I certainly didn’t expect that Lyle or Parmikan would volunteer for the job of helper, but equally I didn’t expect that they’d agree to my doing it — if I were outside, how could they give me disgusting chores? But Parmikan nodded his head at once.

“Right,” said McAndrew. His diffident manner had vanished. “Jeanie, while I get the equipment ready I want you to take the Hoatzin to encounter one of the anomalies. Put us smack in the middle, and set us to hold at zero relative velocity.”

I didn’t argue. But as he went off to the rear of the living-capsule, I did exactly half of what he had requested. A small, watchful region of my brain was awakening, from a slumber it must have been in ever since I decided to reply to McAndrew’s letter by flying straight to the Institute. Now I closed in on one of the objects and set us to zero relative velocity — but I kept our ship a couple of kilometers clear rather than providing McAndrew’s requested encounter. He might know exactly what he was dealing with, and be sure that it was safe. Until I had that knowledge, too, I was going to regard any region of empty space occupied by a mystery as possibly dangerous.

As I was completing my task I noticed a minor oddity in the operations of the Hoatzin’s computer. The program was functioning flawlessly, but as I directed each change in position or speed, a status light indicated that an extra data storage was being performed. The response time was a fraction of a second longer than usual.

I’d probably have caught it when at Parmikan’s insistence we switched the drive on and off every day, but Mac had programmed all those changes as a favor to me. And if McAndrew had been ready to go outside at once, I might have ignored it now. Instead, I took a look to see where the generated data were being stored.

I found a Dummy’s Delight.

The data I was creating were being placed in a trajectory control program of a type much despised by professionals. It was the sort of thing that anyone could use and no one ever did, because it was guaranteed to be inefficient. In a Dummy’s Delight, for every move made by the ship the inverse move was generated and stored. If the program were then executed, the ship would return to its point of origin along whatever convoluted trajectory it may have taken to fly out.

The program’s only advantage was simplicity. One push of a button, and all need for piloting went away.

But there was no way that we would fly back along our original trajectory — there were much more efficient thrust patterns. And I certainly hadn’t given the command to place the required data into the Dummy’s Delight before the Hoatzin left the Penrose Institute.

Had Mac done it? And if so, why?

My wariness node had started to work overtime. On impulse I wiped from memory the whole Dummy’s Delight sequence, and left a message on the control screen: AUTOMATIC RETURN PROGRAM TO TERRA HAS BEEN ERASED BY CAPTAIN ROKER. It seemed reasonable when I did it, but I started to have second thoughts as I hurried to join McAndrew. He had his mass detector survey instruments working as free-standing units and was already in his suit.

“Mac.” I waited until I had my own suit on, and was absolutely sure that we could not be overheard by Van Lyle or Stefan Parmikan. “Did you set up a Dummy’s Delight in the Hoatzin’s computer?”

He was busy, guiding the bulky detectors into the lock. “Now why would I do a daft thing like that?” he said, and vanished into the lock himself.

Why would he do a thing like that? I asked myself. Why would anyone do it? — unless they suspected that no competent pilot, like me or McAndrew, would be around to fly the Hoatzin on its return journey to Sol.

Paranoid? You bet. It’s the only way to fly.

I emerged from the lock into that great forever silence that fills all the space between the stars. Sol was dwindled, indistinguishable from dozens of others. I picked it out from its position, not its superior brightness. The region I floated in appeared totally empty and featureless, despite the suit’s vision enhancement systems. Particles were fewer here, the vacuum a little harder. But the human observer would never know the difference.

I glanced back at Sol. Look at the situation any way you liked; if I blew it and something went wrong, it was a long walk home.

I propelled myself gently away from the ship and towards McAndrew. He was staring at his mass detector readings in great irritation.

“Jeanie, you’ve made a mistake. We’re kilometers away from the source.”

“We certainly are. Two kilometers, to be exact. I know you can see right through it and it looks like nothing’s there, but I want to approach this particular nothing very carefully.”

He gave the patient sigh of a long-suffering martyr. “Ah, Jeanie. There’s no danger here, the way you’re thinking. I know just what this is.”

“Maybe. But you haven’t told me.”

“I will, though, right now. It’s shadow matter.” And then, seeing my stare, “I’m surprised you’ve not heard of it.”

“I know, Mac. I’m a constant disappointment to you. But I haven’t. So tell me.”

“It’s wonderful. Right out of supersymmetry theory. Soon after the Big Bang — about 10^-43 seconds after it, actually, before anything else we know about happened — gravity decoupled from everything else. Sort of like the way that radiation decoupled from matter, but the gravity decoupling happened much earlier. So then you had a symmetry breaking, a sort of splitting, and two types of matter were created: ordinary matter and shadow matter. Just like matter and anti-matter — except that matter and shadow matter can’t interact by strong nuclear forces, or radiation, or the weak nuclear force. They can only interact by gravitational influence. You’d never detect shadow matter by firing particles at it. We proved that for ourselves. The particles feel the gravitational force, but that’s tens of orders of magnitude too weak to do anything noticeable.”

I stared at nothing, in the direction that the mass detectors were pointing. “You’re saying that whatever is out there is as real as we are — but we can’t see it?”

“Can’t see it, never will be able to see it. Seeing depends on interaction with radiation. The only way to learn what we’ve found is through these.” He pointed at the mass detectors. “We’re safe enough, as I said. But we have to do some detailed mapping. Who knows what that is out there? It could be a shadow matter star — we don’t have any idea how big a star might be in that universe, or what the laws of force are. Or maybe we’re detecting a set of interstellar shadow matter spaceships, or a column of shadow matter ants marching in a shadow matter superworld.

“You think I’m joking, but I’m not. It could be anything. The only way we’ll get any idea what we’ve found is by plotting structure. That’s why I need you — it’s a two-person job, to make transects.”

We’re safe enough, he had said. But maybe only while we were outside the Hoatzin.

“Mac, before we start your work we’ve got to talk. I think we have a bad problem.” I told him about the Dummy’s Delight on the ship’s computer.

He frowned through his suit visor. “But why would they waste data storage on a thing like that?”

“So they could get home, even if something happened to both of us.” I took the last mental step, the one I had been resisting. “Mac, we’re not intended to return from this trip. The plan is for us to vanish while we’re out here. If the drive of the ship were turned on now, who’d ever know what had happened to us?”

He turned to stare at the Hoatzin. “They wouldn’t dare.”

“Not now they wouldn’t. I wiped the program they were relying on to fly them back. So they need us.” Or one of us. But I didn’t say that. “We’re safe enough for the moment.”