He looked back at the tractor, with the big red Sun of Japan painted on its side. He thought of a terraformed Moon, of twin blue worlds. He felt tears, hot and unwelcome, prickle his eyes. Damn it. We were here first. We had all this. And we let it go.

Nemoto waited for him, a small figure on the Moon’s folded plain, her face hidden behind her gold-tinted bubble of glass.

She led him into the cluster of buildings. There was a small fission power plant, tanks of gases and liquids. A living shelter was half buried in the regolith.

The center of the site was a crude cylindrical hut, open to the sky, containing a battery of infrared sensors and computer equipment. The infrared detectors themselves were immersed in huge vessels of liquid helium. Robots crawled between the detectors, monitoring constantly, their complex arms stained by moondust.

Nemoto walked up to a processor control desk. A virtual image appeared, hovering over the compacted regolith at the center of the hut. The virtual was a ring of glistening crimson droplets, slowly orbiting.

“Here is a summary of my survey of the asteroid belt,” Nemoto said. “Or ‘belts,’ I should say, for there are gaps between the sub-belts — the Kirkwood gaps, swept clear by resonances with Jupiter’s gravity field.” The Kirkwood gaps were dark bands, empty of crimson drops. “Nishizaki Heavy Industries is very interested in asteroids. There is a mine in Sudbury, Ontario, which for a long time was a rich source of nickel. The nickel seam is disc-shaped. It is almost certainly the scar of an ancient asteroid collision with the Earth.”

“Mineral extraction, then.”

“There is a scheme to retrieve a fragment of the asteroid Geographos, which crosses Earth’s orbit. We may cleave it with controlled explosions. Perhaps we can deliver fragments to orbit, using lunar gravity assists and grazes against the Earth’s atmosphere. Or we may initiate a controlled impact with the Moon. This exercise alone would yield more than nine hundred billion dollars’ worth of nickel, rhenium, osmium, iridium, platinum, gold — so much, in fact, the planet’s economy would be transformed, making estimates of wealth difficult.”

Malenfant walked around the instrument hut. The novelty of his moonwalk was wearing off; his suit scratched, his helmet was hot, and his condom was itching. “Nemoto, it’s time you got to the point.”

“The koan,” she said. The virtual ring shone in her visor, making her face invisible. “Let us look at the stars.”

She took his gloved hand in hers — through the thick layers of glove he could barely feel the pressure of her fingers — and she led him out of the building. The virtual asteroid ring, eerily, followed them out.

They stood in the deep shadow of the structure. With a motion, she indicated he should lift his visor.

He raised his head so he couldn’t see the ground or the buildings, and he turned around and around, as he used to as a kid, on the darkest moonless nights back home.

The stars, of course: thousands of them, peppering the sky all around him, crowding out the bright-star constellations seen from Earth. And now, at last, came that elusive feeling of immensity. From the Moon it was much easier to see that he was just a mote clinging to a round ball of rock, spinning endlessly in an infinite, three-dimensional starry sky.

“Look.” Nemoto, pointing, swept out an arc of the sky, where dusty light shone.

Despite the crowding stars, Malenfant recognized one or two constellations — Cygnus and Aquila, the swan and the eagle. And, where she pointed, a river of light ran through the constellations, a river of stars. It was the Milky Way: the Galaxy, the disc of stars in which Sol and all its planets were embedded, seen edge-on and turned into a band of light that wrapped around the sky. But, as it passed through Cygnus and Aquila, that band of light seemed to split into two, twin streams separated by a dark gap. In fact the rift was a shadow cast by dark clouds blocking the light from the star banks behind.

Nemoto pointed. “See how the darkness starts out narrow in Cygnus, then broadens in Aquila, sweeping wider through Serpens and Opiuchus. This is the effect of perspective. We are seeing a band of dust as it comes from the distance in Cygnus, passing closest to the Sun in Aquila and Opiuchus. Malenfant, we live in a spiral arm of this Galaxy — a small fragment, in fact, called the Orion Arm. And spiral arms typically have lanes of dust on their inside edges.”

“Like that one.”

“Yes. That is the inner edge of our spiral arm, hanging in the sky for all to see.” Her shadowed eyes glimmered, full of starlight. “It is possible to make out the Galaxy’s structure, you see: to witness that we are embedded in a giant spiral of stars — even with the naked eye. This is where we live.”

“Why are you showing me this?”

“Look at the Galaxy, Malenfant. It appears to be a giant machine — no, an ecology — evolved to make stars. And there are hundreds of millions of galaxies beyond our own. Is it really conceivable, given all of that immensity, all that structure, that we are truly alone? That life emerged here, and nowhere else?”

Malenfant grunted. “The old Fermi paradox. Troubled me as a kid, even before I heard of Fermi.”

“Me too.” He could see her smile. “You see, Malenfant, we have much in common. And the logic behind the paradox troubles me still.”

“Even though you think you have found aliens.”

She let that hang, and he found he was holding his breath.

Cautiously, she said, “How would it make you feel, Malenfant, if I was right?”

“If you had proof that another intelligence exists? It would be wonderful. I guess.”

“Would it?” She smiled again. “How sentimental you are. Listen to me: Humanity would be in extreme danger. Remember, by your own argument, the assumption on which a colonizing expedition operates is that it is appropriating an empty system. Such a probe could destroy our worlds without even noticing us.”

He shivered; his spiderweb suit felt thin and fragile.

“Think it through further,” she said. “Think like an engineer. If an alien replicator probe were to approach the Solar System, where would it seek to establish itself? What are its requirements?”

He thought about it. You’ll need energy; plenty of it. So, stay close to the Sun. Next: raw materials. The surface of a rocky planet? But you wouldn’t want to dip into a gravity well if you didn’t have to… Besides, your probe is designed for deep space—

“The asteroid belt,” he said, suddenly seeing where all this was leading. “Plenty of resources, freely floating, away from the big gravity wells… Even the main belts aren’t too crowded, but you’d probably settle in a Kirkwood gap, to minimize the chance of collision. Your orbit would be perturbed by Jupiter, just like the asteroids’, but it wouldn’t require much station-keeping to compensate for that. And some kind of ship or colony out there, even a few kilometers across, would be hard for us to spot.” He looked at her sharply. “Is that what this is about? Have you found something in the belt?”

“The plain facts are these. I have surveyed the Kirkwood gaps with the sensors here. And, in the gap which corresponds to the one-to-three resonance with Jupiter, I have found—” She pointed to her virtual model, to a broad, precise gap.

At the center of the gap, a string of rubies shone, enigmatic, brilliant in the shadows.

“These are sources of infrared,” she said. “Sources I cannot explain.”

Malenfant bent to study the little beads of light. “Could they be asteroids that have strayed into the gap after collisions?”

“No. The sources are too bright. In fact, they are each emitting more heat than they receive from the Sun. I am, of course, seeking firmer evidence: for example, structure in the infrared signature; or perhaps there will be radio leakage.”