It was Jupiter they stopped to look at. It was Jupiter that held them frozen. There was no talk about it, no babble over the helmet radios. It was beyond talk.

Jupiter was a giant globe which, from rim to rim, extended one eighth of the way across the visible sky. Had it been full, it would have been two thousand times as bright as the Earth's full moon, but the night shadow cut a third of it away.

The bright zones and dark belts that crossed it were not merely brown now. They were close enough to show full clear color: pink, green, blue, and purple, amazingly bright. The edges of the bands were ragged and slowly changed shape as they watched, as though the atmosphere were being whipped into gigantic and turbulent storms, as most probably it was. Io's clear, thin atmosphere didn't obscure the smallest detail of that colored shifting surface.

The Great Red Spot was heaving ponderously into sight. It gave the impression of a funnel of gas, swirling lazily.

They watched for a long time, and Jupiter did not change position. The stars moved past it, but Jupiter remained fixed where it was, low in the western sky. It could not move, since Io presented only one side to Jupiter as it revolved. On nearly half of Io's surface Jupiter never rose, and on nearly half it never set. In an in-between region of the satellite, a region making up nearly a fifth of the total surface, Jupiter remained forever on the horizon, part showing, part hidden.

"What a place for a telescope!" murmured Bigman on the wave length allotted to Lucky during the pre-landing briefing.

Lucky said, "They'll have one soon and a lot of other equipment."

Bigman touched Lucky's face-plate to attract his attention and pointed quickly. "Look at Norrich. Poor guy, he can't see any of this!"

Lucky said, "I noticed him before. He's got Mutt with him."

"Yes. Sands of Mars, they go to trouble for that Norrich! That dog suit is a special job. I was watching them put it on the dog when you were keeping tabs on the landing. They had to test to make sure he could hear the orders and obey them and if he'd let Norrich use him once Norrich got into a space suit. Apparently it all worked out."

Lucky nodded. On impulse he moved rapidly in Norrich's direction. Io's gravity was just a trifle over that of the moon, and both he and Bigman could handle that neatly.

A few long, flat strides did the job. "Norrich," said Lucky, shifting to the engineer's wave length.

One cannot tell direction of a sound when it comes out of earphones, of course, and Norrich's blind eyes looked about helplessly. "Who is it?"

"Lucky Starr." He was facing the blind man, and through the face-plate could make out clearly the look of intense joy on Norrich's face. "You're happy to be here?"

"Happy? You might call it that. Is Jupiter very beautiful?"

"Very. Would you want me to describe it to you?"

"No. You don't have to. I've seen it by telescope when-when I had eyes, and I can see it in my mind now. It's just that... I don't know if I can make you understand. We're some of the few people to stand on a new world for the first time. Do you realize what a special group that makes us?"

His hand reached down to stroke Mutt's head and contacted only the metal of the dog's helmet, of course. Through the curved face-plate, Lucky could see the dog's lolling tongue, and his uneasy eyes turning restlessly this way and that, as though disturbed by the strange surroundings or by the presence of his master's voice without the familiar body that went with it.

Norrich said quietly, "Poor Mutt! The low gravity has him all confused. I won't keep him out much longer."

Then, with an increase of passion again, "Think of all the trillions of people in the galaxy. Think how few of them have had the luck to be the first on a world. You can almost name them all off. Janofski and Sterling were the first men on the moon, Ching the first man on Mars, Lubell and Smith on Venus. Add them all up. Even count in all the asteroids and all the planets outside the solar system. Add up all the firsts and see how few there are. And we're among those few. I'm among those few."

He flung his arms out as though he were ready to embrace the whole satellite. "And I owe that to Summers, too. When he worked out a new technique for manufacturing the lead contact point-it was just a matter of a bent rotor, but it saved two million dollars and a year's time, and he not even a trained mechanic -they offered to let him be in the party as reward. You know what he said. He said I deserved it in his place. They said sure, but I was blind, and he reminded them why I was blind and said he wouldn't go without me. So they took us both. I know you two don't think much of Summers, but that's what I think of when I think of him."

The commander's voice sounded ringingly in all helmets: "Let's get to work, men. Jupiter will stay where it is. Look at it later."

For hours the ship was unloaded, equipment was set up, tents unfurled. Temporary air tights were prepared for possible use as oxygen-supplied headquarters outside the ship.

