"Who?" Bigman demanded at once.

But it was Lucky who answered. "Obviously he thinks it's one of us."

"Thanks!" said Panner. "How would you explain it? You mentioned stowaways; you talked about people forcing their way on board the Jovian Moon. Talk about nerve! Aren't there two people who did force their way on board? Didn't I witness the process? You two!"

"True enough," said Lucky.

"And you brought me down here so you could investigate every inch of the ship's workings. You tried to keep me busy with stories about robots hoping I wouldn't notice that you two were going over the whole ship with a microscope."

Bigman said, "We have a right to do it. This is Lucky Starr!"

"He says he's Lucky Starr. If he's a member of the Council of Science, he can prove it and he knows how. If I had any brains, I'd have demanded identification before taking you down."

"It's not too late now," Lucky said calmly. "Can you see clearly from that distance?" He held up one arm, palm forward, and peeled the sleeve back.

"I'm not coming any closer," Panner said angrily.

Lucky said nothing to that. He let his wrist tell the story. The skin along the inner surface of his wrist seemed merely exposed skin, but years before it had been treated hormonally in a most complicated fashion. Responding to nothing more than a disciplined effort of Lucky's will, an oval spot on the wrist darkened and slowly turned black. Within it, little yellow specks formed in the familiar patterns of the Big Dipper and of Orion.

Panner gasped as though the breath had been forcibly knocked out of his lungs. Few human beings had the occasion to see this sign of the Council, but all above the age of childhood knew it for what it was- the final and unforgeable identification insigne of the councilman of science.

Panner was left with no choice. Silently, reluctantly, he released the force field and stepped back.

Bigman came out, raging, "I ought to bend in your skull, you lopsided-"

Lucky pulled him back. "Forget it, Bigman. The man had as much right to suspect us as we had to suspect him. Settle down."

Panner shrugged. "It seemed logical."

"I admit it did. I think we can trust each other now."

"You, maybe," the chief engineer said pointedly. "You're identified. What about this little loudmouth with you? Who identifies him?"

Bigman squawked incoherently and Lucky stepped in between the two. "I identify him and take full responsibility for him... Now I propose that we get back to passenger quarters before a search is organized for us. Everything that went on down here is, of course, strictly confidential."

Then, as though nothing had happened, they resumed the climb upward.

The room assigned to them contained a two-decker bed and a washstand out of which a small trickle of water could be urged. Nothing more. Even the cramped and Spartan quarters on board the Shooting Starr were luxury to this.

Bigman sat cross-legged on the upper bed, while Lucky sponged his neck and shoulders. They talked in whispers, conscious of the listening ears that might be present on the other side of the walls.

Bigman said, "Look, Lucky, suppose I go up to each person on board ship; I mean, each of the ten we don't know about? Suppose I deliberately pick a fight with each one, call them a few names, things like that? Wouldn't it turn out that the guy who doesn't take a punch at me is the robot?"

"Not at all. He might not want to break shipboard discipline, or he might know what a handy fellow you are with a needle-gun, or he might not want to get into a wrangle with the Council of Science, or he might Just not like to hit a man smaller than himself."

"Aw, come on, Lucky." Bigman was silent for a minute, then he said cautiously, ''I've been thinking; how can you be sure the robot is aboard ship? I keep thinking maybe it stayed back on Jupiter Nine. It's possible."

"I know it's possible and yet I'm sure the robot is here on board ship. That's just it. Fm sure and I don't know why I'm sure," said Lucky, his eyes dark with thought. He leaned against the bed and tapped his teeth with the knuckle of one finger. "That first day we landed on Jupiter Nine, something happened."

"What?"

"If I only knew! I had it; I knew what it was, or thought I did, just before I went to sleep that night, and it vanished. I haven't been able to get it back. If I were on Earth, I'd submit to a psycho-probe. Great Galaxy, I swear I would!

''I've tried every trick I could. Thinking hard, getting my mind off it altogether. When we were with Panner down in the engine levels, I tried talking my fool head off. I thought if I would just keep discussing every aspect of the matter, the thought was bound to pop into my head. It didn't.

"But it's there just the same. It's because of the thought that I must feel so sure the robot is one of the men aboard ship. I've made the subconscious deduction. If I could only put my finger on it, I'd have the whole answer. If I could only put my finger on it"

He sounded almost despairing.

