The ship must be there. The ship was not there. I sat and wondered. Where’s Heinrich?

McAndrew emerged from his trance when we still had a full day of flight time ahead. “I’ve got it,” he said. “Or at least, part of it.”

“Well, isn’t that nice.” I should have known better. Sarcasm is wasted on the man. He smiled at me. I went on, “So you know how changing the strong force would allow somebody to do the things that your father claimed?”

He stopped smiling. “Och, I’ve always known that. It’s obvious.”

“Not to me it isn’t. I don’t suppose you’d consider explaining?”

“The strong force holds nuclei together. So suppose you could make it more powerful and act over a longer range, for an indefinitely long period. That would lead to stable compressed matter, and you’d be able to squeeze anything you like into a smaller space. If you used the strong force to overcome electromagnetic repulsion between protons, you’d also be on the road to easy proton-proton fusion at room temperature.

“Now suppose you work the other way round, and know how to weaken the strong force. Then nuclei will be less strongly bound, and a lot of naturally stable elements will become unstable. That gives you induced radioactivity.”

“But that’s everything.” It seemed to me he had covered everything on Heinrich Grunewald’s list. “Why do you say you only have part of it?”

“Because I slipped something in there at the beginning. I said, suppose you could modify the strong force for an indefinitely long period. That’s the killer, Jeanie. I can see a way to make changes, but they’d be unstable. Worse than that, they’d be unpredictable. You’d never know when the effect would reverse itself and things return to the way they were. And when I try to stabilize the situation over time, I need to assume the existence of isolated quarks. I might find a way around that when I give it a bit more of a think, but I still won’t know what went wrong. Something surely did. There must be a missing piece — some trivial point, some fact that I’ve overlooked…”

He was all set to drift off again. I said, loudly, “Mac, we’d be a lot closer to the something you’re missing if we knew how to find the Fafner. In another twenty-four hours we’ll be sitting out in the middle of the Asteroid Belt, wondering what to do next.”

He stared at me with those pale, vague eyes. “Why, I already know what we’ll do. Why do you think I wanted to take the Driscoll, instead of something with a balanced drive?”

“Because you wanted time to think.”

“Maybe. And I got that. But we’d have a problem if we’d used the balanced drive. The compressed matter plate on any of those ships masses trillions of tons.”

“That’s never given us any difficulty before.”

“Because we’ve never had this situation before. Think about it, Jeanie. If the Fafner’s still in one piece, what’s the one thing about itself it can’t hide?”

“Thermal signature?”

“That’s not a bad answer for a ship that’s alive. Anything with people on board has to generate and give off heat. But the Fafner’s more than likely dead, so there would be no thermal signal. A ship can be at the same temperature as its surroundings, or it can change its size and shape, or it can be coated with an absorbing layer so it’s hard to see. The one thing it still has, no matter what it does, is mass.”

When he spoke that one, final word I could see the rest for myself. We knew from the Fafner’s records the ship’s mass at the time of final engine burn: three hundred and thirty tons. Even if Grunewald had jettisoned material into space, the present value would not be far from that. If we were anywhere close to the ship we could locate it using our mass detector. The instrument was highly sensitive and it could be programmed to correct for every nearby object in the right size range. The trillion-ton mass of a compressed matter plate was something else. On a ship with a balanced drive, the plate’s effects would overwhelm the gravity field of everything in the neighborhood.

It’s my one big complaint with McAndrew. He assumes you understand what he understands. Even when you feel sure you know what goes on inside his head, you don’t. One of his colleagues told me that the difference between McAndrew and other people is that Mac knows how to think around corners. It’s probably true, but it doesn’t help much.

I wondered if he got that talent from his father. I had no idea what Heinrich Grunewald had been doing out here, or why he had disappeared thirty years ago from the face of the Solar System. I thought of Mary McAndrew’s words, “he was a wicked, obstinate, reckless man.” Cut out the wicked part, and you had McAndrew.

I can’t think around corners, but I have excellent instincts for danger. And I was feeling uneasy, more nervous than the situation seemed to justify.

We had arrived, exactly at the place where the Fafner was supposed to be. I cut the drive and used the visible wavelength sensors to scan through a full four-pi solid angle.

Did I see the ship? Of course not. You might say I already knew I wouldn’t, because if it had been there the scopes of the Penrose Institute would have found it before we ever left.

I pointed that out to McAndrew.

“Which is why we had to come out here and take a look for ourselves.” He seemed filled with secret glee, his normal reaction when facing a scientific mystery. “Jeanie, keep the drive off and the displays on, and let’s have a go with the mass detector.”

I took a last look at the screens. The optical sensors would find and highlight any unknown body that subtended more than a fiftieth of a second of arc. That meant I would see something as small as a tenth of a meter across, even if it were a hundred kilometers away.

The displays showed absolutely nothing. The Driscoll sat in the middle of a large volume of emptiness.

Convinced that we were wasting our time, I turned on the mass detector. Instantly, a loud buzz from the instrument made me start upright in my seat.

“There we are.” McAndrew clapped his hands in delight. “What do we show for mass and range?”

“Eighteen thousand tons,” I said. “So that’s not the Fafner, it’s far too massive. It’s at eighteen kilometers.” I took another look at the optical sensor outputs. “But Mac, there’s nothing there.”

“We’ll see. Take us that way — slowly.”

I did as he asked, but at one kilometer away from the invisible target I stopped the Driscoll. “No closer, Mac. That’s as near as we’re going.”

“But there’s no possible danger—”

“This ship has to take us home. If you want a closer look, we use suits.”

“Jeanie—”

“On a ship you can only have one captain.” When I feel a certain way I can be as obstinate as McAndrew.

He knows it, too. He scowled at me, but he didn’t argue. He went over and began to put on a suit. I locked the Driscoll to hold a fixed one-kilometer distance from whatever was exciting the mass detector, and went across to do the same.

Keeping an eye on McAndrew? Sure. But I had my own curiosity to satisfy. Why were the optical sensors and the mass detector at odds with each other?

He went a meter or two in front of me, heading for the place where the mass detector insisted that we would find an invisible eighteen-thousand ton object. The suit displays homed us in the location. McAndrew went slower and slower. He was using the light in his suit to illuminate the space ahead of him.

Finally he paused, and said, “There you have it!”

“There you have what?” I, only a meter behind him, saw nothing at all.

“The body — the eighteen-thousand ton body — that Heinrich used to test his strong force modifier. If he compressed it down to three billion tons per cubic centimeter, the radius would be a fraction of a millimeter — just about right.”