Yet the guilt was there, and he knew why. He was taking advantage of a friendship for secret purposes of his own, by suggesting that his interest in the Asymptotic Drive was no more than would be expected from anyone with a scientific or engineering background. But perhaps Warren was not as naieve as he seemed; he could hardly be unaware that the Drive posed a threat to the entire economy of Duncan’s society. He might even be trying to help, in a tactful way.

“You may be disappointed,” said Warren as they passed through the bulkhead floor separating levels Three and Two. “There’s not much to see. But what there is is enough to give some people nightmares which is why we discourage visitors.”

Not the most important reason, thought Duncan. The Drive was not exactly a secret; there was an immense literature on the subject, from the most esoteric mathematical papers down to popularizations so elementary that they amounted to little more than: “You pull on your bootstraps, and away you go.” But it would be fair to say that Earth’s Space Transportation

Authority was curiously evasive when it came down to the practical details, and only its own personnel were allowed on the minor planet where the Drive was assembled. The few photos of Asteroid 4587 were blurred telescopic shots showing two cylindrical structures, more than a thousand kilometers long, stretching out into space on either side of the tiny world, which was an almost invisible speck between them. It was known that these were the accelerators that smashed matter together at such velocities that it fused to form the node or singularity at the heart of the Drive; and this was all that anyone did know, outside the

STA.

Duncan was now floating, a few meters behind his guide, along a corridor lined with pipes and cable ducts-all the anonymous plumbing any vehicle of sea, air, or space for the last three hundred years. Only the remarkable number of handholds, and the profusion of thick padding, revealed that this was the interior of a ship designed to be independent of gravity.

“D’you see that pipe?” said the engineer. “The little red one?”

“Yes-what about it?” Duncan would certainly never have given it a second glance; it was only about as thick as a lead pencil.

“That’s the main hydrogen feed, believe it or not. All of a hundred grams a second. Say eight tons a day, under full thrust.”

Duncan wondered what the old-time rocket engineers would have thought of this tiny fuel line. He tried to visualize the monstrous pipes and pumps of the Saturns that had first taken men to the Moon; what was their rate of fuel consumption? He was certain that they burned more in every second than Sirius consumed in a day. That was a good measure of how far technology had progressed, in three centuries. And in another three … ?

“Mind your head-those are the deflection coils. We don’t trust room-temperature superconductors. These are still good old cryogenics.”

“Deflection coils? What for?”

“Ever stopped to think what would happen if that jet accidentally touched part of the ship? These coils keep it centered, and also give all the vector control we need.”

They were now hovering beside a massive-yet still surprisingly small-cylinder that might have been the barrel of a twentieth-century naval gun. So this was the reaction chamber of the Drive. It was hard not to feel a sense of almost superstitious awe at the knowledge of what lay within a few centimeters of him. Duncan could easily have encircled the metal tube with his arms; how strange to think of putting your arms around a singularity, and thus, if some of the theories were correct, embracing an entire universe…. Near the middle of the five-meter-long tube a small section of the casing had been removed, like the door of some miniature bank vault, and replaced by a crystal window. Through this obviously temporary opening a microscope, mounted on a swinging arm so that it could be moved away after use, was aimed into the interior of the drive unit.

The engineer clipped himself into position by the buckles conveniently fixed to the casing, stared through the eyepiece, and made some

delicate micrometer adjustments. “Take a look,” he said, when he was finally satisfied.

Duncan floated to the eyepiece and fastened himself rather clumsily in place. He did not know what he had expected to see, and he remembered that the eye had to be educated before it could pass intelligible impressions to the brain. Anything utterly i!nfamiliar could be, quite literally, invisible, so he was not too disappointed at his first view.

What he saw was, indeed, perfectly ordinary merely a grid of fine hairlines, crossing at right angles to form a reticule of the kind commonly used for optical measurements. Though he searched the brightly lit field of view, he could find nothing else; he might have been exploring a piece of blank graph paper.

“Look at the crossover at the exact center,” said his guide, “and turn the knob on the left-very slowly. Half a rev will do-either direction.”

Duncan obeyed, yet for a few seconds he could still see nothing. Then be realized that a tiny bulge was creeping along the hairline as he tracked the microscope. It was as if he was looking at the reticule through a sheet of glass with one minute bubble or imperfection in it.

“Do you see it?”

“Yes-just. Like a pin head-sized lens. Without the grid, you’d never notice it.”

“Pinbead-sized! That’s an exaggeration, if ever I heard one. The node’s smaller than an atomic nucleus. You’re not actually seeing it, of course-only the distortion it produces.”

“And yet there are thousands of tons of matter in there.”

“Well, one or two thousand,” answered the engivneer, rather evasively.

“It’s made a dozen trips and is getting near saturation, so we’ll soon have to install a new one. Of course it would go on absorbing hydrogen as long as we fed it, but we can’t drag too much unnecessary mass around, or we’ll pay for it in performance. Like the old seagoing ships-they used to get covered with barnacles, and slowed down if they weren’t scraped clean every so often.”

“What do they do with old nodes when they’re too massive to use? Is it true that they’re dropped into the sun?”

“What good would that do? A node would sail right through the sun and out the other side. Frankly, I don’t know what they do with the old ones.

Perhaps they lump them all together into a big granddaddy node, smaller than a neutron but weighing a few million tons.”

There were a dozen other questions that Duncan was longing to ask. How were these tiny yet immensely massive objects handled? Now that Sirius was in free fall, the node would remain floating where it was-but what kept it from shooting out of the drive tube as soon as acceleration started? He assumed that some combination of powerful electric and magnetic fields held it in place, and transmitted its thrust to the ship.

“What would happen,” Duncan asked, “if I tried to touch it?”

“You know, absolutely everyone asks that question.”

“I’m not surprised. What’s the answer?”

“Well, you’d have to open the vacuum seal, and then all hell would break loose as the air rushed in.”

“Then I don’t do it that way. I wear a spacesuit, and I crawl up the drive tunnel and reach out a finger … to

“How clever of you to hit exactly the right spot! But if you did, when your finger tip got within—oh -something like a millimeter, I’d guess-the gravitational tidal forces would start to tear away at it. As soon as the first few atoms fell into the field, they’d give up all their mass-energy-and you’d think that a small hydrogen bomb had gone off in your face. The explosion would probably blow you out of the tube at a fair fraction of the speed of light.”