“Anyway, you’re right,” Louise said at last.

“What?”

“I’m not happy. Although I’m not sure I could tell you why.”

Mark smiled; the sourceless light of the Britain’s dome smoothed away the lines around his eyes. “Well, if we’re being honest with each other for once, I do kind of enjoy seeing you suffer. Just a bit. But I care as well. Come on, let’s walk.”

He took her arm again, and they walked along the ship’s starboard side. The soles of their shoes made soft sucking sounds as the shoes’ limited processors made the soles adhere to and release the deck surface, unobtrusively reinforcing Port Sol’s microgravity. The shoes almost got it right; Louise felt herself stumble only a couple of times.

Around the ship was a dome of semisentient glass, and beyond the dome — beyond the pool of sourceless light which bathed the liner — the landscape of Port Sol stretched to its close-crowding horizon. Port Sol was a hundred-mile ball of friable rock and water-ice, with traces of hydrogen, helium and a few hydrocarbons. It was like a huge comet nucleus. Port Sol’s truncated landscape was filled with insubstantial, gossamer forms: sculptures raised from the ancient ice by natural forces reduced to geological slowness by the immense distance of the Sun.

Port Sol was a Kuiper object. With uncounted companions, it circled the Sun beyond the orbit of Pluto, shepherded there by resonances of the major planets’ gravitational fields.

Louise looked back at the Great Britain. Even against the faery background of Port Sol, still Brunel’s ship struck her as a thing of lightness, grace and elegance. She remembered going to see the ship in her dry dock on Earth; now, as then, she compressed her eyes, squinting, trying to make out the form of the thing — the Platonic ideal within the iron, which poor old Isambard had tried to make real. The ship was three thousand tons of iron and wood, but with her slim, sharp curves and fine detail she was like a craft out of fantasy. Louise thought of the gilded decorations and the coat-of-arms figurehead around the stern, and the simple, affecting symbols of Victorian industry carved into the bow: the coil of rope, the cogwheels, the set-square, the wheat-sheaf. It was impossible to imagine such a delicate thing braving the storms of the Atlantic…

She tilted back her head, and looked for the brightish star in Capricorn that was Sol, all of four billion miles away. Surely even a visionary like old Isambard never imagined that his first great ship would make her final voyage across such an immense sea as this.

Mark and Louise climbed down a steep staircase amidships to the promenade deck; they strolled along the deck past blocks of tiny cabins toward the engine-room bulkhead.

Mark ran a fingertip over the surface of a cabin wall as they passed. He frowned, rubbing his fingertips together. “The surface feels odd… not much like wood.”

“It’s preserved. Within a thin shell of semisentient plastic, which seals it, nourishes it… Mark, the damn boat was launched in 1843. Over two thousand years ago. There wouldn’t be much left of her without preservation. Anyway, I thought you weren’t interested.”

He sniffed. “Not really. I’m more interested in why you wanted to come down here: now, in the middle of all the celebrations for the completion of the starship.”

“I try to avoid introspection,” she said heavily.

“Oh, sure.” He turned to her, his face picking up the soft glow of the ancient wood. “Talk to me, Louise. The bit of me that cares about you is outvoting the bit that enjoys seeing you suffer, just for the moment.”

She shrugged. She couldn’t help sounding sour. “You tell me. You always were good at diagnosing the condition of the inside of my head. At great and tedious length. Maybe I’m feeling melancholy after completing my work on the Northern. Could that be it, do you think? Maybe I’m going through some equivalent of a post-coital depression.”

He snorted. “With you, it was post, pre and during, frankly. No, I don’t think it’s that… And besides,” he said slowly, “your work on the Northern isn’t finished yet. You’re planning to leave with her. Aren’t you? Spend subjective decades hauling her out to Tau Ceti.”

She heard herself growl. “How did you find out about that? No wonder you drove me crazy, all those years. You’re too damn interested in me.”

“I’m right, though, aren’t I?”

Now they reached the Britain’s dining room. It was a fantastic Victorian dream. Twelve columns of white and gold, with ornamental capitals, ran down its spine, and the room was lined by two sets of twelve more columns each. Doorways between the columns led off to passageways and bedrooms, and the door archways were gilded and surmounted by medallion heads. The walls were lemon-yellow, relieved by blue, white and gold; omnipresent, sourceless light shone from the cutlery and glassware on the three long tables.

Mark walked across the carpet and ran his hand over a table’s gleaming, polished surface. “You should do something about this semisentient plastic: have it give the surfaces some semblance of their natural texture. The touch is half the beauty of a thing, Louise. But you always were… remote, weren’t you? Happy enough with the surface of things — with their look, their outer form. Never interested in touching, in getting closer.”

She ignored that. “Brunel had a lot of style, you know. He worked on a tunnel under the Thames, with his father.”

“Where?” Mark had been born in Port Cassini, Titan.

“The Thames. A river, in England… on Earth. The tunnel was flooded, several times. Once, when it had been pumped out, Brunel threw a dinner party right up against the working face for fifty people. He got the band of the Coldstream Guards to — ”

“Hmm. How interesting,” Mark said dryly. “Maybe you should put some food on these tables. Why not? It could be preserved, by your sentient plastic. You could have segments of dead animals. As devoured by the great Brunel himself.”

“You never did have any taste, Mark.”

“I don’t think your mood has anything to do with the completion of the Northern.”

“Then what?”

He sighed. “It’s you, of course. It always is. For a long time, while we were together, I thought I understood your motivation. There would always be another huge, beautiful GUTship to build; another immense undertaking to lose yourself in. And since we’re all immortal now, thanks to AntiSenescence, I thought that would be enough for you.

“But I was wrong. It isn’t like that. Not really.”

Louise was aware of intense discomfort, somewhere deep within her; she felt she wanted to talk, read a bookslate, bury herself in a Virtual — anything to drown out his words.

“You always were smarter than me, Mark.”

“In some ways, yes.”

“Just say what you’ve got to say, and get it over.”

“You want immortality, Louise. But not the dreary literal immortality of AS — not just a body-scouring every few years — but the kind of immortality attained by your idols.” He waved a hand. “By Brunel, for instance. By achieving something unique, wonderful. And you fear you’ll never be able to, no matter how many starships you build.”

“You’re damn patronizing,” she snapped. “The Northern is a great achievement.”

“I know it is. I’m not denying it.” He smiled, triumph in his eyes. “But I’m right, aren’t I?”

She felt deflated. “You know you are. Damn you.” She rubbed her eyes. “It’s the shadow of the future, Mark…”

A century and a half earlier, the future had invaded the Solar System.

It had been humanity’s own fault; everyone recognized that. Under the leadership of an engineer called Michael Poole the Interface project — a wormhole link to a future a millennium and a half ahead — had been completed.