I listened, wondering who could possibly be sending a call for help from so far away. Two hundred and eighty billion kilometers, sixty times the distance of Neptune, way south of the ecliptic and far beyond normal Solar System runs. No cargo or passenger vessel ever ventured in that direction, or so far out.

Help, help, help. The standard Mayday distress signal was the only clear part of the message. “… limited chance for transmission… every year or two…” The voice was thin, scratchy, and distorted. “We transmit when we can, aim at Sol…” Help, help, help. “… we’ll keep sending as long as possible. We have no idea what’s happening outside… are trapped… except for this chamber. No control of resources except this unit…”

Help, help, help. The automated Mayday signal bleated on, over and over. I heard nothing more from the desperate human voice.

McAndrew was watching me closely as I removed the headphones. “Well?”

“It couldn’t be clearer. There’s a ship out there in bad trouble, even if we don’t know what kind of trouble.”

Mac said, “Not a ship.”

“What, then?”

“I’d guess it’s one of the Arks.”

That made me catch my breath. The Arks were part of history. Before McAndrew and I were born, seventeen of the great space habitats had been launched by the United Space Federation. Self-contained and self-supporting, they were multi-generation ships, crawling through the interstellar void at a tiny fraction of light speed. Their destinations were centuries away. But even at minimal speeds, they ought by now to be well on their way to the stars. They should be far beyond the place where the signal had been picked up.

McAndrew’s suggestion that it was one of the Arks seemed unlikely for another reason. “I don’t think it can be,” I said. “As I recall it, none of the Arks was launched in a direction so far south of the ecliptic. And I don’t believe they were capable of significant changes of direction.”

“Perfectly true. They could start and stop, and that was about all.” McAndrew gazed at me blank-eyed as the Sphinx. He knew something he wasn’t telling.

“But in any case,” I went on, “I can’t believe that Fogarty would simply leave them like that, and keep on going. They said they were in trouble.”

“They also implied it isn’t a new emergency — they’ve been transmitting for some time. Anyway, Paul Fogarty didn’t just listen and run. There’s more from him. He stayed far longer than he expected, searching and searching; but he couldn’t track down an origin for the signal.”

“But that’s ridiculous. He must have been right on top of it, to receive it like that.”

“You think so?” McAndrew, that great ham, was full of poorly-disguised satisfaction. “If somebody knew where that signal was coming from, do you think that they should choose to go out on a rescue mission?”

“It wouldn’t be a question of choice. They’d have to go.”

“Exactly.” McAndrew didn’t rub his hands together, but only I think because he was tapping away at the keys of the console. “I’m checking out the status of the reconditioned Merganser. If it’s ready to fly, you and I will be on our way. And don’t worry, we’ll be going with Director Rumford’s blessing. I’ve already asked.”

“But if Fogarty couldn’t find the ship—”

“Then he must have been looking in the wrong place, mustn’t he? In a very wrong place. Wait and see, Jeanie. Wait and see.” And beyond that, for all my coaxing and urging and outright cursing, McAndrew the mule would not for the moment go.

As I say, he’s more human than most people give him credit for. He likes to talk about what he does, but only in his own sweet time and in his own backhanded way.

I waited until we were on board the new Merganser and heading out of the ecliptic. The balanced drive was on. The ship was accelerating at a hundred gees, while the disk of condensed matter in front of the life capsule drew us toward it at close to a hundred gees, leaving us with a residual quarter-gee field. Very comfortable, great for sleeping.

And sleeping is what we might be doing, much of the time. Even accelerating at a hundred gees, we had a lengthy flight ahead of us. We could lie side by side in the cramped life capsule and sleep, relax, play — and talk.

McAndrew had been a clam when it came to our destination, but it had not escaped me that we were going in exactly the wrong direction, toward Cassiopeia rather than away from it. The solar focus for the supernova lay on the other side of the Sun. I mentioned that fact casually, as though it was something of minor interest.

“Quite right.” He was in his bare feet, wriggling his long toes and staring at them with apparent fascination. “If light comes toward Sol from a very long way away, so it’s close to being a parallel beam, then the gravity field of the Sun acts as a great big lens. Light that passes close to the Sun is converged. It is brought to a focus eighty-two billion kilometers away, on the far side of the Sun. So if you want to observe the Cassiopeia supernova, which is way north of the ecliptic, you have to go south.”

“Which is what Paul Fogarty did.”

“Aye. Him, and that Geoffrey Benton.” McAndrew gave me a strange look, which I could not interpret.

“And the place where they heard the signal was south, too,” I said. “But we’re going north.”

“We are indeed.” McAndrew looked smug. “Here’s a question for you, Jeanie. Suppose that you are in trouble, and you can only send out a distress signal now and again.”

“Once every year or two, they said.”

“Right. Now, you’re way out in deep space. Where would you beam the signal?”

“Where people were most likely to hear it. Back toward the Sun.”

“Indeed you would. But if you’re a long way out, and the signal is weak, chances are no one will hear you. Unless there’s some way you can amplify the signal, or you can focus it.”

I’m no McAndrew, but I’m not an idiot. I almost had it. “A signal, sent back toward the Sun — a radio signal. That would be focused just the way that a light beam is focused. But what Paul Fogarty heard wasn’t at the solar focus.”

“No more it was. You’ve had courses in optics, Jeanie, you must have. The Sun acts like a lens, one that takes a beam of light that comes from infinity and converges it to a focus at eighty-two billion kilometers. Now suppose you have a radio signal, but instead of focusing at eighty-two billion kilometers from the Sun it focuses itself at two hundred and eighty billion kilometers. Where would the origin of the radio signal have to be, to make that happen?”

“On the other side of the Sun from where you receive it.” I tried to recall the relevant formula — and failed. I said, “How far out? It’s a standard result in geometrical optics…”

“It certainly is. If a lens converges a parallel beam of light at a distance F from the lens, then light starting at a distance S from the lens will be converged at a distance D beyond it, where the reciprocal of S plus the reciprocal of D equals the reciprocal of F.”

“Don’t gibber at me, McAndrew. I asked you a question. How far out?”

“You’re not listening, Jeanie.” The wretch went on regardless, probably imagining that he was speaking English. “Take F as eighty-two billion, and D as two hundred and eighty billion — that’s where Paul Fogarty caught the distress signal — and you find that S, the distance from Sol where the signal originated, is a hundred and seventeen billion kilometers from Sol. That’s where the distress signal came from, the other side.”

“The other side of what?” As usual, he was turning my head into a muddled mess.

“Of the Sun — the signal was generated on the opposite side of the Sun.”

“You mean Fogarty and Geoffrey Benton have been searching in the wrong place?”