It took a day to get there.

They came at the planet with the Sun behind them, so it showed a nearly full disc. It glared, brilliant white, just a solid mass of cloud from pole to pole, blinding and featureless. And it was surrounded by a pearly glow of interstellar hydrogen, like an immense, misshapen outer atmosphere.

The flower-ship’s petals opened wide, the lasers working vigorously, and it decelerated smoothly into orbit.

They could see nothing of the surface. Their instruments revealed a world that was indeed like Venus: an atmosphere of carbon dioxide, kilometers thick, scarcely any water.

There was, of course, no life of any kind.

The Chaera spun in its tank, volunteering nothing.

Ben was troubled. “There’s no reason for a Venus to form this far from the Sun. This world should be temperate. An Earth.”

“But,” Nemoto hissed, “think what this world has that Earth doesn’t share.”

“The gas cloud,” Madeleine said.

Ben nodded. “All that interstellar hydrogen. Madeleine, we’re so far from the Sun now, and the gas is so thick, that the hydrogen is neutral — not ionized by sunlight.”

“And so—”

“And so the planet down there has no defense against the gas; its magnetic field could only keep it out if it was charged. Hydrogen has been raining down from the sky, into the upper air.”

“Once there, it will mix with any oxygen present,” Nemoto said. “Hydrogen plus oxygen gives—”

“Water,” Madeleine said.

“Lots of it,” Ben told her. “It must have rained like hell, for a million years. The atmosphere was drained of oxygen, and filled up with water vapor. A greenhouse effect took off—”

“All that from a wisp of gas?”

“That wisp of gas was a planet killer,” Nemoto whispered.

“But why would anyone kill a planet?”

“It is the logic of growth,” Nemoto said. “This has all the characteristics of an old system, Meacher. Caught behind a wave of colonization — all its usable resources dug out and exploited…”

Madeleine frowned. “I don’t believe it. It would take a hell of a long time to eat up a star system.”

“How long do you think?”

“I don’t know. Millions of years, perhaps.”

Nemoto grunted. “Listen to me. The growth rate of the human population on Earth, historically, was two percent a year. Doesn’t sound like much, does it? But it’s compound interest, remember. At that rate your population doubles every thirty-five years, an increase by tenfold every century or so. Of course after the twentieth century our growth rates collapsed; we ran out of resources.”

“Ah,” Ben said. “What if we’d kept on growing?”

“How many people could Earth hold?” Nemoto whispered. “Ten, twenty billion? Meacher, the whole of the inner Solar System out to Mars could supply only enough water for maybe fifty billion people. It might have taken us a century to reach those numbers. Of course there is much more water in the asteroids and the outer system than in Earth’s oceans, perhaps enough to support ten thousand trillion human beings.”

“A huge number.”

“But not infinite — and only six tenfold jumps away from ten billion.”

“Just six or seven centuries,” Ben said.

“And then what?” Nemoto whispered. “Suppose we start colonizing, like the Gaijin. Earth is suddenly the center of a growing sphere of colonization whose volume must keep increasing at two percent a year, to keep up with the population growth. And that means that the leading edge, the colonizing wave, has to sweep on faster and faster, eating up worlds and stars and moving on to the next, because of the pressure from behind…”

Ben was doing sums in his head. “That leading edge would have to be moving at light speed within a few centuries, no more.”

“Imagine how it would be,” Nemoto said grimly, “to inhabit a world in the path of such a wave. The exploitation would be rapid, ruthless, merciless, burning up worlds and stars like the front of a forest fire, leaving only ruins and lifelessness. And then, as resources are exhausted throughout the light-speed cage, the crash comes, inevitably. Remember Venus. Remember Polynesia.”

“Polynesia?”

“The nearest analog in our own history to interstellar colonization,” Ben said. “The Polynesians spread out among their Pacific islands for over a thousand years, across three thousand kilometers. But by about A.D. 1000 their colonization wave front had reached as far as it could go, and they had inhabited every scrap of land. Isolated, each island surrounded by others already full of people, they had nowhere to go.

“On Easter Island they destroyed the native ecosystem in a few generations, let the soil erode away, cut down the forests. In the end they didn’t even have enough wood to build more canoes. Then they went to war over whatever was left. By the time the Europeans arrived the Polynesians had just about wiped themselves out.”

“Think about it, Meacher,” Nemoto said. “The light-speed cage. Imagine this system fully populated, a long way behind the local colonization wave front, and surrounded by systems just as heavily populated — and armed — as they were. And they were running out of resources. There surely were a lot more space dwellers than planet dwellers, but they’d already used up the asteroids and the comets. So the space dwellers turned on the planet. The inhabitants were choked, drowned, baked.”

“I don’t believe it,” Madeleine said. “Any intelligent society would figure out the dangers long before breeding itself to extinction.”

“The Polynesians didn’t,” Ben said dryly.

The petals of the flower-ship opened once more, and they receded from the corpselike planet into the calm of the outer darkness.

It was time to talk to the icosahedral God again. The second X-ray punch laser was launched.

After studying the records of the last encounter, Ben had learned how the configuration of the icosahedral artifact anticipated the direction of the resulting beam. Now Madeleine watched the core squint into focus. The killer beam would again lance through the accretion disc — and, this time, right into one of the largest of the Chaera worldlets.

Millions of Chaera were going to die. Madeleine could see them, infesting their accretion disc, swarming and living and loving.

In its tank, their Chaera passenger drifted like a Dali watch.

“Nemoto,” Madeleine said, “we can’t go ahead with the second firing.”

“But they understand the consequences,” virtual Nemoto said blandly. “The Chaera have disturbed the artifact a few times in the past, with their mirrors and smoke signals. Every time it’s killed some of them. But they need the X-ray nourishment… Meacher,” she warned, “don’t meddle as you did at the burster. If you meddle, the Gaijin may not allow human passengers on future missions. And we won’t learn about systems like this. We’ll have no information; we won’t be able to plan… Besides, the laser is already deployed. There’s nothing you can do about it.”

“It is the Chaera’s choice, Madeleine,” Ben said gently. “Their culture. It seems they’re prepared to die to attain what they believe is perfection.”

Nemoto quoted the Chaera. “It knows we’re arguing here. ‘Where there are prophecies, they will cease. Where there are tongues, they will be stilled; where there is knowledge, it will pass away. For we know in part and we prophesy in part, but when perfection comes, the imperfect disappears.’ ”

“Who’s the philosopher?” Madeleine asked sourly. “Some great Chaera mind of the past?”

Ben smiled. “Actually, it was quoting Saint Paul.”

Nemoto looked startled, as Madeleine felt.

“But there remain mysteries,” Ben said. “The Chaera look too primitive to have constructed that artifact. After all, it manipulates a black hole’s gravity well. Perhaps their ancestors built this thing. Or some previous wave of colonists, who passed through this system.”