"True. This ship can only accommodate a very short-ranged sensor suite. But that is all that is required.

You see, we commenced our temporal displacement at no great distance from where Novak did-and objects in this state of dimensional phasing conserve their spatial relationships in our own universe. And, for reasons concerning which I'll have to refer you to the specialists, the gravitational 'shadow' which makes the sensors possible is detectable across the dimensional divide."

"But," I persisted, "we won't be able to see her ship until we 'become contemporaneous' with it?"

"Not altogether true. Once we detect it, we will immediately begin to come into phase with it. Once we get 'close' enough to it, dimensionally speaking, it will become visible, albeit indistinctly. Indeed, under these conditions we should be able to appear to physically interpenetrate with it-a somewhat unsettling sensation, I would imagine." Khorat paused reflectively. "Come to think of it, if the two ships came into

full dimensional synchronicity, permitting actual physical interaction, while such an interpenetration was taking place, it would end the problem rather definitively.""It would end us rather definitively," I pointed out. "You're not really planning to . . . ?""Oh, no. It was only a thought." But Khorat's tone was altogether too d.a.m.ned thoughtful for my taste."Then what is the plan?" Chloe sounded like she wanted to get Khorat's mind off this particular track without delay.

"The impellers, being reactionless in nature, will function under the conditions of temporal displacement, though at a reduced level of efficiency. In other words, we can maneuver, albeit

sluggishly. We will establish synchronicity with Novak's ship at the closest range we can safely manage, and . . . use the laser weapon."

"And what then?" Chloe persisted. "a.s.suming that all goes according to plan, where does that leave us?"

"We will immediately disengage the temporal displacement field generator, and drop back into the

normal universe. As I have indicated, we should be only twenty years or less into the past."

"But Khorat," Chloe continued remorselessly, "our history has no mention of an Ekhemasu ship appearing in Earth orbit. And your history doesn't include slightly older versions of you and Thramoz

and the rest showing up on Khemava. So won't we be doing the same thing you told us Novak would do by changing the past?"

Khorat tried to look away, but Chloe met those huge dark eyes and held them. There was an urgent

intensity in her that could be sensed even from beyond the gulf of alienness. He spoke expressionlessly. "We have antic.i.p.ated the possibility that this situation might arise ever since we . . . dealt with Imhaermekh. So we took steps. Some time ago the Medjavar prepared a habitat in a hollowed-out asteroid in the trailing-Trojan position of our sun's fourth planet. Then it was left to robotic maintenance, unoccupied and unapproached. Only a few individuals know of its existence. Only a very few, of whom I am one, know its location. We will remain cloaked when we appear in Earth orbit. We will then proceed back to the Khemava system, to that habitat. There we Ekhemasu will pa.s.s the remainder of our lives, unable to influence history."

By now, I knew the capabilities of the translator software too well to doubt that what I'd just heard was precisely what Khorat had said, and meant. So I tried to cope with it on its own terms. I failed. The philosophy behind it was simply too foreign. Compared to the Medjavar, the monks of Mount Athos were regular guys.

"So," I finally managed, "you and the rest of this ship's crew have been sitting tight inside some asteroid for the last couple of decades, while at the same time you-that is, the you I'm talking to right now-have been on Khemava, and on Antyova II meeting me, and . . ." My head was starting to spin.

"We can only hope so." Khorat's bland insouciance was returning. "That would mean that our present mission succeeded."

My head spun faster.

"And what about Bob and me?" I silently applauded Chloe for not losing sight of that little matter. But

her tone was still strange, and her face wore the look of a soldier advancing into a minefield.

"We are not devoid of ethics. Our intention is to return the two of you to your own world as soon as it is safe to do so-that is to say, as soon as we can manufacture ident.i.ties for you. Among Earth's teeming billions, you should not be able to alter history in any measurable way . . . and I hope what I have told

you will persuade you not to try. At a minimum, we will require your solemn promise that you will make no attempt to contact your own younger selves."

"So," I breathed, "was this what you really had in mind all along when you had Nafayum do a makeover

on us?"

"By no means. What you were told about our plans was quite truthful . . . at the time. Of course, I cannot deny that some such contingency as this entered our minds."

A contingency you never thought to mention to us, I thought darkly. Chloe remained inscrutable.

Thramoz got up from behind a computer station-the weapon station, I a.s.sumed-and spoke matter-of-

factly. "We should all get into survival suits, while there is nothing for anyone except watch-standers to do."

"A sensible suggestion," said Khorat, obviously grateful for the interruption.

Suits had been prepared to my and Chloe's measurements. Aside from their size and shape, they were like the Ekhemasu models, which means they bore little resemblance to your visualization of a "s.p.a.ce suit." They were flexible, lightweight, form-fitting jumpsuits made of quasi-alive, self-repairing nanoplastic. No bulky air tanks or anything; just a small belt-mounted bottle with enough air to pressurize the suit, after which it could keep you alive for weeks on your own recycled exhalations and wastes. All you had to do was slip on gloves and pull over a flexible transparent hoodlike helmet. As everyone got suited up, Chloe and I were never alone. (Was it my imagination, or did she arrange it that way?)

