waves. She wondered how it was for Floyd, whose head was free of twinkling machines. What was he thinking now that he was so close to home, and simultaneously so close to seeing everything he knew destroyed?

"By my estimation," Tunguska said, "the Molotov impact will happen in fifty seconds. I'm deploying expendable sensors, but closing off all our usual channels. No one's ever seen a big matter-antimatter explosion up close, and there's no telling what kind of reaction the blast will provoke from the ALS itself."

"How close is that shuttle to the impact point?" Auger asked.

"About half our present distance," Tunguska replied. "His shielding had better be good if he wants to be alive at the end of this. Thirty seconds-"

"I can do without the countdown, Tunguska," Auger said, bracing herself. "Just tell us if we're still alive at the end of it."

She felt, when it happened, some ghostly report of the blast, even though Tunguska a.s.sured her that no signals could possibly reach her through the barricades he had put in place. It was long and drawn out, like a distant peel of thunder.

"The Molotov device has detonated," Tunguska said. "And we, self-evidently, are still alive."

"I was being sarcastic."

"I wasn't. It's always good to confirm these things."

When the expendable sensors deemed it safe, Tunguska unshuttered the ship's more delicate eyes and turned them on the scene of the crime. It took a little while for them to make sense of the data, for the view was obstructed by a slowly expanding debris plume, spreading away from the impact point like a cherry-red fountain. Auger grappled with the scale, but she still couldn't adjust to the mind-numbing size of the ALS object. The plume was huge-hundreds of thousands of kilometres across and still growing-but it was just a tiny detail on the surface of the sphere.

"Debris is clearing near the epicentre," Tunguska said. "The view is foreshortened, so it isn't easy to see exactly what damage has been done."

"Just show us what you've got," Auger said.

They had to wait twenty minutes until the plume had dissipated sufficiently, and their angle of observation improved enough, to allow a clearer view. By then, Tunguska's ship was following the same arcing trajectory as Niagara's, curving around for a hard interception with the ALS. They were still sustaining five gees, coc.o.o.ned against harm.

"They've broken through," Tunguska said.

He pushed an image into Auger's head. The Molotov device had punched a surprisingly neat little entry

wound into the skin of the ALS. The hole was a hundred kilometres across and nearly circular. The kilometre-thick skin glowed painfully brightly around the edge of the hole, shading down through blue and yellow and charred red out to a distance of perhaps two or three hundred kilometres from the epicentre. There were hints of wild, lashing structures in the exposed cross section, flailing like severed nerve endings.

"Dear Christ," Auger said. "They did it. The d.a.m.ned thing didn't put up any kind of fight at all."

"Did you expect it to?" Floyd asked.

"I expected something."

"What about the other ship?"

"Still tracking it," Tunguska said. "She's under thrust and maintaining the course she was following

before the blast. It will take her through the wound in under ten minutes."

Maybe he shouldn't have been so concerned about the state of Niagara's shielding, she thought. "I take it we're still not within beam range?"

"No." Tunguska sounded genuinely embarra.s.sed. "We'll have to follow her in for that."

"Through the wound?"

"Yes," Tunguska said. "Into the ALS. I'm afraid it's the only course available to us."

FORTY.

By the time they were about to pa.s.s through the hole that Niagara had punched into the ALS, the debris cloud had completely cleared. The wound remained raw and bright, spilling a faint shaft of re-radiated golden-white light back into s.p.a.ce, twinkling off the few remaining shards of hot matter still hanging around the impact site.

"That light has the spectrum of solar radiation," Tunguska said, when they were falling down the column of light. "It's a perfect match for the Sun, at the limit of our instruments."

The transition between outside and inside happened in an eyeblink. One kilometre of sh.e.l.l thickness was nothing compared to their speed. One moment the surface of the sphere was swelling larger, with the wound growing rapidly from a searing, white-rimmed eye to a swallowing mouth...and then they were through, falling towards the heart of the ALS.

Tunguska's sensors took immediate stock of the interior. Behind his ship, the receding wound embraced a circle of the perfect blackness of interstellar s.p.a.ce. It was rimmed with bright, agonised matter on this side as well. But instead of the quilted blue-grey material of the outer skin, the inner surface of the ALS was made of something far stranger; something far less susceptible to easy interrogation by Tunguska's instruments.

They had always known that the inner surface of the sh.e.l.l had to function as a kind of near-perfect planetarium, projecting an image of the sky that would have been seen from the original Earth. There were false stars, their brightness and colours reproduced precisely, aligned into exactly the right constellations that the inhabitants of E2 had learned to expect. Some fraction of the stars must even have been programmed for variability-to dim and brighten according to intricate astrophysics-rich algorithms. They were all required to move with respect to each other, following the slow, stately currents of proper motion, or the wheeling gyre of binary orbits.

