understood what was about to happen to the atmosphere when the first fusion engine ignited.

"The readings," she said. Her voice was steady, but her hands were shaking. "I've run the calculations again. When Engine One fires, the ionization of the upper atmosphere will create auroral phenomena visible across the entire northern hemisphere. The aurora will persist for approximately seventy-two hours. There will be —" She paused. A cough. She tucked the blood-stained handkerchief into her sleeve. "There will be localized electromagnetic interference with the telegraph network."

"Will it affect the engine?" Silas asked.

"No. The engine is shielded. But the telegraphs —"

"Let the telegraphs break," Silas said. He looked back at the engine. "When the engine fires, we won't need telegraphs."

Isabella Crawford studied his face. She had known Silas Noah for four years — since the Royal Society had recruited him, a self-taught engineer from a coal-mining family, to consult on the fusion engine schematics. He had looked at her equations and corrected three errors in the magnetic confinement calculations without picking up a pen. She had recommended him for chief engineer against the objections of twelve men who had spent their entire careers dismissing anyone who had not attended Cambridge.

"You understand what this means?" she asked.

"Yes."

"The Earth will stop rotating."

"The Earth will stop rotating."

"The oceans will —"

"Flooding begins in T-minus forty-eight hours. I know the evacuation figures. London: two point three million transferred to the under-city. Paris: two point one million. New York: eight point four million. All coastal cities — evacuated or submerged."

Isabella Crawford was silent for a long time. The wind blew from the east, carrying the smell of coal smoke and frozen Thames and the distant hiss of steam vents from the under-city heating systems.

"I spent last night in the garden," she said. "My grandmother's garden in Kew. I sat on the bench where she used to read to me, and I watched the frost form on the rose bushes. I counted twelve varieties. Twelve varieties of roses, Silas. Twelve varieties that will be dead when the temperature drops to minus fifty in six months."

"I know."

"I want you to know that I counted them. That someone counted them. That I sat there and counted twelve varieties of roses before the world ended."

Silas Noah nodded. He did not have words for what she was feeling. He had never been good with words. He was a man of measurements and calculations and the weight of iron and the temperature of steam. Words were for people like Isabella, who could articulate the inarticulable.

"I will count them again," he said. "After the engine fires. When it is cold. I will go to the garden. I will count the roses."

She smiled. It was a sad smile, and a brave one, and it was the most beautiful thing he had ever seen.

Engine Number One stood on a plain outside London, one of ten thousand fusion engines being constructed simultaneously around the globe. Each engine was identical: a towering cylinder of brass and iron, three hundred meters tall, powered by a controlled fusion reaction that used magnetic fields to confine plasma at temperatures exceeding one hundred million degrees Celsius. The technology was theoretical — Silas had derived the magnetic confinement equations from first principles, combining Maxwell's electromagnetic theory with a new understanding of nuclear binding energy that he had developed in the quiet hours between shifts in the coal mine.

The engineers called it impossible. The Royal Society called it brilliant. Empress Victoria Adelaide called it Project Noah.

Because the sun was dying.

Not dying — expanding. The Royal Observatory's solar observations had detected an anomalous increase in the sun's outer envelope. In three years, the sun would engulf Earth's orbit. In five years, the Earth would be vaporized. In ten years, the solar system would be empty.

Project Noah was humanity's answer: ten thousand fusion engines, arranged in a global network, pushing the Earth out of the solar system. Turning the planet into a wandering world, a nomad carrying its home on its back into the interstellar dark.

Captain Joseph Whitaker arrived at dawn. He was a Royal Navy officer, fifty-two years old, with a face like weathered leather and a posture so straight it looked painful. He led thirty volunteers — all over fifty, all healthy, all volunteers — in a column that stretched from the engine base to the horizon.

"Chief Engineer Noah," Captain Whitaker said, and his voice was the voice of a man who had given orders at sea and expected them to be obeyed. "The volunteers are assembled."

Silas turned. Thirty men and women stood in formation, their faces grim, their eyes bright with the strange mixture of terror and resolve that only people who are about to do something irreversible can muster.

"These are the calibration team," Captain Whitaker said. "When the engine fires, the magnetic field will need manual adjustment in the first sixty seconds. The automatic systems cannot handle the initial surge. One of us has to go inside the core and make the adjustments by hand. It will be —" He paused. It was the first time Silas had ever heard Captain Whitaker hesitate. "It will be fatal. The radiation inside the core at full fusion intensity is sufficient to kill a human being in approximately four minutes. But sixty seconds of manual adjustment is all we need. After that, the automatic systems take over."

Sixty seconds. Four minutes. A suicide mission dressed up as a calibration procedure.

Silas Noah looked at the thirty faces. He saw fear. He also saw something else: acceptance. These were people who had lived fifty, fifty-five, sixty years. They had worked the mines. They had served in the navy. They had raised families and built homes and planted gardens. They had earned their rest. And yet they had volunteered to walk into a radiation furnace and die in four minutes so that humanity could live.

"Captain," Silas said. "You do not have to ask them."

"I know," Captain Whitaker said. "But I will. Because every volunteer deserves to know exactly what they are signing up for."

He turned to the formation. His voice rang out across the frozen plain, clear and commanding and carrying the weight of thirty lives placed squarely on his shoulders.