Then we waited. The hours crawled by with an unnatural slowness that had nothing to do with the peculiarities of the s.p.a.ce in which we were falling backwards through time.

When it finally came, the alarm seemed deafening.

"Novak's ship has been detected," said Khorat unnecessarily, amid the crackling tension that suddenly pervaded the control room. "We will now begin to close with it, in dimensional terms."

"Why can't we just go directly to the same dimension Novak is in?" I wondered aloud.

"That is theoretically possible. But interdimesnsional energy differentials make it unacceptably risky. It is safer to transition gradually from one dimension to another, as we are doing now."

Even as he spoke, the disturbing blankness in the viewscreen began to change.

It was barely perceptible at first, as though a mist was trying to solidify into the form of a s.p.a.cecraft a

few thousand yards to port and a little ahead of us. Then it came into focus . . . but a ghostly, transparent kind of focus. Then it began to flicker and go out. The control room crew went silently but unmistakably to a higher pitch of concentration.

"Novak, or her pilot, is doubtless trying to evade us by shifting dimensionally," Khorat explained. "Of course, she can only shift 'downward,' inasmuch as her ship was already at its highest attainable level of temporal displacement. This simplifies our problem in matching her shifting." And even as we watched, the phantom ship began to firm up again.

Novak tried a new tack, attempting physical evasive action now that the dimensional variety had failed.

But I could now see what Khorat had meant about the sluggishness of impeller-driven maneuvering in this strange medium. Our quarry-it looked like a ship made of gla.s.s-swung away from us with no more apparent swiftness than a sailing vessel beating into the wind. We followed suit, but such maneuvers were evidently hard to control as well as slow. Before our pilot could compensate, he had overshot Novak's ship . . . and, for a second we occupied the same s.p.a.ce it did, in a slightly different dimensional phase.

That interpenetration was utterly indescribable. For an instant, I was only a few yards away from what looked like the ghost of Renata Novak, her mouth open and shouting inaudible commands. Then we were past.

"Stand by for dimensional synchronicity," the pilot called out. Thramoz, at the weapon station from

which he could directly control the laser cannon, stiffened into a posture of tense expectancy.

But I was conscious of none of this, for my mind was empty of all save the stunning realization of what I had, at the last split second, glimpsed through the transparent inner complexities of Novak's ship. There could be no mistake, for it was a shape I'd come to know lately.

"Khorat," I blurted, "she's got a laser cannon!"

The old Ekhemar stood immobilized by shock. With a curse, I turned away from him toward the pilot's station.

"Abort!" I yelled.

It was too late. At that moment, the two ships came into full dimensional synchronicity. With shocking

abruptness, Novak's ship was there in all its solid reality.

Thramoz shouted something and jabbed at his controls.

With a hideous din of tearing metal, the ship cried out its agony. Concussion threw us to the deck.

* * * Even laser beams using the wavelengths detectable by human eyes are invisible in vacuum. Still more so are the X-ray lasers that are standard shipboard armament.

Otherwise, though, the Hollywood version was surprisingly accurate. Rather than a continuous beam-impractical for a lot of reasons-a weapons-grade laser delivers a short, intense pulse of energy to a small area of its target, which it vaporizes, causing an explosion like the one whose shock waves had sent us sprawling.

But I'd underestimated the Ekhemasu . . . or at least Thramoz, an atypical specimen of the race. He must have fired in the same nanosecond as Novak's human gunner, because--incredibly-the two beams had struck with such simultaneity that both laser weapons were disabled.

"Furthermore," Thramoz reported to Khorat, "we believe the target has lost its ability to shift dimensionally, or to maneuver under impeller drive. Otherwise, it would be doing one or the other now, to evade us."

"But it can still return to the normal universe?" Khorat asked anxiously.

"Presumably. That merely requires disengaging the temporal displacement field generator. But Novak will not want to do so yet, as we are still less than twenty years into the past, far short of the era she is aiming for."

"But she can still do it whenever she chooses." Khorat's body language held more despair than the

translator could put into a voice.

"What about us?" I demanded. The ship's air held an acrid smell of electrical fires, and I could hear a distant commotion that smacked of damage control.

"Our life support, navigational and drive systems are in no danger," Thramoz told me. "Only our laser weapon is damaged beyond hope of repair. Presumably, Novak targeted it first, intending to finish us off at her leisure afterwards."

"You prevented that," said Chloe. "You nailed them to the wall!"

Thramoz looked about as comfortable with the compliment as a human would have been with praise for

virtuosity as a cannibal chef. "Well, computer a.n.a.lysis indicates that our laser cannon was more powerful than theirs, so our one shot caused more damage than theirs."

"So," I summarized for him, "they're sitting out there dead in the water, incapable of firing on us or