Beyond the stars, there were galaxies, vast shoals of them in every direction. Each and every galaxy had to stand up to the same scrutiny as the stars. Novae and supernovae had to flare and die...whether they were noticed or not.

It was awesome and astonishing. It was also doomed to failure, for no such tapestry could ever have withstood arbitrarily close study using the kind of astronomical tools available in Auger's era. Even a simple interplanetary probe would have eventually sniffed out something odd about those stellar positions...just before it dashed itself to atoms by colliding with the inner surface of the sh.e.l.l. No: it wasn't perfectly foolproof, nor must that ever have been the intention of its builders. It was good enough to withstand examination using the crude science of Floyd's time, but it was never the intention that the sh.e.l.l should form an utterly convincing illusion. Sooner or later, it must have been a.s.sumed, the inhabitants of E2 were bound to discover the truth. The function of the ALS was to protect them from outside interference until precisely that moment. After that-at which point they would probably direct their energies into breaking through the sh.e.l.l-they were on their own.

But there was already something amiss with the view of the heavens around the inside edge of the open wound. For thousands of kilometres in all directions, the stars were distorted, elongated and spermlike, their stretched, tapering tails pointing like accusing fingers towards the hole Niagara had made.

"The zone of distortion is spreading," Tunguska said. "Frankly, they're going to have a hard time not noticing that on the Earth, even if they somehow missed the initial flash."

"What will they make of it?" Auger wondered.

"I don't know. But if an inexplicable astronomical puzzle is all they have to worry about by the end of the day, they'll be doing rather well."

"Can we shoot that shuttle down yet?" she asked.

"No," he replied. "But I'm ready to squeeze a little more out of the bleed-drive. If my estimates are good, we still have a chance of intercepting her before she hits the atmosphere."

"Don't hesitate, Tunguska."

"I won't. There is something I feel I should mention, though. It's just an observation, and it may be misleading."

Auger didn't like the sound of that at all. "Tell us anyway."

"The wound appears to be healing itself. The aperture was more than a hundred kilometres across immediately after the detonation of the Molotov device."

"And now?"

"A shade under a hundred. It may not mean anything-it's rather difficult to define the precise boundary -but I thought I should draw it to your attention."

"Keep an eye on it," she said. "I don't want that d.a.m.ned thing closing on us while we're still inside."

"I'll have a better idea of the closure rate in a little while," Tunguska said.

"Squeeze as much speed out of this thing as you can. Then we can all go home."

For the next hour, they pushed deeper into the ALS, following the lone echo of Niagara's shuttle. All attempts at communication were ignored, although that did not stop Tunguska from making repeated offers of negotiation. He was, he said, prepared to consider any proposal that would stop the deployment of Silver Rain. But no acknowledgement of his messages ever returned.

Despite the urgent need to intercept the shuttle before it reached Earth's atmosphere, Auger could not help marvelling at the experience of being inside the ALS sphere and seeing her world as it should have been. This was an Earth that had never known nuclear war, or runaway climatic catastrophe, or smart weather, or a Nanocaust. The sight of it made her want to weep. No image had ever come close to the heartbreaking beauty of this small blue world, a beauty all the more acute now that she knew how exquisitely fragile it was. It was the beauty of a b.u.t.terfly's wing.

E2 hung at the exact geometric centre of the ALS. Orbiting it, or at least moving in a convincing simulacrum of Newtonian motion, was what appeared to be an identical copy of the Moon. Auger presumed it had been captured in the same quantum snapshot as E2, but it would take close-up investigation to verify this. The Moon could just as easily be a mocked-up representation, imbued with enough detail to fool surface observers and enough gravity to lift tides on the Earth. The remaining contribution to the tides-the solar component-must have been achieved through some deft trickery of gravitational manipulation-invisible small orbiting ma.s.ses, perhaps-for there was no sun. Instead, there was a golden-yellow disc of exactly the right temperature and apparent brightness shining out from the inner surface of the sphere. But it was only designed to look convincing from the vantage point of the Earth's surface, and close to they saw how its shape was distorted by the sphere's concavity.

"There's your source of solar-spectrum radiation," Auger said. "From outside the sphere we were seeing its light, leaking through the hole. How long do you think it would have taken Floyd's people to figure that out?"

"Even without s.p.a.ceflight, they'd have begun to notice some puzzling things about it within a few decades," Tunguska said. "In our timeline, a great deal of attention was focused on measuring the circularity of the solar disk, since it turned out to be a way of discriminating between competing cosmological models. With that kind of attention, the illusion probably couldn't have been sustained for long."