The men were not to be kept from watching the unusual sky, though. As it happened, all three of Jupiter's other large satellites were in the sky.

Europa was closest, appearing somewhat smaller than Earth's moon. It was a crescent, near the eastern horizon. Ganymede, appearing smaller still, was nearer zenith and half full. Callisto, only a quarter the width of Earth's moon, was nudging close to Jupiter and, like Jupiter, was some two thirds full. All three together gave not one quarter the light of Earth's full moon and were completely inconspicuous in the presence of Jupiter.

Bigman said exactly that.

Lucky looked down at his small Martian friend after having studied the eastern horizon thoughtfully. "You think nothing could beat Jupiter, do you?"

"Not out here," Bigman said stoutly.

''Then keep watching," said Lucky.

In Io's thin atmosphere there was no twilight to speak of and no warning. There was a diamondlike sparkle along the frost-covered top-line of the ridge of low hills, and seven seconds later the sun had topped the horizon.

It was a tiny seed-pearl of a sun, a little circle of brilliant white, and for all the light that giant Jupiter cast, the pigmy sun cast much, much more.

They got the telescope up in time to catch Callisto vanishing behind Jupiter. One by one, all three satellites would do the same. Io, although it kept only one face to Jupiter, revolved about it in forty-two hours. That meant that the sun and all the stars seemed to march around Io's skies in those forty-two hours.

As for the satellites, Io moved faster than any of them, so it kept overtaking them in the race about Jupiter. It overtook the farthest and slowest, Callisto, most rapidly; so Callisto circled Io's heavens in two days. Ganymede took four days and Europa seven. Each traveled from east to west and each in due turn was to pass behind Jupiter.

The excitement in the case of the Callisto eclipse, which was the first to be witnessed, was extreme. Even Mutt seemed to be affected by it. He had grown increasingly used to low gravity, and Norrich gave him periods of freedom during which he floundered grotesquely about and tried vainly to inspect by nose the numerous strange things he encountered. And in the end, when Callisto reached Jupiter's glowing curve and passed behind, and all the men grew silent, Mutt, too, sat on his swathed haunches and, tongue lolling, stared upward at the sky.

But it was the sun they were really waiting for. Its apparent motion was faster than that of any of the satellites. It gained on Europa (whose crescent thinned to nothingness) and passed behind it, remaining in eclipse for something less than thirty seconds. It emerged, and then Europa was a crescent again, with its horns facing in the other direction now.

Ganymede had plunged behind Jupiter before the sun could reach it, and Callisto, having emerged from behind Jupiter, was below the horizon.

It was the sun and Jupiter now, those two.

The men watched greedily as the seed-pearl sun climbed higher in the sky. As it did, Jupiter's phase grew narrower, its lighted portion always, of course, facing the sun. Jupiter became a "half-moon," then a fat crescent, then a thin one.

In Io's thin atmosphere the sunlit sky was a deep purple, and only the dimmer stars had been blotted out. Against that background there burnt the gigantic crescent in the sky, bulging out toward the relentlessly approaching sun.

It was like David's pebble hurled from some cosmic slingshot toward Goliath's forehead.

The light of Jupiter shrank still further and became a yellowish curved thread. The sun was almost touching.

It did touch and the men cheered. They had masked their face-plates in order to watch, but now that was no longer necessary, for the light had dimmed to bearable dimensions.

Yet it had not vanished entirely. The sun had moved behind the edge of Jupiter but it still shone murkily through that giant planet's thick, deep atmosphere of hydrogen and helium.

Jupiter itself was now completely blanked out, but its atmosphere had sprung to life, refracting and bending the sunlight through itself and around the curve of the planet, a smoothly bending film of milky light.

The film of light spread as the sun moved farther behind Jupiter. It curved back on itself until faintly, very faintly, the two horns of light met on Jupiter's other side. Jupiter's vanished body was outlined in light and one side bulged with it. It was a diamond ring in the sky, big enough to hold two thousand globes the size of the moon as seen from Earth.

And still the sun moved farther behind Jupiter so that the light began to fade and grow dim, and dimmer, until finally it was gone and, except for the pale crescent of Europa, the sky was black and belonged to the stars.

"It will stay like this five hours," said Lucky to Big-man. "Then everything will repeat itself in reverse as the sun comes out"

"And this happens every forty-two hours?" said Big-man, awed. ''That's right," said Lucky.

