In the control room he took off his helmet and dropped into the pilot's seat. He looked at Caroline. "Good to get the helmet off," he said.

She nodded, lifting her own off her head.

His fingers tapped out a firing pattern. He hesitated for a moment, his thumb poised over the firing lever.

"Listen, Caroline," be asked, "how much chance have we got?"

"We'll get there," she said.

"No," he snapped, "don't tell me that. Tell me the truth. Have we any chance at all?"

Her eyes met his and her mouth sobered into a thin, straight line.

"Yes, some," she said. "Not quite fifty-fifty. There are so many factors of error, so many factors of accident. Mathematics can't foresee them, can't take care of them, and mathematics are the only signposts that we have."

He laughed harshly.

"We're shooting at a target, don't you see?" she said. "A target millions of light-years away, and millions of years away as well, and you have to have a different set of co-ordinates for both the time and distance. The same set won't do for both. It's difficult."

He looked at her soberly. She said it was difficult. He could only faintly imagine how difficult it might be. Only someone who was a master at the mathematics of both time and space could even faintly understand — someone, say, who had thought for forty lifetimes.

"And even if we do hit the place," he said, "it may not be there."

Savagely he plunged his thumb against the lever. The rockets thundered and the ship was arcing up. Another pattern and another. They were plunging upward now under the full thrust of rocket power and still the ruined city was all around them, cragged, broken towers shattered by the blasting of atomic energy.

The soft swirl of light that marked the opening of the time-space tunnel lay between and beyond two blasted towers. Gary fired a short, corrective pattern to line the nose of the ship between the towers and then depressed a stud and fired a blast that drove them straight between the towers, up and over the city, straight for the whirl of light.

The ship arrowed swiftly up. The directional crossbars lined squarely upon the hub of spinning light.

"We're almost there," he said, his breath whistling between his teeth. "We'll know in just a minute."

The cold wind out of space was blowing on his face again; the short hairs on his neck were trying to rise into a ruff. The old challenge of the unknown. The old glory of crusading.

He snapped a look at Caroline. She was staring out of the vision plate, staring straight ahead, watching the rim of the wheel spin out until only the blackness of the hub remained.

She turned to him. "Oh, Gary!" she cried, and then the ship plunged into the hub and blackness as thick and heavy and as stifling as the ink of utter space flooded into the ship and seemed to dim the very radium lamps that burned within the room. He heard her voice coming out of the blackness that engulfed them. "Gary, I'm afraid!"

Then the black was gone and the ship rode in space again — in a star-sprinkled space that had, curiously, a warm and friendly look after the blackness of the tunnel.

"There it is!" Caroline cried, and Gary expelled his breath in a sigh of relief.

Below them swam a planet, a planet such as they had seen in the spinning bowl back in the city of the Engineers. A planet that was spotted with mighty mountains weathered down to meek and somber hills, a planet with shallow seas and a thinning atmosphere.

"The Earth," said Gary, looking at it.

Yes, the Earth. The birthplace of the human race, now an old and senile planet tottering to its doom, a planet that had outlived its usefulness. A planet that had mothered a great race of people — a race that always strove to reach what was just beyond, always reaching out to the not-as-yet, that met each challenge with a battle cry. A crusading people.

"It's really there," said Caroline. "It's real."

Gary glanced swiftly at the instruments. They were only a matter of five hundred miles above the surface and as yet there was no indication of atmosphere. Slowly the ship was dropping toward the planet, but still there was no sign of anything but space.

He whistled softly. Even the slightest presence of gases would be registered on the dials and so far the needles hadn't even flickered.

Earth must be old! Her atmosphere was swiftly being stripped from her to leave her bare bones naked to the cold of space. Space, cold and malignant, was creeping in on mankind's cradle.

He struck the first sign of atmosphere at slightly under two hundred miles.

The surface of the planet was lighted by a Sun which must have lost much of its energy, for the light seemed feeble compared to the way Gary remembered it. The Sun, behind them, was shielded from their vision.

Swiftly they dropped, closer and closer to the surface. Eagerly they scanned the land beneath them for some sign of cities, but they saw only one and that, the telescope revealed, was in utter ruins. Drifting sands were closing over its shattered columns and once mighty walls.

"It must have been a great city in its day," said Caroline softly. "I wonder what has happened to the people."

"Died off," said Gary, "or left for some other planet, maybe for some other sun."

The telescopic screen mirrored scene after scene of desolation. Vast deserts with shifting dunes and mile after mile of nothing but shimmering sand, without a trace of vegetation. Worn-down hills with boulder-strewn slopes and wind-twisted trees and shrubs making their last stand against the encroachment of a hostile environment.

Gary turned the ship toward the night side of the planet, and it was then they saw the Moon. Vast, filling almost a twelfth of the sky, it loomed over the horizon, a monstrous orange ball in full phase.

"How pretty!" gasped Caroline.

"Pretty and dangerous," said Gary.

It must be approaching Roche's limit, he thought. Falling out of the sky, year after year it had drawn closer to the Earth. When it reached a certain limit, it would be disrupted, torn to bits by the stresses of gravitation hauling and tugging at it. It would shatter into tiny fragments and those fragments would take up independent orbits around old Earth, giving her in miniature the rings of Saturn. But the same forces which would tear the Moon to bits would shake up the Earth, giving rise to volcanic action, world-shattering earthquakes, monstrous tidal waves. Mountains would be leveled, new continents raised. Earth's face would be changed once again, as it probably had been changed many times before. As it had been changed since early Man had known it, for search as he might, Gary could find no single recognizable feature, not a single sea or continent that seemed familiar.

He reflected on the changes that must have come to pass. The Earth must have slowed down. Probably a night now was almost a month long, and a day equally as long. Long scorching days and endless frigid nights. Century after century, with the moon tides braking the Earth's motion, with the addition of mass due to falling meteors, Earth had lost her energy. Increase of mass and loss of energy had slowed her spin, had shoved her farther and farther away from the Sun, pushing her out and into the frigidness of space. And now she was losing her atmosphere. Her gravity was weakening and the precious gases were slowly being stripped from her. Rock weathering also would have absorbed some of the oxygen.

"Look!" cried Caroline.

Aroused from his daydreaming, Gary saw a city straight ahead, looming on the horizon, a great city a-gleam with shining metal.

"The Engineer said we would find people here," Caroline whispered. "That must be where we'll find them."

The city was falling into ruin. Much of it, undoubtedly, already had been covered by the creeping desert that crawled toward it from every direction. Some of the buildings were falling apart, with great gaping holes staring like empty, hopeless eyes. But part of it, at least, was standing, and that part gave a breath-taking hint to the sort of city it had been when it soared in full pride of strength at its very prime.