"Nanomachines can build things, too, y'know," Cardenas said.

At first Paul thought he was looking at a child's sand castle, the kind that kids build on the beach. The chamber he was looking into was hardly larger than a phone booth, dimly lit by a single bare bulb in the ceiling. Its floor was covered with sand or a grayish brown powder of some sort. In the middle of it stood a half-built tower.

"Here we've got assemblers at work," Cardenas said, her voice low, almost a reverential whisper.

Paul studied the tower. It was about three feet tall. It wasn't made of sand, he realized. It was gray, almost the same color as the stuff strewn over the floor, but it looked smoother, metallic.

"What is this?"

In the dim light from the display screens Cardenas' expression was difficult to read. But her voice was vibrating with barely-suppressed excitement.

"Last week Mr. Masterson phoned me with a special request. This is the result."

"What'd Greg want?"

"That sand is from the Moon," Cardenas said. "We've put in a few simple assemblers and the tower is what they're building."

"Assemblers? You mean nanomachines?"

She nodded eagerly. "Actually, we put one hundred assemblers into the sand, five days ago."

"And they've built the tower," Paul said.

"They're still building it. Watch real careful and you can see new features being added."

Paul turned and stared at the tower rising out of the lunar sand. It rose perpendicularly from a wide, low base, its flanks smooth and featureless except for small setbacks every foot or so.

"Nothing seems to be happening," he said.

Cardenas peered at the tower. "They stop every once in a while, like they're taking a coffee break. Then they get busy again."

"Don't you know why they stop?"

"For sure." She grinned. "Each time they reach a change in the blueprint we've programmed into them, they stop until the proper members of the team are in the right position to start the new phase of the building."

Paul's eyes widened. "You make them sound as if they're intelligent."

"About as intelligent as bacteria," Cardenas replied.

Paul grunted.

"The assemblers spent the first four days building more of themselves out of the aluminum and silicon in the sand. Yesterday that tower wasn't here."

"No shit," Paul breathed.

"The tower is mainly titanium, y'know. The assemblers are taking titanium atoms preferentially from the sand and using them to build the tower."

"How do they know-"

"It's all programmed into them," Cardenas said. "We did this sort of thing last year, as a demonstration for Mr. Masterson and his father, when they visited here. We didn't use lunar sand then; just beach sand. We built a really neat castle for them."

Paul looked at her. "Could you build more complex stuff?"

Without an eyeblink's hesitation, Cardenas said, "We could build a whole base on the Moon for you, if you give us the time to program the assemblers."

Paul saw that there were a couple of little wheeled typist's chairs by the consoles. He 'pulled one up and sank onto it Cardenas took the other one facihg him.

"Instead of sending tons and tons of heavy machinery to the Moon," she said, leaning toward him, "all you'd have to do is send a sampling of the necessary assemblers. They'll build more of themselves out of the raw materials in the Moon's soil-"

"Regolith," Paul corrected automatically.

"-and then they'll construct your base out of the regolith, " she stressed the word, "all by themselves."

"One shipload of nanomachines," Paul mused.

"Could build your whole base for you," she said.

"How long would it take you to develop the nanomachines? Specifically for Moonbase, I mean."

She waved her hands in the air. "Simple tasks, like building airtight shelter shells and other construction forms, that's pretty easy. When you get down to complicated equipment, like air regenerators and pressure pumps, we'll need a while to program the assemblers."

"A while? How long?"

"Months. Maybe years. We've never tried to build anything very complicated. Not yet It'll take some time."

A new thought struck Paul. "Most of the compounds in the regolith are oxygen-bearing. And there's hydrogen imbedded in the top layers of the regolith, blown in on the solar wind. Could your machines-"

"Produce water out of those atoms? For sure. That's no problem!"

"Jesus H. Christ on a motorcycle."

"You want a motorcycle, we'll build you a motorcycle." Cardenas laughed.

"Maybe we ought to be talking with Harley," Paul kidded back.

"Or General Motors." She was suddenly completely serious.

INFLIGHT.

Paul was in the company helicopter heading back to San Francisco International Airport when his pocket phone beeped.

It was Melissa. "Delta's flight's been cancelled," she said, "and there's nothing connecting to Savannah until late tonight. Can I ride back with you?"

"Sure," Paul said, knowing it was a mistake, not knowing how to say no without feeling like a jerk.

Melissa was waiting for him in the hangar where his twin-engined jet was sheltered. The same plane in which he and Joanna had made love for the first time. Melissa was standing beside the plane, looking slightly forlorn in a baggy pair of tan slacks and a light sweater that hung loosely on her.

"Sorry to impose on you," she said as soon as Paul got to within arm's reach. I'd have to fly the redeye to Atlanta and then make a connection at six tomorrow morning, otherwise."

