“I get it. And the crater you’ve found on Cronos—”

“ — is about twenty miles wide. Below the turndown limit for the icy bodies. Carbonaceous chondrite meteorites would have four or five times the density, and a hundred times the internal strength.”

She tried to think it through. “So the crater you found can’t have been caused by a cometary-ice impact.”

“It’s possible, but unlikely.”

It was typical of Rosenberg to play the cautious scientist when he was asking her to make a decision that would put all their lives on the line.

“But it could be something else,” she said doggedly. “A stony or iron meteorite.”

“Yes, that’s possible. But the flux rates for objects like that, out here so far from the sun, are small,” he said. “Much smaller than at Earth. Really, Paula, a carbonaceous chondrite is the best explanation. And the crater I’ve found is the most likely chondrite crater for a few hundred miles. Paula, we’re lucky to have found something so close.”

She sighed. “So we have to go there.”

“You know it. Paula, you’ve seen the figures. We just aren’t able to achieve closure of the life support loops, particularly of the amino acids and some of the trace elements — sulphur, potassium, chlorine.

Even Titan itself can’t supply everything. So we have to look for manna from heaven, in the crater of a carbonaceous chondrite meteorite. “We need that kerogen.” He smiled, his thin face dreamy. “Before it fell out of the sky, the meteorite must have drifted around for five billion years, a fragment of the original circumsolar nebula. Food, cooked up in the interior of the first generation of stars…” This kind of stuff was what worried her. This was Rosenberg’s personal escape hatch, his way of retreating from the dull horrors of their life on Titan. Her worry was, what if there were other options for survival which he wasn’t considering, because he was caught up with the idea of digging out the celestial stuff of life from some crater on Cronos?

“A hundred and twenty miles, across the surface of Titan. My God, Rosenberg. Do you really think it’s feasible? The longest surface EVAs in NASA history were the last Moonwalks. Seven or eight hours outside the Lunar Module; a traverse of a few miles, every minute timelined, in those damn Lunar Rovers. All controlled from the ground, and all of it within a walk-back limit of the LM. Now, we’re going to have to figure out how to survive independently of Discovery for two weeks or more.”

He shrugged. “It will take some preparation. But I think it’s possible, Paula. We’ll need the sleds, of course, with food and water and stuff, and some kind of surface shelter. But remember we should be able to haul along a lot of mass, in the low gravity—”

“Rosenberg, we haven’t reached a crisis yet. Maybe we should wait.”

He looked confused. “What good would waiting do? We don’t have any smarter options. It’s better to attempt this now. While we’re still reasonably healthy. Before the equipment starts to wear out. Before the life support loops start failing.”

“You’ve worked this out, haven’t you, Rosenberg?”

“Paula, I really don’t think we have a choice,” he said seriously.

He started talking about more expeditions they could mount later. For instance to the crater of an iron meteorite. Maybe they could find some way to refine the metal, and…

She listened with weary patience. He was off in his dream-world of technology and science and achievement, that realm where all his schemes came to magical life, and where Tartarus became the hub of a spreading, glittering complex of science and technology.

None of it had anything to do with the real problem they faced about this EVA, she thought. Which was what to do with Bill Angel.

“El Dorado,” he was saying now. “That’s what we’ll call the crater.”

“Whatever you say, Rosenberg.”

In the chaos, it wasn’t difficult for Barbara Fahy to get out of the complex. She rode a steel elevator to the surface, and emerged into the early hours of a spring morning in Washington DC.

She checked her watch. It was nearly 6:00 a.m.; the briefings had gone on all night.

The streets were all but empty: there were a couple of street cleaners, a girl in a short skirt and inert image-tattoos making her way home, maybe from some club, one or two tense-looking office workers in suits, strutting anxiously towards their workplaces. The traffic lights were working, but randomly, it seemed to her.

She wondered where the President was this morning. Nowhere near here.

She walked.

She reached the Tidal Basin, and walked among the cherry trees near the Jefferson Memorial, around the reflecting pool walkway. The canopy of white blossoms filtered the morning light, so that it was like the glow of a skylight, shadowless, diffuse, warming.

She passed a small colony of homeless, huddled under paper and cardboard against the softscreen-coated wall of a bank. The softscreen shed flickers of light over clothes that had been reduced by rain and sunlight to shapeless, colorless pulp. But this morning there was no pattern to the softscreen’s display, just formless grey static.

Maybe, she thought, she should warn someone. But what was the point? Let them enjoy the morning. Let them sleep, if they could.

Maclachlan had said he’d sweep the homeless from the streets. At the end of two terms — and as Maclachlan aimed to change the Constitution to allow him to run for a third — there were more of them than ever. And malnutrition in the Bronx, and cholera in Georgia…

But, she thought, all these problems would soon be swept away, more rapidly and effectively than even Xavier Maclachlan, in his wildest dreams, could have planned.

She felt she’d lived through an immense paradox. After that steel cavern, she could understand why people felt that science was a terrible thing. Maybe even an evil thing. But the fact was that one nuke, in the right place at the right time, could have deflected this incoming, the Chinese rock. There was the paradox. What do we do when the dinosaur-killer comes? Accept it as inevitable? Throw philosophy books at it?

But in the end it was science and technology which had delivered the evil on their heads. The paradox deepened.

She just hoped there would be people around to debate this tomorrow.

According to the projections prepared by her staff, everything depended on the geometry of the impact.

A hell of a lot of kinetic energy would be released downwards, into the crust, and upwards, into the atmosphere, first as a vapor plume and then as an airblast. If there was an ocean strike there would be earthquakes: Richter eight or nine. A lot of dust and salt water would be injected into the middle atmosphere; nobody cared to guess what that would do to the weather. And they were going to get global oscillations of the atmosphere and ionosphere. Upper atmosphere heating, high intensity atmospheric disturbances. Hydrogen-mixing would wreck the ozone layer, for good and all. A lot of nitrogen would be burned, into nitrogen dioxide, nitric acid. Acid rain. And the high-speed plasma plumes from the shock, reaching up to the geomagnetic field, were going to play hell with the radiation belts…

Funny weather. Storms. Auroras. Lousy communications. Stunning sunsets, from all that dust. The skies would be spectacular.

Even if the impact itself wasn’t too severe, secondary effects could do a lot of damage. Nuclear waste repositories. Hydroelectric power stations and dams. Chemical plants. Nuclear power stations. She imagined a dozen Chernobyls, scattered along the eastern seaboard…

Still, it was possible humanity — even civilization — could survive the impact itself and its consequences. But then, everything would depend on the war that would surely follow, when the Chinese came over in their clumsy ships, and Al Hartle and his boys emerged from their bunkers in Cheyenne, and they started the work of finishing off whatever the asteroid left behind. Even given enough survivors, she thought bleakly, it might be impossible to climb back. The post-impact world would not be a blank slate for a new civilization, now that they’d used up ail the most accessible raw materials — ore, coal, oil. And besides the biosphere was already unstable. This might trigger the final plankton collapse, for instance…