I was attacked with violent fever attended by extreme cold; and taking to my bed, I made my mind up that I was sure to die. Nature in me was utterly debilitated and undone; I had not strength enough to fetch my breath back if it left me; and yet my brain remained as clear and strong as it had been before my illness. Nevertheless, although I kept my consciousness, a terrible old man used to come to my bedside, and make as though he would drag me by force into a huge boat he had with him. This made me call out, and Signor Giovanni Gaddi, who was present, said, “The poor fellow is delirious, and has only a few hours to live.” His fellow, Mattio Franzesi, remarked: “He has read Dante, and in the prostration of his sickness this apparition has appeared to him.”

—The Autobiography of Benevenuto Cellini

ON THE TERRACE IN BELLOSGUARDO, Galileo lay sprawled over the tiles. Cartophilus had shoved blankets under him, and laid blankets over him, but he still lay there awkwardly, seemingly paralyzed, chest rising and falling in shallow irregular breaths. His feet and hands felt cold. La Piera came out with a jug of mulled wine.

“Can you get any of this in him?”

Cartophilus shook his head. “We’ll just have to wait.”

They floated among the stars, just Galileo and Hera, with dark Jupiter majestically scrolling beside them. Ahead of them a white half-moon, covered with a black craquelure, grew visibly larger. Galileo shook his head hard, shocked by such a vivid immersion in his seldom-remembered past. Marina …

“From that point on you saw her as often as you could?” Hera said, looking at the pad in her lap.

“That’s right,” he said.

“You had an understanding.”

“Yes.”

“You were in love.”

“I suppose so.”

It wasn’t a feeling he remembered very well. It hadn’t lasted long. But now it was right there in him, hard to deny. “Yes. But listen—you sent me back into my past, but—” He gestured at the teletrasporta, on the floor between them. “Where was the one in Italy? Where was Cartophilus?”

She regarded him calmly. “These experiences aren’t like your fiery alternative, where the entangler was in fact on hand, and I sent you back into yourself at that time. With the mnemonic helmet here, I don’t send you back into the past, but into your own mind. Everything that happens to us with a strong enough emotional charge is remembered in full. But that ability to record events turns out to be much stronger than our ability to recall them at will. Recollection is the weak link. So, I was a mnemosyne, yes. It’s a kind of doctor for the mind. Perhaps also what your priests do in confession. A kind of therapist. With the help of the mnemonic helmet I can locate memories in your brain and cause them to abreact in you.”

“You caused me to remember?”

“Yes.”

He touched her celatone. “All your machines … they make you into a sorceress.”

“Brain scanning and stimulation is not that hard. Let’s go back to Marina. You spent ten years with her and had three children with her, but you never married her, and when you moved to Florence, you left her behind.”

“Yes.”

“Do you know why you did that?”

“We fought.”

“Do you know why you fought?”

“No.”

She was staring at him, and he looked away uncomfortably. He saw that one of the Jovian moons, either Ganymede or Callisto, was now a large half-moon. “We arrive, it seems.”

“Yes, I have to attend to the ship. Then we’ll continue. It’s important. Your mind is parcellated into many little archipelagoes. It’s partly you, partly the structure of feeling in your time. But you’re going to have to put yourself together, like a puzzle, if you want to live. Which means you’ll have to remember the pieces that matter.”

“How can I forget them?” Galileo complained. “Why do you think I can’t sleep at night?”

But now she was focused on piloting their craft toward the looming moon, running her forefingers over the pad in her lap. Again Galileo felt the pressure against him, pushing him into his chair. Ahead the moon grew even more quickly. To their right and behind them, space glowed and then seemed to split in a great arc, as if a red blade were slicing into the black firmament—a crescent thin as could be, but immense in circumference. The lit side of Jupiter was coming back into view. The crescent thickened quickly, revealing the latitudinal bands, which made it look like a piece of brocade. The whole great ball was shrinking perceptibly, although not as fast as the half-moon ahead of them was growing, which of course made sense in terms of perspective.

“This is Callisto?” His Moon IV had often seemed the brightest of the four.

“No, this is Ganymede. Our Ganymede’s home world, as you might have guessed. He and his followers came from the big city there, before they were exiled.”

The moon Ganymede bloomed in front of them; they were going to pass over the sunlit half of it. “That’s the city there, in that crater.” She pointed. “Memphis Facula. The dark area around it is called the Galileo Regio, I’m sure you’ll be pleased to know.”

Galileo frowned at the jab, though in fact he was pleased. “Will we stop there?”

“No, we’re just passing by. We’re using Ganymede for a redirect and some sling. See there, the big star out there? That’s Callisto.”

They shot just over the surface of bright Ganymede. It was big and rocky, lined with an orthogonal crackle almost everywhere, also pockmarked with many round impact scars, like a smallpox survivor. There was an infinite litter of rocks and boulders scattered over the lined plains, which were in some places very dark, in other places a blasted brilliant white, though the landscape seemed basically level. Long strips of different kinds of terrain, lined or smooth or rocky, were laid beside and over each other like gallery carpets.

“The white areas are called palimpsests,” Hera said. “Now we’re over Osiris, that’s the big crater with the white marks radiating from it. And now we’re coming over Gilgamesh.”

“Why was Ganymede exiled from his world?” Galileo asked.

Her expression grew sad and forbidding. “He is a charismatic, the leader of a sect with a lot of power on Ganymede. The sect did something forbidden by Ganymede’s government. Strange to say, I think they made an incursion into the Ganymedean ocean. This is the biggest of the four moons, the biggest moon in the solar system, in fact, and it has the biggest ocean too, much bigger than Europa’s. The ice layer here is thicker too. So—something happened down there. Ganymede was at that time the Ganymede, a kind of religious leader, so that made it especially shocking, that he would initiate such a transgression.”

“You don’t know what happened?”

“No. Afterward I was assigned to be his mnemosyne, when he and his group were exiled to Io, but after a few sessions he refused to continue working with me, and the judgment has not been enforced. He has to be careful around me because of that, and pretends even to accommodate me, as when I joined you during the trip into Europa’s ocean. But in truth he keeps his distance.” She shook her head, watching the big moon gloomily as they angled swiftly away from it, then shot into the night toward Callisto. “Maybe he got somebody killed down there, or encountered something like what we ran into inside Europa. Whatever happened, he must have come to think it was a bad idea to make the incursion, judging by the way he tried to stop the Eu-ropans from doing the same.”

“So you think he found a creature in Ganymede’s ocean? Given there is one inside Europa, it seems possible.”

“Yes, it does. But the government in Memphis Facula says there isn’t anything down there. None of the Ganymede’s people has ever said anything about their incursion, and he refused to work with me, as I said. He and his circle have moved to a more distant massif on Io.”