"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 it 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 unfamiliar 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 from 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 to the left — very slowly. Half a rev will do — either direction."
Duncan obeyed, yet for a few seconds he could still see nothing. Then he 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 pinhead-sized lens. Without the grid, you'd never notice it."
"Pinhead-sized! That's an exaggeration, if I ever 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 engineer, 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..."