Lucky jumped recklessly into the exposed river, drifting gently downward under the pull of Io's weak gravity. He was angry at the slowness of his fall, at Bigman for the childish enthusiasms that seized him so suddenly, and-unpredictably-at himself for not having stopped Bigman when he might.

Lucky hit the stream, and ammonia sprayed high in the air, then fell back with surprising quickness. Io's thin atmosphere could not support the small droplets even at low gravity.

There was no sense of buoyancy to the ammonia river. Lucky had not expected any to speak of. Liquid ammonia was less dense than water and had less lifting power. Nor was the force of the current great under Io's weak pull. Had Bigman not damaged his air hose, it would have been only a matter of walking out of the river and through any of the drifts that might have packed it round.

As it was...

Lucky splashed downstream furiously. Somewhere ahead the small Martian must be struggling feebly against the poisonous ammonia. If the break in the hose was large enough, or had grown large enough, to allow liquid ammonia to enter, Lucky would be too late.

He might be too late, already, and his chest constricted and tightened at the thought.

A form streaked past Lucky, burying itself in the powdered ammonia. It disappeared, leaving a tunnel into which ammonia slowly collapsed.

"Panner," Lucky said tentatively.

"Here I am." The engineer's arm fell upon Lucky's shoulder from behind. "That was Mutt. He came running when you yelled. We were both on his wave length."

Together they forged through the ammonia on the track of the dog. They met him, returning.

Lucky cried eagerly, "He's got Bigman."

Bigman's arms feebly enfolded the dog's suit-encased haunches, and though that hampered Mutt's movements, low gravity enabled the dog to make respectable headway through use of shoulder muscles alone.

Even as Lucky bent for Bigman, the little Martian's straining hold relaxed and he fell.

Lucky scooped him up. He wasted no time on investigation or talk. There was only one thing to do. He turned up Bigman's oxygen flow to full capacity, slung him over his shoulders, and ran for the ship. Even allowing for Io's gravity he had never run so recklessly in his life. With such haste did he kick the ground away when coming down from each hurtling, horizontal stride that the effect was almost one of low-level flying.

Panner pumped along in the rear, and Mutt stayed excitedly at Lucky's heels.

Lucky used the communal wave length to alert the others even as he was running and one of the air tights was made ready.

Lucky hurtled inside the air tight, scarcely breaking his stride. The flap closed behind him and the interior flooded with additional air under pressure to make up the loss during the flap's opening.

With flying fingers he unbuckled Bigman's helmet, then more slowly drew off the rest of the suit.

He felt for the heartbeat and, to his relief, found it. The air tight was equipped, of course, with a first-aid kit. He made the necessary injections for general stimulation and waited for warmth and plentiful oxygen to do the rest.

And eventually Bigman's eyes fluttered and focused with difficulty on Lucky. His lips moved and made the word "Lucky," though no sound was involved.

Lucky laughed with relief and finally took the time to remove his own space suit.

On board the Jovian Moon Harry Norrich stopped at the open door of the compartment within which Big-man was completing his recuperation. His unseeing, china-blue eyes were warm with pleasure.

"How's the invalid?"

Bigman struggled up in his bunk and shouted, "Fine! Sands of Mars, I feel great! If it weren't that Lucky wants to keep me down, I'd be up and around."

Lucky grunted his disbelief.

Bigman ignored that. He said, "Hey, let Mutt come in. Good old Mutt! Here, boy, here!"

Mutt, the hold on his harness released, trotted over to Bigman, his tail wagging furiously and his intelligent eyes doing everything but talk a greeting.

Bigman's small arm embraced the dog's neck in a bear hug. "Boy, there's a friend. You heard what he did, Norrich, didn't you?"

"Everyone did," and it was plain to see that Norrich took a great personal pride in his dog's accomplishment.

