by Isaac Asimov
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!
12. The Skies and Snows of Io
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.