"Or maybe they'd just pick another theory," Auger said.

"Perhaps."

"Anyway, Floyd's world hasn't achieved the science ours did even by nineteen fifty-nine."

"They could quickly make up lost ground," Tunguska said. "And then they might put up too much of a fight if someone attempted to do what Niagara is attempting now."

"Which means that whoever was working behind the scenes had serious co-ordination," Auger said. "Enough to change the outcome of the Second World War before it became truly global. And whoever did that is still down there."

"You think they deserve retribution, don't you?" Tunguska asked.

"Of course. Don't you?"

"They stopped a war in which millions died in our timeline, Auger. No Final Solution, no Russian Front, no Hiroshima, Nagasaki."

"They didn't stop that war out of the goodness of their hearts, Tunguska. They stopped it because it interfered with their plans for global genocide. And now I think they should pay for it."

"Well, we're almost within attack range, if that's any consolation. That little shuttle is having to decelerate in preparation for atmospheric flight. If it released Silver Rain at this speed, even the ablative jackets wouldn't protect the nanomachinery at the heart of the weapon. There's some uncertainty, but I can begin attempting the strike within three minutes."

"What about the missiles you promised us?" she asked.

"Nearly ready. Patience, please." She heard a note of diffidence in his voice. "Concerning the other matter...."

"What other matter?"

"The healing of the wound. I've been keeping a close eye on it and I can now state with some authority that-"

"Is there still time for us to get out?"

"Yes, allowing for-"

"I don't need anything else to worry about, Tunguska."

"Good. In which case I won't mention the bleed-drive."

Tunguska was as good as his word. Barely two minutes later, Auger felt the slight change in the ship's attack posture that indicated it was bringing its beam weapons to bear. When they powered up and fired, discharging in timed salvos, she felt the surging and ebbing of ma.s.sive acc.u.mulators somewhere in the ship's gut.

"How long can we sustain this fire rate?" she asked.

"For as long as it takes. Energy isn't a problem."

The shuttle had antic.i.p.ated a beam-weapon strike-Tunguska said this was almost inevitable-but it was limited in its defensive options. It could drop reflective chaff by shedding discrete layers of its hull, but not indefinitely. It could change its course randomly, making it more difficult for the beams to lock on to the bright aura of its drive flame-which was pointed away from them now, but still visible against the background of E2 and the inner surface of the ALS sphere-but every course correction cost it some of its hard-won lead. For the pilot of the shuttle, it was the trickiest of trade-offs to balance.

"Whatever Niagara does," Tunguska said, "it will hurt him in the long run. All my simulations now point to a successful interception before he's within drop-range of the atmosphere."

There was something about the c.o.c.ksure confidence of that statement that gave Auger goose pimples. It was like an invitation to fate.

That was when the bleed-drive chose to fail again.

She felt the ship stall in its chase, suddenly losing ground on its victim. The drive stuttered, pushing hard and then cutting out. The cushioning embrace of the ship did its best to smooth over the sudden changes in acceleration, but Auger still felt several lapses in consciousness as the blood in her brain sloshed around like mud in a bucket.

"Tunguska..." she gasped, when she was able, "maybe you want to rethink..."

The ship was in free fall. The drive had died completely, shut down by emergency control systems before instabilities opened a drooling mouth in the flesh of s.p.a.ce itself.

Over the next several minutes, repair estimates began to trickle in. The drive was still fixable, but the patches put in place since the missile attack had now outlived their usefulness. It would take many hours before even a moderate push of one gee could be achieved.

Sensing that its charges no longer needed to be buffered against the jolts and swerves of combat, the ship relinquished its hold on Floyd, Auger and Tunguska, the white coc.o.o.ns collapsing back into the familiar forms of table, chairs, floor, walls and ceiling.

"I hope," Auger said, "that you have a backup plan, Tunguska. Because otherwise we're screwed."

Tunguska, to his credit, still managed to retain a credible gloss of authority. "I've already reviewed the options," he said. "You'll be pleased to hear that there is still a way of intercepting that s.p.a.cecraft."

"The missiles?" Floyd asked.

"No." He gave a self-critical grimace. "Well, yes. But it's not quite that simple."

Auger looked at Floyd and rolled her eyes. "It never is. What's your plan?"

"The missiles don't have the range from here. My internal repair factories have license to manufacture almost anything except complete bleed-drive a.s.semblies. I had to settle for small, crude fusion power plants. They're fast and agile enough for the task and they'll double as warheads, but only if they're given a helping hand."

The tone of his voice said beware. Whatever he was offering them, it was not without its costs.

"Such as?" Auger asked.

"They'll need a delivery system. We can't get close enough at the moment, and by the time the ship's