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."