He regularly tapped into the ExoBio seminars, mostly to use the interactive data bases and 3-D choice-theory-outcome representations. These last were visualizations of the overall consequences of any theory of extraterrestrial life, tracing the many strands of planetary evolution, biology, and socioeconomics. Earthside’s spotty flow of news on the Swarrners and Skimmers had to be folded into what Lancer and the independent probes found. There were competing schools of thought, led by specialist analysts among the crew. Nigel seldom met these savants. They existed for him as disembodied constellations of theory in the seminar representations, ways of organizing the data. Their command of interconnections was formidable. They could relate the structure of the Marginis wreck to the swim patterns of the Swarmers, fold it into a theory of universal languages, and come up with (a) an estimate of the probability that most galactic lifeforms still lived exclusively in oceans, (b) a best-choice scheme for achieving radio contact through use of gigawatt-level radio beacons, (c) a recalculated optimum-search strategy for probes to stars within a hundred light-years. Nigel recalled Mark Twain’s remark that the wonder of science was how vast a return of speculation you got for such a trifling investment of fact.

The snag was that you had to have some initial premise to fit it all together. Shipboard, the running consensus was that all earlier alien contacts—the Snark craft that Nigel spoke to briefly, and the Marginis wreck—had been feelers. Something, probably the Swarmers and Skimmers themselves, had probed Earth for a long time, sizing up its suitability as a biosphere. The conventional wisdom of the past, that no species would bother to invade another world, seemed no longer true. Lancer had found that most planets were blasted relics. It would be far easier to adapt to an existing biosphere like Earth, than to start at zero with a smashed, barren planet. So the Swarmers had probably been bioengineering themselves to adapt to Earth’s oceans, ever since they discovered it in the expedition that left the Marginis wreck.

The theory even explained Walmsley’s Rule. The Swarmers—or the civilization they represented, the technology that built the starships they came in—made the Watchers, to keep track of other possible life sites, other developing societies. Some Watchers survived the final war that scraped some worlds free of life; others didn’t. Man was coming late upon the galactic stage; he should expect to find some props from earlier acts—most of them tragedies. Thus went the conventional wisdom, new edition.

Nigel’s point of view was duly heard, discussed, footnoted in later work—and then the stream of theories and models and self-consistency cheeks flowed on around it, a consensus river skirting an island. He did not know enough about analysis to integrate his model with the wealth of data. He thought it probable that the Marginis wreck had died while destroying Earth’s Watcher. Over half a million years after its crash, the crumpled eggshell vessel had demonstrated powerful weapons—which was how Moon Operations found it. At full capability, the wreck could have blown apart whole asteroids—and Nigel suspected that was precisely what it was designed to do. Many of the worlds they’d seen by probe—and Isis, too—had been pulverized by bombardment. It was the cheapest way to damage a planetary surface in terms of energy invested. So the Marginis wreck had laid there as man evolved up from apes. The wreck could detect and smash any large asteroid falling toward the biosphere. But its strength ebbed. It had stood up to battering attacks, only to fade slowly as time wore it down.

Now humanity could defend itself against asteroids or even worse weapons. As long, Nigel thought to himself, as we can recognize them as weapons.

Six

Luyten 789–6 had only one world, circling near one of the two small suns, and it was devoured by fire.

As the probe swung near it, the spectral traces and photometry showed a pall of smoke and sheets of flame. The planet was Earth-size, comfortably warm, 80 percent ocean. Above the seas the oxygen content of the air was 25.4 percent, and over the continents, 23.7 percent.

It did not take much analysis to see what had happened. Warm surface temperatures made sea life abundant. Microorganisms there exhaled large amounts of oxygen. On Earth the same process ran, too, but oxygen was only 21 percent of the air.

The probability of forest fire nearly doubles with each 1 percent rise in oxygen. On the sole world of Luyten 789–6, the sea life poured oxygen into the forever burning tropical forests. Even Arctic tundra ignited. In the planet’s winter season plants grew despite the cold, driven by the high chemical reaction rates, and by processes in the soil. With summer came worldwide fires.

On Earth, methane belched up from mud ponds soaks oxygen from the air, keeping a stable balance. Somehow that mechanism had failed here. There was evidence from the chem sampling that this world was older than Earth; the grow-and-burn cycle had been running for billions of years. No animal life moved on the land; none could survive the fires. Yet a Watcher circled the world—impassive, scarred, and ancient.

“Carlotta!”

She turned. Nigel walked faster with obvious effort and caught up at a Y in the corridors. “Time for some talk?”

She grinned. “Sure. I’ve been wanting to bring something up myself. Just haven’t had an opportunity.”

They made their way to a viewpod that looked out on the base of the ship’s axis. Here the centrifugal gravity was low. Nigel’s face showed relief at the lessened strain. Beyond, they could see a globe of water ejected at the axis. People swam in it as it wobbled and flowed along the axis in free fall. They had thin rubber bands fixed to their ankles, in case they broke the surface tension and fell outward, Few did; they were adept fish, showering droplets and laughter.

“I miss that,” Nigel mused. “Haven’t done it for years.”

“Well, soon you’ll be able to again and we can—”

“No. I’ve been putting off my medical, but I can tell matters aren’t improving.”

“Chem?”

“Right. Radicals in the blood, so the body leaps to my defense”—a wry shrug—”and overcompensates.”

“Cancer.”

“That’s the homey name for it, yes. I’ve been doing a lot of blood filtering on my own—don’t look so shocked, it’s a simple trick, really—but I can’t get past the med-mon sniffer anymore.”

“Some therapy—”

He shook his head. “I know what Medical and Ted will say. I’m too much a bloody precious relic to risk. They’ll pop me into a Sleepslot until we’re Earthside.”

“Look, landfall at Ross is nearly a year away. I’m sure they’d let you last through that.”

“Um. Risk me dying from inadequate treatment? Unlikely.”

“You’re valuable to us, too. Didn’t Luyten 789–6 prove Walmsley’s Rule?”

“The first law of management is: Cover your ass. This shall ye honor before all else. Ted doesn’t want to haul me back to Earth a corpse.”

“You don’t want that either. There’s nothing you can do except take the luck you’re handed. Look, you know time in the Slots isn’t so bad. I’m going in myself for four months, next Friday.”

“What for?”

“I … A tune-up, sort of. I … We all three should talk about it, I guess …” She paused and then went on briskly. “You have no choice.”

“I’ve ducked by Medical before.”

She saw what he meant. “Uh-oh …”

“Right.” He grinned. “You took me out, put me on self-serve, remember, years ago? Do it again. Please.”

“I … You know I care for you, I still do, even if we aren’t … together now … but …”

“Please.”

“Do you really care that much about making landfall?”