There were other problems, too. There was a variation, like a tide, of the size of the force along the length of her body; her head was a good deal closer to the axis of spin than her feet. The centrifuge’s arm couldn’t have been much shorter than it was, or that difference would rise above a few percent, and cause damaging hydrostatic pressure differences in her tissues.

There were two fold-up exercise devices in here, a cycle ergometer and a treadmill, both folded away against the wall. Moving carefully, she reached down now and pulled out the bike.

The fake gravity was still so low that she had some trouble starting; her pedal motions tended to lift her off her seat. She had brought a pillow which she braced now against the ceiling of the cabin, and wedged herself in place with her head. She held tightly to the handlebars. Her feet were in pedal straps, so she could pull down with one pedal while pushing with the other, and that helped keep her in place.

Nobody had run a mission in microgravity much beyond a few hundred days. Nobody knew for sure what the impact of very long term exposure to microgravity would be, or if any of the countermeasures they were taking would work. And nobody had tried to live for years under one-seventh G, as they would have to on Titan. The surgeons didn’t know if that was even survivable. For sure, the crew had to expect a long-term loss of bone mass of maybe a quarter, even after they had reached Titan.

Exercise, which would help combat the other damaging micro-gravity deconditioning processes — muscle atrophy, bone marrow loss, reduction in T-lymphocytes — was no use with the real show-stopper, the cumulative loss of bone calcium. And although the crew would be treated with osteogenic drugs — and there was hopeful talk, which had so far come to nothing, of finding ways to stimulate bone growth with electromagnetic fields — the surgeons on the ground had agreed that the only practical solution was to remove the cause: to restore the crew, periodically, to gravity.

So this centrifuge had been improvised. Every crew member was supposed to work out in here, in conditions of nearly a G, for several hours a day.

She didn’t really object to the exercising, uncomfortable as it was. Unlike some of the others. It got a lot of the stiffness out of her underused muscles, especially her legs. It was as if her body had an agenda of its own, every now and again demanding that she give it some work to do. And she enjoyed the glow of rude health she experienced after a tough work-out.

It made her look better, too — more like herself — because the extra flow of blood to her legs reduced the puffiness around her eyes.

Anyhow, she thought, it was better than rickets.

And she enjoyed the privacy of this snug, enclosed little bay, the isolation from the others.

As she worked, she thought about her crew.

Rosenberg seemed relatively content with his restricted life: pursuing his own research, bitching at the others when some disturbance wrecked one of his careful experiments. But he was drawing inwards, she thought.

So, too, was Nicola Mott, Mott seemed moody, perhaps depressive, ground down already — despite her experience on Station — by the dullness of the interplanetary trajectory, without even the glowing skin of Earth sliding past the windows as a distraction.

But Siobhan Libet, who of all of them was closest to Mott, seemed to be hanging on to her cheerfulness — her sense of wonder — longer than the rest, and she seemed to be doing a good job of keeping Mott back from whatever abyss of depression was threatening her.

Then there was Bill Angel: tough, competent, but restless — a pilot, Benacerraf thought, without any piloting to do, for two thousand days. Of all of them it was Angel who had most rebelled against their daily regime, bitching at the others and Mission Control in Houston. He was a monkey rattling the bars of his cage.

And as for herself, Benacerraf tried to avoid too much introversion, as she had throughout her life. She, like Angel, felt the chafing frustration of being stuck in here with nothing meaningful to do.

Early in the mission, during the euphoria that had followed their hair-raising launch and injection onto this long interplanetary trajectory — and the delight of becoming the first humans to leave cislunar space — they had all been a lot more sociable with each other. They had made a point, for example, of planning meal times to be together.

But that had worn off as soon as the dull daily slog of the mission unfolded.

She’d read of Antarctic scientists who, after a winter snowed into their huts, would throw open the doors as soon as spring came, and just walk off, heading so far into the distance, away from each other, that they might disappear over the horizon.

The crew of Discovery, in their space-going shack, faced a winter that would last six long years. As far as Benacerraf was concerned, anything that they found to help them all endure that and keep from driving each other crazy, like fragments of privacy and broken-up shift patterns, was fine by her.

She pressed her eye to the coelostat eyepiece. The coelostat, an old British invention, was an arrangement of spinning mirrors that compensated for the whirl of the centrifuge, and the barbecue roll of Discovery, to deliver a reasonably steady telescopic view.

She had the coelostat centered on Earth and Moon. The image was slightly blurred, and prone to drift.

Discovery’s trajectory was a complicated double orbit around the sun, in which she would complete two passes past Venus, and then a final close approach to Earth, coming within a few hundred miles of the surface, achieving powerful gravity assists each time.

Only then, after two years, having accumulated the velocity its chemical rockets could not impart, would Discovery leave the inner Solar System behind, and be hurled towards Jupiter — for a further assist — and on to Saturn.

Thus, right now, Discovery was spiralling in towards the sun, on its way to the first rendezvous with Venus. But the energy provided by its injection burn was so low that the ship’s orbit was pretty much tracking that of Earth around the sun, drawing almost imperceptibly away from the home world, in towards the solar fire. So even now, after eighty days, Earth and Moon showed fat, gibbous discs, their faces turned in parallel to the sun. The blue-white of Earth was much brighter, almost overwhelming the faint brown sheen of its smaller companion.

Benacerraf could still study Earth. She was looking at the area from Tibet across Mongolia: northern China and the Gobi desert, one of the bleakest, most barren parts of the planet.

Her perspective was evolving, as Earth receded.

She’d tried to follow, even participate in, the inquisitions that had followed the Endeavour launch. The country had gone into a kind of weary agony when it had been discovered that the X-15 operation had been mounted by a rogue USAF faction, and heads were rolling. There seemed to be a mood of sourness among the public, engendered by the X-15 incident, as if NASA and the USAF were all of a piece. And besides — as Jackie had predicted — the public had rapidly grown bored with the unchanging news from space.

Xavier Maclachlan was growing ever stronger, his lead in the polls consolidating. Jake Hadamard was already fighting a rearguard action to maintain the RLV and other programs he had started, in the wake of the Columbia crash.

It became steadily harder to believe that there would ever be a meaningful attempt at a retrieval.

But it was too late to turn back. Benacerraf had committed herself to traversing this long dark tunnel, leading only to the frigid wastes of Titan. And she suspected she’d always known — in her heart of hearts — it would turn out this way.