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The look on Wahram’s face seemed to suggest he felt much the same.

T he visit with Wahram’s creche on Iapetus was just a matter of dropping in on one of the ordinary meals in their communal kitchen. “These are some of my friends and family,” Wahram said when introducing Swan to the small group at a long table. Swan nodded as they chorused hello, and then Wahram walked her around the room and introduced her to people. “This is my wife Joyce; this is Robin. This is my husband Dana.”

Dana nodded once, in a way that reminded Swan of Wahram, and said, “Wahram is funny. I seem to recall that I was the wife when it came to us.”

“Oh no,” Wahram said. “I was the wife, I assure you.”

Dana smiled with a little squint of suppressed disagreement. “Maybe we both were. It was a long time ago. In any case, Miss Swan, welcome to Iapetus. We’re happy to be hosting such a famous designer. I hope you’ve enjoyed Saturn so far?”

“Yes, it’s been interesting,” Swan said. “And now Wahram is going to take me down into the rings.”

She followed them to the central dining table, and Wahram introduced her to some more people, whose names she forgot, and they waved or nodded without attempting to say more. After a while they chatted with her a bit, then went back to their conversations and left Wahram and his guest alone. Wahram’s cheeks sported little spots of red, but he seemed pleased too and was easy with his crechemates as they drifted by on their way out. Maybe on Saturn, Swan thought, this was a rousing party.

S oon after that they took a shuttle to Prometheus, the inner shepherd moon of the F ring. The gravitational sweeps of Prometheus and Pandora, F’s outer shepherd moon, changed in relation to each other in ways that ended up braiding the F ring’s billions of ice chunks into complex streamers, very unlike the smooth sheets of the bigger rings. In effect the F ring was being swirled in the tides created by its two shepherd moons, making for some waves. And where there were waves, there were surfers.

Prometheus proved to be a potato moon, 120 kilometers long. Its biggest crater dimpled the end closest to the F ring and had been domed, and a station set just inside the rim.

Inside the dome a group of ring surfers greeted them and described the local wave, of which they were very proud. Prometheus reached its apoapse, meaning its farthest point away from Saturn, every 14.7 hours; each time it did so, it almost brushed the slowly tumbling wall of ice chunks that composed the inner border of the F ring. Prometheus was moving faster in its orbit than the ice chunks were in theirs, so it tugged a streamer of chunks out behind it as it passed, in a gravitational effect called Keplerian shear. The curving strand of tugged ice always appeared at a regular distance behind Prometheus, as predictable as the wake behind a boat. The wave at each apoapse appeared 3.2 degrees farther along than the previous one, so it was possible to calculate both when and where to drop in and catch it.

“One wave every fifteen hours?” Swan asked.

That was enough, the locals assured her, grinning crazily. She wouldn’t need more. The rides went on for hours.

“ Hours? ” Swan said.

More crazy grins. Swan turned to Wahram, and as usual could not read his stone face.

“You’re going out too?” she asked.

“Yes.”

“Have you done it before?”

“No.”

She laughed. “Good. Let’s do it.”

T he rings could be modeled mathematically as a fluid, and from any distance they looked like a fluid, grooved by tight concentric waves. Up close one could see that the F ring, like the others, was made of ice chunks and ice dust, layered in ribbons that thickened and thinned in masses of individual bodies, all flying at almost the same speed. Gravity: here one saw its effects in a pure state, unobstructed by wind or solar radiation or anything else-just the sling of spinning Saturn and a few small competing tugs, all creating this particular pattern.

Prometheus was a perfect put-in spot for the surfers, and the ones going out with Swan and Wahram informed them they were both going to be launched into the wave with experienced veterans going before and after, to keep tabs and give help if needed. They offered tips for how to catch the wave, but Swan nodded agreeably and forgot their advice: surfing was surfing. You needed to catch the break at its own particular speed, and off you went.

Then they were all suited up and jetting out a lock. The white jumbled wall of the F ring was right there next to them; streamers of denser clusters of rubble were braided and kinked, but the entire mass was extremely flat-no more than about ten meters north to south relative to Saturn. Those ten meters were not the height of their wave but its width-which meant one could pop out of the ice at any point and be spotted and picked up if one was having any kind of trouble. Most of the waves Swan had ridden before were not like that, and she found it reassuring.