Lucky's smile expanded, and he rapped Bigman's skull with two knuckles. "Anybody home?"

Bigman's fist whipped toward Lucky's abdomen, but Lucky caught it in midair and held the little fellow motionless.

"If s as simple as this, Bigman. Before space travel was invented, men were restricted to Earth and all they knew about Jupiter was what they could see in a telescope. The satellites are numbered in the order they were discovered, see?"

"Oh," said Bigman, and yanked free. "Poor ancestors!" He laughed, as he always did, at the thought of human beings cooped up on one world, peering out longingly, even as he struggled to free himself from Lucky's grip.

Lucky went on. "The four big satellites of Jupiter are numbered One, Two, Three, and Four, of course, but the numbers are hardly ever used. The names Io, Europa, Ganymede, and Callisto are familiar names. The nearest satellite of all, a small one, is Jupiter Five, while the farther ones have numbers up to Twelve. The ones past Twelve weren't discovered till after space travel was invented and men had reached Mars and the asteroid belt… Watch out now. We've got to adjust for landing."

It was amazing, thought Lucky, how you could consider tiny a world eighty-nine miles in diameter as long as you were nowhere near it. Of course, such a world is tiny compared to Jupiter or even to Earth. Place it gently on Earth and its diameter is small enough to allow it to fit within the state of Connecticut without lapping over; and its surface area is less than that of Pennsylvania.

And yet, just the same, when you came to enter the small world, when you found your ship enclosed in a large lock and moved by gigantic grapples (working against a gravitational force of almost zero but against full inertia) into a large cavern capable of holding a hundred ships the size of the Shooting Starr, it no longer seemed so small.

And then when,you came across a map of Jupiter Nine on the wall of an office and studied the network of underground caverns and corridors within which a complicated program was being carried out, it began to seem actually large. Both horizontal and vertical projections of the work volume of Jupiter Nine were shown on the map, and though only a small portion of the satellite was being used, Lucky could see that some of the corridors penetrated as much as two miles beneath the surface and that others spread out just under the surface for nearly a hundred miles.

"A tremendous job," he said softly to the lieutenant at his side.

Lieutenant Augustus Nevsky nodded briefly. His uniform was spotless and gleaming. He had a stiff little blond mustache, and his wide-set blue eyes had a habit of staring straight ahead as though he were at perpetual attention.

He said with pride, "We're still growing."

He had introduced himself a quarter of an hour earlier, as Lucky and Bigman had stepped from the ship, as the personal guide assigned them by Commander Donahue.

Lucky said with some amusement, "Guide? Or guardian, Lieutenant? You are armed."

Any trace of feeling was carefully washed out of the other's face. "My arms are regulation for officers on duty, Councilman. You will find you will need a guide here."

But he seemed to relax, and there was ordinary human feeling about him as he listened to the visitors' awed praise of the project. He said, "Of course the absence of any significant gravitational field makes certain engineering tricks feasible that wouldn't work on Earth. Underground corridors require practically no support."

Lucky nodded, then said, "I understand that the first Agrav ship is about ready for take-off."

The lieutenant said nothing for a moment. His face blanked free, again, of emotion or feeling. Then he said stiffly, "I will show you your quarters first. It can be most easily reached by Agrav, if I can persuade you to use an Agrav cor-"

"Hey, Lucky," called Bigman in sudden excitement. "Look at this."

Lucky turned. It was only a half-grown cat, gray as smoke, with the look of solemn sadness that cats usually have, and a back that arched readily against Bigman's curved fingers. She was purring.

Lucky said, "The commander said they went for pets here. Is this one yours, Lieutenant?"

The officer flushed. "We all have shares in it. There are a few other cats around, too. They come on the supply ships sometimes. We've got some canaries, a parakeet, white mice, goldfish. Things like that. Nothing like your whatever-it-is, though." And his eyes, as they looked quickly at the V-frog's bowl tucked under Lucky's arm, contained a spark of envy.

But Bigman was concentrating on the cat There was no native animal life on Mars and the furry pets of Earth always had the charm of novelty to him.

"He likes me, Lucky."

"It's a she," said the lieutenant, but Bigman paid no attention. The cat, tail hoisted into a stiff vertical with only the tip drooping, walked past him, doubling sharply so as to present first one side, then the other, to Bigman's gentle stroking

And then the purring stopped, and through Bigman's mind stabbed one pure touch of fevered and hungry desire.

It startled him for a moment, and then he noticed that the cat had stopped purring and was squatting slightly in the tense hunting posture dictated by its millions-of-years-old instincts.

Her green slitted eyes stared directly at the V-frog. But the emotion, so feline in its touch, was gone almost as soon as it had come. The cat padded softly over to the glass container Lucky was holding and stared in curiously, purring with contentment. The cat, too, liked the V-frog. It had to. Lucky said, "You were saying, Lieutenant, we would have to reach our quarters by Agrav. Were you going to explain what that means?"

The lieutenant, who had also been staring fondly at the V-frog, paused to gather his wits before answering. "Yes. It's simple enough. We have artificial gravity fields here on Jupiter Nine as on any asteroid or on any space ship for that matter. They are arranged at each of the main corridors, end to end, so that you can fall the length of them in either direction. It's like dropping straight down a hole on Earth." Lucky nodded. "How fast do you drop?" "Well, that's the point. Ordinarily, gravity pulls constantly and you fall faster and faster…"

"Which is why I ask my question," interposed Lucky dryly.

"But not under Agrav controls. Agrav is really A-grav: no gravity, you see. Agrav can be used to absorb gravitational energy or store it or transfer it. The point is you only fall so fast, you see, and no faster.

With a gravitational field in the other direction, too, you can even slow down. An Agrav corridor with two pseudo-grav fields is very simple and it has been used as a steppingstone to an Agrav ship which works in a single gravitational field. Now Engineers' Quarters, which is where your rooms will be, is only a little over a mile from here and the most direct route is by Corridor A-2. Ready?"

"We will be once you explain how we're to work Agrav."

"That's hardly a problem." Lieutenant Nevsky presented each with a light harness, adjusting them over the shoulders and at the waist, talking rapidly about the controls.

And then he said, "If you'll follow me, gentlemen, the corridor is just a few yards in this direction."

Bigman hesitated at the opening of the corridor. He was not afraid of space in itself, or of drops in themselves. But all his life he had been used to bridging gaps under Martian gravity or less. This time the pseudo-grav field was at full Earth-normal, and under its influence the corridor was a brilliantly lighted hole, plummeting, apparently, straight downward, even though in actuality (Bigman's mind told him) it paralleled the satellite's surface closely.

The lieutenant said, "Now this is the lane for travel in the direction of Engineers' Quarters. If we were to approach from the other side, 'down' would appear to be in the other direction. Or we could make 'up' and 'down' change places by appropriate adjustments of our Agrav controls."