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!

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.