"Volunteers of the Royal Noachian Calibration Corps — the engine will ignite in forty-eight hours. The magnetic field will require sixty seconds of manual adjustment inside the core. The radiation level will be lethal within four minutes. You will have sixty seconds. You will die. But the Earth will live. Do you volunteer?"

Thirty voices answered. Thirty voices, loud and clear and steady.

"We volunteer."

Silas Noah closed his eyes. He thought of his father, who had died in a coal mine collapse when Silas was fourteen. He thought of his mother, who had raised him alone, working as a laundress, teaching him to read by candlelight. He thought of the equations he had derived in the dark, after eighteen hours of shifts, writing magnetic confinement formulas on the backs of coal sacks.

He opened his eyes. He looked at Captain Whitaker.

"Thank you," he said.

Captain Whitaker nodded. "That is what men like us do, Chief Engineer. We make the sacrifice so that men like you can build the future."

The ignition happened at noon on November 20th, 1893.

Silas stood at the control bunker, two kilometers from Engine Number One. Isabella stood beside him, her notebook open, her pen ready, her face turned toward the engine.

Below them, the Earth trembled.

The ignition sequence began: magnetic fields activated. Plasma injected. Containment achieved. Temperature rising: one million degrees. Five million. Fifty million. One hundred million.

And then — ignition.

The engine roared.

It was not a sound. It was a physical force, a pressure wave that hit the control bunker like a fist, rattling the steel walls, vibrating the teeth in Silas's skull. Through the reinforced viewing port, Silas saw the engine: a pillar of blue-white light, three hundred meters tall, rising from the barrel like a pillar of the gods.

The ground heaved. The fusion reaction was producing thrust — seven billion newtons of force, pushing the Earth in the direction opposite to the engine's exhaust. In forty-eight hours, this thrust would be multiplied by nine thousand nine hundred and ninety-nine other engines, and the total force would be sufficient to accelerate a planet.

Isabella Crawford was weeping. She did not try to hide it. She stood at the viewing port, tears streaming down her face, and she watched the engine roar and she wept for the twelve varieties of roses in the Kew garden and for the frozen Thames and for London, which was flooding beneath twenty meters of ice and seawater and which would never see another spring.

Captain Whitaker and his thirty volunteers were inside the engine.

Silas could not see them. He could only hear them, through the communication channel that Captain Whitaker had insisted on maintaining. The channel crackled with static — the electromagnetic interference from the fusion reaction was drowning out the signal — but through the noise, Silas heard Captain Whitaker's voice, calm and steady, reading the calibration readings.

"Alpha field: nominal. Beta field: nominal. Gamma field: adjusting. Delta field — adjusting now. Chief Engineer, the Gamma field is drifting. I am adjusting manually."

A pause. The static grew louder.

"Gamma field corrected. Silas — can you hear me?"

"I hear you, Captain."

"Tell my daughter — Eleanor — that I —"

The static swallowed the rest. But Silas heard what came after the static: a cough. A wet, rattling cough. And then Captain Whitaker's voice, quieter now, weaker:

"Epsilon field: critical. Adjusting. I can feel — the heat. It is — very hot. But the field is stable. The Earth is —"

A scream. Not of pain. Of effort. The effort of turning a wheel by hand inside a fusion reactor, of holding a lever against seven billion newtons of force, of keeping the magnetic field stable while radiation cooked your cells from the inside out.

"Field — stable. Silas. Tell Eleanor — I loved —"

The channel went silent.

Silas Noah stood at the viewing port and watched the engine roar. He watched the blue-white pillar of light. He watched the smoke and steam and ice rising from the frozen plain around the engine base. He watched the sky turn grey and then black and then, as the engine's exhaust ionized the upper atmosphere, a deep and impossible green — the aurora, beginning early, painting the sky in colors that would not fade for seventy-two hours.

He did not move. He did not speak. He stood there and watched the engine that had killed thirty people and saved eight billion, and he understood, for the first time in his life, what it meant to be small.

The Earth was moving.

Seven billion newtons of thrust. The acceleration was imperceptible — a fraction of a millimeter per second squared — but it was real. The Earth was leaving the solar system. Slowly. Inevitably. Like a ship casting off from the harbor.

Isabella Crawford wiped her eyes. She closed her notebook. She turned to Silas.

"Count the roses," she said.

"I will."

She left. She went back to the Royal Observatory, where she would spend the rest of her days mapping the new climate, the frozen Earth, the world that would never warm again.

Silas Noah went to the garden in Kew.

The roses were dead. Frost had killed them within hours of the ignition. The stems were black. The petals had fallen and frozen and turned to shards of crimson glass on the ground.

He knelt beside the bush. He counted the stems. Twelve varieties. He counted them all.

Then he stood up. He looked at the sky. The aurora was green and vast and beautiful, stretching from horizon to horizon, a curtain of ionized oxygen and nitrogen drawn across the sky by the engine's electromagnetic exhaust.

"We put our home on our backs," Silas whispered, "and then we walked."

He turned and walked away from the garden, toward the under-city, toward the world that would never see sunlight again but would carry on anyway, beneath the ice, in the dark, with ten thousand engines burning on its surface, pushing it forward into the interstellar night.

The frozen nomads had begun their journey.