Bigman had never seen Lucky with quite that look of frustrated loss in his face. He said, worried, "Hey, we'd better get some sleep."

"Yes, we'd better."

Minutes later, in the darkness, Bigman whispered, "Hey, Lucky, what makes you so sure I'm not the robot myself?"

Lucky whispered back, "Because the Sirians couldn't bear to build a robot with such an ugly face," and lifted his elbow to ward off a flying pillow.

The days passed. Halfway to Jupiter, they passed the inner and more sparsely populated belt of small moons, of which only Six, Seven, and Ten were numbered. Jupiter Seven was visible as a bright star, but the others were far enough away to melt into the background of the constellations.

Jupiter itself had grown to the size of the moon as seen from Earth. And because the ship was approaching the planet with the sun squarely to its rear, Jupiter remained in the "full" phase. Its entire visible surface was ablaze with sunlight. There was no shadow of night advancing across it.

Yet though the size of the moon, it was not so bright as the moon by any means. Its cloud-decked surface reflected eight times as much of the light that reached it, as did the bare powdered rock of the moon. The trouble was that Jupiter only received one twenty-seventh of the light per square mile that the moon did. The result was that it was only one third as bright at that moment as the moon appeared to be to human beings on Earth.

Yet it was more spectacular than the moon. Its belts had become quite distinct, brownish streaks with soft fuzzy edges against a creamy-white background. It was even easy to make out the flattened straw-colored oval that was the Great Red Spot as it appeared at one edge, crossed the face of the planet, then disappeared at the other.

Bigman said, "Hey, Lucky, Jupiter looks as though it isn't really round. Is that just an optical illusion?"

"Not at all," said Lucky. "Jupiter really isn't round. It's flattened at the poles. You've heard that Earth is flattened at the poles, haven't you?"

"Sure. But not enough to notice."

"Of course not. Consider! Earth is twenty-five thousand miles about its equator and rotates in twenty-four hours, so that a spot on its equator moves just over a thousand miles an hour. The resulting centrifugal force bulges the equator outward so that the diameter of the Earth across its middle is about twenty-seven miles more than the diameter from North Pole to South Pole. The difference in the two diameters is only about a third of one per cent so that from space Earth looks like a perfect sphere."

"Oh."

"Now take Jupiter. It is 276,000 miles about its equator, eleven times the circumference of Earth, yet it rotates about its axis in only ten hours; five minutes less than that, to be exact. A point on its equator is moving at a speed of almost twenty-eight thousand miles an hour; or twenty-eight times as fast as any point on Earth. There's a great deal more centrifugal force and a much larger equatorial bulge, especially since the material in Jupiter's outer layers is much lighter than that hi the Earth's crust. Jupiter's diameter across its equator is nearly six thousand miles more than its diameter from North Pole to South Pole. The difference in the diameters is a full fifteen per cent, and that's an easy thing to see."

Bigman stared at the flattened circle of light that was Jupiter and muttered, "Sands of Mars!"

The sun remained behind them and unseen as they sank toward Jupiter. They crossed the orbit of Callisto, Jupiter Four, outermost of Jupiter's major satellites, but did not see it to advantage. It was a world one and a half million miles from Jupiter and as large as Mercury, but it was on the other side of its orbit, a small pea close to Jupiter and heading into eclipse in its shadow.

Ganymede, which was Jupiter Three, was close enough to show a disc one third as wide as the moon seen from Earth. It lay off to one side so that part of its night surface could be seen. It was three quarters full even so, pale white, and featureless.

Lucky and Bigman found themselves ignored by the rest of the crew. The commander never spoke to them or even looked at them, but moved past with eyes fixed on nothingness. Norrich, when he was led past by Mutt, nodded cheerfully as he always did when he detected the presence of humans. When Bigman answered the greeting, however, the pleasant look vanished from his face. A gentle pressure on Mutt's harness started the dog moving and he was gone.

The two found it more comfortable to eat in their own quarters.

Bigman grumbled. "Who in space do they think they are? Even that guy Panner gets busy all at once when I'm around."

Lucky said, "In the first place, Bigman, when the commander makes it so obvious that we're in his bad books, subordinates don't fall over themselves being friendly. Secondly, our dealings with a few of the men have been unpleasant."

Bigman said thoughtfully, "I met Red Summers today, the cobber. There he was coming out of the engine room and there I was, facing him."

"What happened? You didn't..."