"I just barely remember it," Bigman said, "before I blacked out altogether. I got that lungful of ammonia and couldn't seem to straighten out. I rolled downhill, just going through the ammonia snow as though it were nothing. Then there was this thing coming at me and I was sure it was Lucky when I heard the sound of something moving. But he knocked enough of the snow off us to let some of the Jupiter light come in and I could just make out it was Mutt. The last thing I remember was grabbing him."

"And a good thing, too," Lucky said. "The extra time that would have been required for me to find you would have been your finish."

Bigman shrugged. "Aw, Lucky, you make such a big deal out of it. Nothing would have happened if I hadn't just caught the hose on a rock and torn it. At that if I had had enough brains to turn up my oxygen pressure, I could have kept the ammonia out. It was just the first lungful that seemed to put me out of kilter. I couldn't think."

Panner passed by, just then, and looked. "How are you, Bigman?"

"Sands of Mars! Looks like everyone thinks I'm an invalid or something. There's nothing wrong with me.

Even the commander stopped by and managed to find his tongue long enough to grunt at me."

"Well," said Panner, "maybe he's getting over his mad."

"Never," said Bigman. "He just wants to make sure his first flight won't be spoiled by a casualty. He wants his record pure white, that's all."

Panner laughed. "All set for the take-off?"

Lucky said, "Are we leaving Io?"

"Any hour. The men are reloading the equipment we're taking with us and securing what we leave behind. If you two can make the pilot room once we're underway, do so. We'll get a better look at Jupiter than ever."

He tickled Mutt behind one ear and left

They radioed Jupiter Nine that they were leaving Io, as days earlier they had radioed that they had surfaced on the satellite.

Bigman said, "Why don't we call Earth? Chief Councilman Conway ought to know we've made it."

"Officially," said Lucky, "we haven't made it all the way until we've returned to Jupiter Nine."

He did not add aloud that he was not at all anxious to return to Jupiter Nine, still less anxious to talk to Conway. He had, after all, accomplished nothing on this trip,

His brown eyes surveyed the control room. The engineers and crewmen were at their stations for the takeoff. The commander, his two officers and Panner, however, were in the control room.

Lucky wondered again about the officers as time and again he had wondered about each of the ten men whom the V-frog had not had a chance to eliminate.

He had spoken to each of them on occasion, as had Panner even more frequently. He had searched thek quarters. He and Panner together had gone over thek records. Nothing had resulted.

He would be going back to Jupiter Nine with the robot unlocated, and thereafter location would be harder than ever and he might have to report back to Council headquarters with news of failure.

Once more, desperately, the thought of X rays entered his mind, or some other means of forceful inspection. As always, he thought at once of the possibility of triggering off an explosion, probably a nuclear explosion.

It would destroy the robot. It would also kill thir-teen men and blow up a priceless ship. Worst of all, it would show no safe way of detecting the humanoid robots which, Lucky felt certain, were preying in other parts of the Solar Confederation.

He was startled by Panner's sudden cry, "Here we go!"

There was the familiar distant whoosh of the initial thrust, the gathering backward press of accelerations, and Io's surface dropped away, faster and faster.

The visiplate could not center Jupiter in its entirety:" it was too large. It centered the Great Red Spot instead and followed it in its rotation about the globe.

Panner said, "We've gone into Agrav again, yes, but it's only temporary, just to let Io pull away from us."

"But we're still falling toward Jupiter," Bigman said.

"That's right, but only till the proper moment is reached. Then we go into hyperatoroic drive and plunge toward Jupiter on a hyperbolic orbit. Once that is established, we cut the drive and let Jupiter do the work. Our closest approach will be about 150,000 miles. Jupiter's gravity will zoom us around as though we were a pebble in a slingshot and shoot us out again. At the proper point our hyperatomic drive cuts in again. By taking advantage of the slingshot effect, we actually save a bit on energy over the alternative of leaving directly from Io, and we get some super close-ups of Jupiter."

He looked at his watch. "Five minutes," he said.

