Quake was changing. Not in the way that Max Perry had warned, moving as Summertide approached from a parched but peaceful world of high seismic activity to a trembling inferno of molten lava flows and fissured ground. Instead, Quake in this year of the Grand Conjunction had become — unpredictable.

And in its own way, Opal might be changing just as much. More than anyone on the planet realized.

That thought had come to Rebka as they were flying back around Opal, from the foot of the Umbilical to the Starside spaceport where Darya Lang would be waiting for them.

Six days earlier the journey around the clouded planet to the Umbilical had been dull, with no turbulence and little to see but uniform gray above and below. Now, with Summertide still twenty-seven days away, the car was buffeted and beaten by swirling and violent winds. Sudden updrafts ripped at the lifting surfaces and jolted the fuselage. Max Perry was forced to take the aircar higher and higher to escape the driving rain, black thunderheads, and whirling vortices of air and water.

So the inhabitants of Opal were convinced that they would be safe, were they, even with tides far greater than normal?

Hans Rebka was not so sure.

“You’re making a big assumption,” he told Perry, as they began a descent through choppier air for their approach to Starside port. “You think Opal’s tides this year will be just the same as at other Summertides, but bigger.”

“That’s overstating things.” Once all sight of Quake had been lost under Opal’s ubiquitous cloud layer, Perry’s other personality had surfaced again: cool, stiff, and indifferent to most events. He did not want to discuss their experiences on the surface of Quake, nor his mystification at what was happening there. “I do not say that nothing different will occur on Opal,” he went on. “Yet I believe that is not far from the truth. We may get forces too great for some of the bigger Slings, and one or two of them may break up. But I see no danger to people. If necessary, everyone on Opal can take to the water and ride out Summertide at sea.”

Rebka was silent, holding on to the arms of his seat as they dropped through an air pocket that left both men floating free for a second or two. “It may not be like that,” he said, as soon as his heart was no longer rising to stick in his throat.

Again and again he had the urge to poke and probe at Max Perry and watch his reactions. It was like control theory, feeding a black box with a defined set of inputs and monitoring the output. Do that often enough, and the theory said one could learn precisely all of the boxes’ functions, though not, perhaps, why it performed them. But in Perry’s case, there seemed to be two boxes. One of them was inhabited by a capable, thoughtful, and likable human. The other was a mollusc, retreating into its protective and impervious shell whenever certain stimuli presented themselves.

“This situation reminds me of Pelican’s Wake,” Rebka went on. “Did you hear what happened there, Commander?”

“If I did, I forgot it.” That was not the sort of reaction that Rebka was seeking, but Max Perry had an excuse. His attention was on the automatic stabilization system as it fought to bring them down to a smooth landing.

“They had a situation not too different from Opal,” Rebka continued. “Except that it involved a plant-to-animal mass ratio not sea tides.

“When the colonists first landed there, everything was fine. But every forty years Pelican’s Wake passes through part of a cometary cloud. Little bodies of volatiles, mostly small enough to vaporize in the atmosphere and never make it to the ground. The humidity and temperature take a quick jump, a few percent and a few degrees. The plant-animal ratio swings down, oxygen drops a bit, then in less than a year it all creeps back to normal. No big deal.

“Everyone thought so. They went on thinking it, even when their astronomers predicted that on the next passage through the cloud, Pelican’s Wake would pick up thirty percent more material than usual.”

“I think I remnember it now.” Perry was showing a distant and polite interest. “It’s a case we studied before I came to Dobellel. Something went wrong, and they came close to losing the whole colony, right?”

“Depends who you talk to.” Rebka hesitated. How much should he say? “Nothing could be proved, but I happen to think you’re correct. They came close. But my point is this: Nothing went wrong that could have been predicted with anybody’s physical models. The higher level of comet material influx changed the Pelican’s Wake biosphere to a new stable state. Oxygen went from fourteen to three percent in three weeks. It stayed there, too, until a terraforming gang could get in and start to change it back. That sudden switch would have killed almost everybody, because in the time available they wouldn’t have had a hope of shuttling everyone out.”

Max Perry nodded. “I know. Except that one man down on Pelican’s Wake decided to move people offworld anyway, long before they got near the comet shower. He’d seen fossil evidence for changes, right? It’s a classic case — the man on the spot knew more than anyone light-years away could know. He overrode instructions from his own headquarters, and he was a hero for doing it.”

“Not quite. He got chewed out for doing it.” The car had touched down and was taxiing toward the edge of the port, and Rebka was ready to let the subject drop. It was not the right time to tell Max Perry the identity of the man involved. And although he had been reprimanded in public, he had been congratulated in private for his presumption in countermanding a Sector Coordinator’s written instructions. The fact that his immediate supervisors had deliberately left him ignorant of those written instructions was never mentioned. It seemed to be part of the Phemus Circle’s government philosophy: Troubleshooters work better when they do not know too much. More and more, he was convinced that he had not been given all the facts before he was sent to Dobelle.

“All I’m saying is that you could face a similar situation on Opal,” he went on. “When a system is disturbed by a periodic force, increasing the force may not simply lead to a bigger disturbance of the same kind. You may hit a bifurcation and change to a totally different final state. Suppose the tides on Opal become big enough to interact chaotically? You’ll have turbulence everywhere — whirlpools and waterspouts. Monstrous solitons, maybe, isolated waves a mile or two high.

“Boats wouldn’t live through that, nor would the Slings. Could you evacuate everyone if you had to, during Summertide? I don’t mean to sea — I mean right off-planet?”

“I doubt it.” Perry was switching off the engine and shaking his head. “I can be more definite than that. No, we couldn’t. Anyway, where would we take them to? Gargantua has four satellites nearly as big as Opal, and a couple of them have their own atmospheres. But they’re methane and nitrogen, not oxygen — and they’re far too cold. The only other place is Quake.” He stared at Rebka. “I assume we’ve given up on the idea that anyone should go there?”

The torrential rain that had plagued their approach to Starside had eased, and the car had come to a halt close to the building that Perry had assigned to Darya Lang as living quarters.

Hans Rebka stood up stiffly from his seat and rubbed at his knees. Darya Lang was supposed to be waiting to meet them, and she must surely have heard the aircar’s approach. But there was no sign of her at the building. Instead, a tall skeletal man with a bald and bulging head was standing half-clear of the overhanging eaves, staring at the arriving car. He was holding a garish umbrella above his head. The shimmering white of his suit, with its gold epaulets and light-blue trim, could have come only from the spun fiber cocoon of a Ditron.