"It's okay," Paul said. Last night they had been in bed together. But that was last night Melissa picked up her single garment bag. "I know I look a mess. This is my airline outfit. It's for comfort, not looks."

He made a smile for her. "You look fine, kid.

As he walked toward the plane beside Melissa, Paul remembered his elderly grandfather on the day the news broke that the first black president of the United States had been caught in the sack with a woman who was not his wife.

His grandfather had shaken his head mournfully. "See the trouble a man's cock can get him into?"

Yeah, I see, Gramps. But Seeing ain't the same thing as doing.

Paul let Melissa sit in the co-pilot's seat as he slipped on the headset and checked out the plane's instruments. She did not say a word to him as he taxied out to the runway, got clearance for takeoff, and then shoved the throttles forward.

The engines howled joyfully and the plane surged down the runway, faster, faster, the ground blurring as Paul watched the digital airspeed display, then pulled back with an artist's delicate touch to rotate the nose wheel off the concrete. The plane seemed to leap into the air and Paul's heart soared with it.

Once they cleared the airport traffic and Paul put the twin-jet on course eastward, he slipped his earphones down around his neck and turned to Melissa.

"Too bad there's no Clippership service to Savannah," he said.

"When will we get there?"

"Eleven-thirty, eastern time, the way things look now. We'll have to make a pit stop in Amarillo. Gas up."

Melissa nodded. "Beats the redeye."

For a while neither of them said anything. Paul watched the shadows lengthening below as they flew over the mountains with the sun setting behind them.

"Lake Tahoe," he said.

"Uh-huh."

Time went by in silence. Then he pointed out the Grand Canyon, barely visible off in the distance in the twilight haze.

Melissa stared out the window on her side of the cockpit until a cloud bank obscured the ground altogether.

Finally, Paul said hesitantly, "About last night Melissa turned sharply toward him. "Forget it," she said.

"Forget it?"

"It never happened."

Paul felt puzzled. "What d'you mean?"

"You're a married man and you're worried I'm going to shoot my mouth off to Greg or somebody. Well, don't worry about it."

"That's not what I meant."

"Isn't it?"

"Shit, Mel, I wasn't thinking you were a spy for Greg."

"The hell you weren't."

"You told me you two had broken up."

"Yes. That's right."

Paul's befuddlement deepened. Melissa seemed irritable, almost angry.

"Look," he said. "I'm sorry about last night. I shouldn't have done it. I am a married man and-"

"Oh, Paul, it's not your fault. I..." She seemed to want to say more, but stopped.

Paul didn't know what to say. If anything. It was a stupid thing to do, he told himself. If Joanna finds out I'll have hurt her just as bad as Gregory hurt her in the past.

"Do you know why Greg and I broke up?" Melissa asked, her voice so low Paul had to strain to hear her over the muted rumbling of the engines.

"You said it was because of Joanna."

Melissa shook her head slowly. "That's only part of it. I mentioned the magic word, and that drove him off the deep end."

"The magic word?"

"Baby."

Paul wasn't certain he had heard her correctly.

"I told Greg that I wanted his baby," Melissa said sorrowfully. "I told him that when a man and a woman love each other they make a baby together."

"He didn't like the idea."

"I thought he was going to punch me out."

"If he ever lays a hand on you-"

Melissa silenced Paul by laying a slender finger on his lips. "I can take care of myself," she said. "You've got a wife to think about. You can't go around fighting my battles."

But Paul pictured Greg hitting Melissa. Just like the spoiled sonofabitch, he thought. He doesn't love anybody except himself. If he ever touches her I'll punch out his lights, but good.

After they stretched their legs in Amarillo and took off again, Melissa curled up in one of the capacious reclining seats in the plane's cabin and fell asleep. Paul put the plane on autopilot, but flayed in the cockpit, awake, his mind churning with thoughts of Greg and Melissa and Joanna and the nanomachines that could make Moonbase a going proposition if only he could hammer the idea through the board of directors. But Greg was going to use the next meeting to accuse him of murder, or at least fornication. How can he attack me without attacking his mother? Then Paul realized that Greg was so furious with blind hate that he wanted to hurt Joanna, punish her for falling in love with a black man.

It was almost midnight when Paul finally put the twin-jet down on the company's airstrip, a few miles from Savannah. He was tired, drained physically and emotionally. Gratefully, he saw that the limo was there at the apron in front of the hangar, waiting for him.

Paul helped Melissa down the little metal ladder to the concrete of the apron. When he turned back toward the limousine, he saw that Joanna was standing beside it, staring at them.

MARE NUBIUM.

Do I have enough oxygen to make it? Paul asked himself that question again and again as he struggled across the rocky undulating lunar plain, trying to make up for the time and distance he had lost by straying so far off course.

He pushed himself harder. "Gotta get smokin' now," he told himself. "Gotta get there before the oxy runs out."