Panner approached them the next day and called out to them, "How are you? We're almost done here." He spread his arm about in a broad circle to indicate the loan valley, now littered with equipment. "We'll be leaving soon, you know, and we'll leave most of this stuff here."

"We will?" said Bigman, surprised.

"Why not? There's nothing living on the satellite to disturb the stuff and there's no weather to speak of. Everything's coated for protection against the ammonia in the atmosphere and it will keep nicely till a second expedition comes round." His voice was suddenly lower. "Is there anyone else on your private wave length, Councilman?"

"My receivers don't detect anyone."

"Do you want to take a walk with me?" He headed out, out of the shallow valley and up the gentle slope of the surrounding hills. The other two followed.

Panner said, "I must ask your pardon if I seemed unfriendly on board ship. I thought it better so."

"There are no hard feelings," Lucky assured him.

"I thought I'd try an investigation of my own, you see, and I thought it safer not to seem hand in glove with you. I was sure that if I only watched carefully, I would catch someone giving himself away, doing something non-human, if you know what I mean. I failed, I'm afraid."

They had reached the top of the first rise and Panner looked back. He said with amusement, "Look at that dog, will you? He's getting the real feel of low gravity."

Mutt had learned a lot in the past few days. His body arched and straightened as he lunged in low, twenty-foot leaps, and he seemed to indulge in them for the sheerest pleasure.

Panner switched Ms radio to the wave length that had been reserved for Norrich's use in calling Mutt and shouted, "Hey, Mutt, hey, boy, come, Mutt," and whistled.

The dog heard, of course, and bounded high in the air. Lucky switched to the dog's wave length and heard Ms delighted barking.

Panner waved Ms arm and the dog headed toward them, then stopped and looked back as though wondering if he did right to leave his master. He approached more slowly.

The men walked onward again. Lucky said, "A Sirian robot built to fool a man would be a thorough job. Casual examination wouldn't detect the fraud."

"Mine wasn't casual examination," protested Pan-ner.

Lucky's voice held more than a tinge of bitterness. ''I'm beginning to think that the examination by anyone but an experienced robotics man can be nothing but casual."

They were passing over a drift of snowlike material, glittering in Jupiter light, and Bigman looked down upon it in amazement.

"This thing melts if you look at it," he said. He picked some up in his gauntleted hand, and it melted down and ran off like butter on a stove. He looked back, and where the three had stepped were deep indentations.

Lucky said, "It's not snow, it's frozen ammonia, Bigman. Ammonia melts at a temperature eighty degrees lower than ice does, and the heat radiating from our suits melts it that much faster."

Bigman lunged forward to where the drifts lay deeper, gouging holes wherever he stepped, and shouted, "This is fun."

Lucky called, "Make sure your heater is on if you're going to play in the snow."

"It's on," yelled Bigman, and running down a ridge with long low leaps, he flung himself headlong into a bank. He moved like a diver in slow motion, hit the drifted ammonia, and, for a moment, disappeared. He floundered to his feet.

"It's like diving into a cloud, Lucky. You hear me? Come on, try it. More fun than sand skiing on the moon."

"Later, Bigman," Lucky said. Then he turned to Panner. "For instance, did you try in any way to test any of the men?"

Out of the corner of his eye Lucky could see Bigman plunging into a bank for a second time, and, after a few moments had elapsed, his eyes turned full in that direction. Another moment and he called out anxiously, "Bigman!" Then, more loudly and much more anxiously, "Bigman!"

He started running.

Bigman's voice came, weak and gasping. "Breath... knocked out... hit rock... river down here..."

"Hold on,I'll be with you." Lucky and Panner, too, were devouring space with their strides.

Lucky knew what had happened, of course. The surface temperature of Io was not far removed from the melting point of ammonia. Underneath the ammonia drifts, melting ammonia could be feeding hidden rivers of that foul-smelling, choking substance that existed so copiously on the outer planets and their satellites.

There was the rattle of Bigman's coughing in his ear. "Break in air hose... ammonia getting in... choking."

Lucky reached the hole left by Bigman's diving body and looked down. The ammonia river was plainly visible, bubbling slowly downhill over sharp crags. It must have been against one of those that Bigman's air hose had been damaged.

"Where are you, Bigman?"

And though Bigman answered feebly, "Here," he was nowhere to be seen.