"Good deal," said Bigman fiercely. "Then we're on our way."

"Hold it," said Lucky at once. "Hold everything, my friend. This is a non-specific weapon. We'll sense strong emotion but we may never sense one that will give us the key to the mystery. It's like having eyes. We may see, but we may not see the right thing, not ever."

"You will," said Bigman confidently.

Dropping down toward Jupiter Nine reminded Bigman very strongly of similar maneuvers in the asteroid belt. As Lucky had explained on the voyage outward, most astronomers considered Jupiter Nine to have been a true asteroid to begin with; a rather large one that had been captured by Jupiter's tremendous gravity field many millions of years previously.

In fact, Jupiter had captured so many asteroids that here, fifteen million miles from the giant planet, there was a kind of miniature asteroid belt belonging to Jupiter alone. The four largest of these asteroid satellites, each from forty to a hundred miles in diameter, were Jupiter Twelve, Eleven, Eight, and Nine. In addition there were at least a hundred additional satellites of more than a mile in diameter, unnumbered and unregarded. Their orbits had been plotted only in the last ten years when Jupiter Nine was first put to use as an anti-gravity research center, and the necessity of traveling to and from it had made the population of surrounding space important.

The approaching satellite swallowed the sky and became a rough world of peaks and rocky channels, un-softened by any touch of air in the billions of years of its history. Bigman, still thoughtful, said, "Lucky, why in Space do they call this Jupiter Nine, anyway? It isn't the ninth one out from Jupiter according to the Atlas. Jupiter Twelve is a lot closer."

Lucky smiled. "The trouble with you, Bigman, is that you're spoiled. Just because you were born on Mars, you think mankind has been cutting through space ever since creation. Look boy, it's only a matter of a thousand years since mankind invented the first spaceship."

"I know that," said Bigman indignantly. "I'm not ignorant. I've had schooling. Don't go shoving your big brain all over the place."

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."

He looked at the expression on Bigman's face and said, "You'll get the idea as you go along. It becomes second nature after a while."

He stepped into the corridor and didn't drop an inch. It was as though he were standing on an invisible platform.

He said earnestly, "Have you set the dial at zero?''

Bigman did so, and instantly all sensation of gravity vanished. He stepped into the corridor.

Now the lieutenant's hand on the central knob of his own controls turned it sharply, and he sank, gathering speed. Lucky followed him, and Bigman, who would sooner have fallen the length of the corridor under double gravity and been smashed to pulp than fail to do anything Lucky did, took a deep breath and let himself fall.

"Turn back to zero," called the lieutenant, "and you'll be moving at constant velocity. Get the feel of it"

Periodically they approached and passed through luminous green letters that glowed keep to this side, Once there was the flash of a man passing (falling, really) in the other direction. He was moving much more rapidly than they were.

"Are there ever any collisions, Lieutenant?" asked Lucky.

"Not really," said the lieutenant. "The experienced dropper watches for people who might be overtaking him or whom he might be overtaking, and it's easy enough to slow down or speed up. Of course the boys will bump on purpose sometimes. It's a kind of rowdy fun that ends with a broken collarbone sometimes." He looked quickly at Lucky. "Our boys play rough."

Lucky said, "I understand. The commander warned me."

Bigman, who had been staring downward through the well-lit tunnel into which he was sinking, cried in sudden exhilaration, "Hey, Lucky, this is fun when you get used to it," and turned his controls into the positive region.

He sank faster, his head moving down to a level with Lucky's feet, then farther down at an increasing rate.

Lieutenant Nevsky cried out in instant alarm, "Stop that, you fool. Turn back into the negatives!"

Lucky called out an imperious, "Bigman, slow down!"

They caught up to him, the lieutenant angrily exclaiming. "Don't ever do that! There are all sort of barriers and partitions along these corridors, and if you don't know your way, you'll be slamming into one just when you think you're safe."

"Here, Bigman," said Lucky. "Hold the V-frog. That will give you some responsibility and make you behave, perhaps,"

"Aw, Lucky," said Bigman, abashed. "I was just kicking my heels a bit. Sands of Mars, Lucky..."

"All right," said Lucky. "No harm done," and Bigman brightened at once.

Bigman looked down again. Falling at a constant rate was not quite the same as free fall in space. In space, nothing seemed to move. A space ship might be traveling at a velocity of hundreds of thousands of miles an hour and there would still be the sensation of motion-lessness all about. The distant stars never moved.

Here, though, the sense of motion was all about. The lights and openings and various attachments that lined the corridor walls flashed past.

In space, one expected that there would be no "up" and "down," but here there was none either and it seemed wrong. As long as he looked "down" past his feet, it seemed "down" and that was all right. When he looked "up," however, there would be a quick sensation that "up" was really "down," that he was standing head downward falling "up." He looked toward his feet again quickly to get rid of the sensation.

The lieutenant said, "Don't bend too far forward, Bigman. The Agrav works to keep you lined up in the direction of fall, but if you bend over too much, you'll start tumbling."

Bigman straightened.

The lieutenant said, "There's nothing fatal about tumbling. Anyone who's used to Agrav can straighten himself out again. Beginners would find it troublesome, however. We'll decelerate now. Move the dial into the negatives and keep it there. About minus five."

He was slowing as he spoke, moving above them. His feet dangled at Bigman's eye level.

Bigman moved the dial, trying desperately to line himself up with the lieutenant. And as he slowed, "up" and "down" became definite, and in the wrong way. He was standing on his head.

He said, "Hey, the blood's rushing to my head."

The lieutenant said sharply, "There are footholds along the sides of the corridor. Hook one with the toe of your foot as you reach it and let go quickly."

He did so as he said this. His head swung outward, and head and feet reversed position. He continued swinging and stopped himself with a quick hand tap against the wall.

Lucky followed suit, and Bigman, flailing widely with his short legs, managed to catch one of the footholds at last. He whirled sharply and caught the wall with his elbow just a trifle too hard for comfort but managed to line up properly.

At least he was head-up again. He wasn't falling any more, but rising, as though he had been shot out of a cannon and rising against gravity more and more slowly; but at least he was head-up.

When they were moving at a slow crawl, Bigman, looking uneasily toward his feet, thought: We're going to be falling again. And suddenly the corridor looked like an endlessly deep well and his stomach tightened.

But the lieutenant said, "Adjust to zero," and at once they stopped slowing down. They just moved upward, as though in a smooth, slow elevator, until they reached a cross-level at which the lieutenant, seizing a foothold with one toe, brought himself to a feathery stop.

"Engineers' Quarters, gentlemen," he said.

"And," added Lucky Starr gently, "a reception committee."

For men were waiting for them in the corridor now, fifty of them at least.

Lucky said, "You said they liked to play rough, Lieutenant, and maybe they want to play now."

He stepped firmly out into the corridor. Bigman, nostrils flaring with excitement and grateful to be on the firm pseudo-grav of a solid floor, clutched the V-frog's cage tightly and was at Lucky's heels, facing the waiting men of Jupiter Nine.