"I didn't do anything. I just stood there waiting for him to start something, hoping he would start something, but he just smiled and moved around me."

Everyone aboard the Jovian Moon was watching the day Ganymede eclipsed Jupiter. It wasn't a true eclipse.

Ganymede covered only a tiny part of Jupiter. Ganymede was 600,000 miles away, not quite half the size of the moon as seen from Earth. Jupiter was twice the distance, but it was a swollen globe now, fourteen times as wide as Ganymede, menacing and frightening.

Ganymede met Jupiter a little below the latter's equator, and slowly the two globes seemed to melt together. Where Ganymede cut in, it made a circle of dimmer light, for Ganymede had far less of an atmosphere than Jupiter had and reflected a considerably smaller portion of the light it received. Even if that had not been so, it would have been visible as it cut across Jupiter's belts.

The remarkable part was the crescent of blackness that hugged Ganymede's rear as the satellite moved completely onto Jupiter's disk. As the men explained to one another in breathless whispers, it was Ganymede's shadow falling on Jupiter.

The shadow, only its edge seen, moved with Ganymede, but slowly gained on it. The sliver of black cut finer and finer until in the mid-eclipse region, when Jupiter, Ganymede, and the Jovian Moon all made a straight line with the sun, the shadow was completely gone, covered by the world that cast it.

Thereafter, as Ganymede continued to move on, the shadow began to advance, appearing before it, first a sliver, then a thicker crescent, until both left Jupiter's globe.

The entire eclipse lasted three hours.

The Jovian Moon reached and passed the orbit of Ganymede when that satellite was at the other end of its seven-day orbit about Jupiter.

There was a special celebration when that happened. Men with ordinary ships (not often, to be sure) had reached Ganymede and landed on it, but no one, not one human being, had ever penetrated closer than that to Jupiter. And now the Jovian Moon did.

The ship passed within one hundred thousand miles of Europa, Jupiter Two. It was the smallest of Jupiter's major satellites, only nineteen hundred miles in diameter. It was slightly smaller than the moon, but its closeness made it appear twice the size of the moon as seen from Earth. Dark markings could be made out that might have been mountain ranges. Ship's telescopes proved they were exactly that. The mountains resembled those on Mercury, and there was no sign of moon-like craters. There were brilliant patches, too, resembling ice fields.

And still they sank downward, and left Europa's orbit behind.

Io was the innermost of Jupiter's major satellites, in size almost exactly equal to Earth's moon. Its distance from Jupiter, moreover, was only 285,000 miles, or little more than that of the moon from Earth.

But there the kinship ended. Whereas Earth's gentle gravitational field moved the moon about itself in the space of four weeks, Io, caught in Jupiter's gravity, whipped about in its slightly larger orbit in the space of forty-two hours. Where the moon moved about Earth at a speed of a trifle over a thousand miles an hour, Io moved about Jupiter at a speed of twenty-two thousand miles an hour, and a landing upon it was that much more difficult.

The ship, however, maneuvered perfectly. It cut in ahead of Io and wiped out Agrav at just the proper moment.

With a bound, the hum of the hyperatomics was back, filling the ship with what seemed a cascade of sound after the silence of the past weeks.

The Jovian Moon curved out of its path, finally, subject once again to the accelerating effect of a gravitational field, that of lo. It was established in an orbit about the satellite at a distance of less than ten thousand miles, so that lo's globe filled the sky.

They circled about it from dayside to nightside, coming lower and lower. The ship's batlike Agrav fins were retracted in order that they might not be torn off by Io's thin atmosphere.

Then, eventually, there was the keen whistling that came with the friction of ship against the outermost wisps of that atmosphere.

Velocity dropped and dropped; so did altitude. The ship's sidejets curved it to face stern-downward toward Io, and the hyperatomic jets sprang into life, cushioning the fall. Finally, with one last bit of drop and the softest jar, the Jovian Moon came to rest on the surface of Io.

There was wild hysteria on board the Jovian Moon. Even Lucky and Bigman had their backs pounded by men who had been avoiding them constantly all voyage long.

One hour later, in the darkness of Io's night, with Commander Donahue in the lead, the men of the Jovian Moon, each in his space suit, emerged one by one onto the surface of Jupiter One.

Sixteen men. The first human beings ever to land on Io!

Correction, thought Lucky. Fifteen men.

And one robot!