He was referring, as Lucky knew, to the moment when the ship would switch from Agrav to hyper-atomic drive and begin to curve off into the planned orbit about Jupiter.

Still staring at his watch, Panner said, "The time is selected so that we come out heading toward Jupiter Nine as squarely as possible. The fewer side adjustments we have to make, the more energy we save. We've got to come back to Jupiter Nine with as much of our original energy store as possible. The more we come back with, the better Agrav looks. I've set my goals at eighty-five per cent. If we can come back with ninety, that would be superlative."

Bigman said, "Suppose you come back with more energy than you had when you left? How would that be?"

"Super-superlative, Bigman, but impossible. There's something called the second law of thermodynamics that stands in the way of making a profit on the deal or, for that matter, of breaking even. We've got to take some loss." He smiled broadly and said, "One minute."

And at the appropriate second the sound of the hyperatomics filled the ship with its muted murmurings, and Panner placed his watch in his pocket with a satisfied expression.

"From here on in," he said, "until actual landing maneuvers at the Jupiter Nine approach, everything is quite automatic."

He had no sooner said that when the humming ceased again, the lights in the room flickered and went out. Almost at once they went on again, but now there was a little red sign on the control panel that said, emergency.

Panner sprang to his feet. "What in Space...?''

He left the pilot room at a run, leaving the others staring after him and at one another in various degrees of horror. The commander had gone dead-white, Ms lined face a tired mask.

Lucky, with sudden decision, followed Panner, and Bigman, of course, followed Lucky.

They came upon one of the engineers clambering out of the engine compartment. He was panting. "Sir!"

"What is it, man?" snapped Panner. "The Agrav is off, sir. It can't be activated."

"What about the hyperatomics?"

"The main reserve is shorted. We cut it just in time to keep it from blowing. If we touch it, the whole ship will go up. Every bit of the stored energy will blow."

''Then we're working on the emergency reservoir?"

''That's right."

Panner's swarthy face was congested with blood. "What good is that? We can't set up an orbit about Jupiter with the emergency reservoir. Out of the way. Let me down there."

The engineer stepped aside, and Panner swung into the shaft. Lucky and Bigman were at his heels.

Lucky and Bigman had not been in the engine compartment since that first day aboard the Jovian Moon. The scene was different now. There was no august silence, no sensation of mighty forces quietly at work.

Instead, the puny sound of men rose high about them.

Panner sprang off into the third level. "Now what's wrong?" he called. "Exactly what's wrong?"

Men parted to let him through and they all huddled over the gutted insides of a complex mechanism, pointing things out in tones of mingled despair and anger.

There were sounds of other footsteps coming down the rungs of the shaft, and then the Commander himself made his appearance.

He spoke to Lucky, who was standing gravely to one side. "What is it, Councilman?" It was the first time he had addressed Lucky since they had left Jupiter Nine.

Lucky said, "Serious damage of some sort, Commander."

"How did it happen? Panner!"

Panner looked up from the close examination of something that had been held out to him. He shouted in annoyance, "What in space do you want?"

Commander Donahue's nostrils flared. "Why has something been allowed to go wrong?"

"Nothing has been allowed to go wrong."

"Then what do you call this?"

"Sabotage, Commander. Deliberate, murdering sabotage!"

"What!"

"Five gravitic relays have been completely smashed and the necessary replacements have been removed and can't be located. The hyperatomic thrust-control has been fused and shorted beyond repair. None of it happened by accident."

The commander stared at his chief engineer. He said, hollowly, "Can anything be done?"

"Maybe the five relay replacements can be located or cannibalized out of the rest of the ship. I'm not sure. Maybe a makeshift thrust-control can be set up. It would take days anyway and I couldn't guarantee results."

"Days!" cried the commander. "It can't take days. We're fatting toward Jupiter!"

There was a complete silence for a few moments, and then Panner put into words what all of them knew. "That's right, Commander. We're failing toward Jupiter and we can't stop ourselves in time. It means we're through, Commander. We're all dead men!"