McAndrew had solved Vandell’s Eleventh Problem when he was a very young man. From that work had emerged the whole theory for the existence and location of the kernel ring, the torus of Kerr-Newman black holes that circles the sun ten times as far out as Pluto. Nine years later, Wenig’s partial solution of the Fourteenth Problem had given McAndrew the clue that led him to the vacuum-energy drive. Now, assuming that Jan’s report was correct, the Fifth Problem had fallen to Wicklund’s analysis.

“But why is it so important?” I asked McAndrew. “The way you describe it, I don’t see practical uses. It’s just a way of amplifying an observed signal without amplifying background noise — and it only applies when the original signal is minute.”

He shook his head in vigorous disagreement. “It has a thousand applications. Vandell already proposed one when he first set the problem, and I’m sure Wicklund will tackle it as soon as his experimental equipment is working. He’ll use the technique to look for solitaries — rogue planets.”

Rogue planets.

With those last two words, McAndrew brought the explanation along to the point where it made sense to me. I could draw on my own formal training in classical celestial mechanics.

The possible existence of rogues went back a long way, farther than 1900. Probably all the way to Lagrange, who in his analysis of the three-body problem set up a mathematical framework to look at the motion of a planet moving in the gravitational fields of a binary star system. By 1880, that case was known to be “stable against ejection.” In other words, the planet could have close approaches to each of the stars, and might suffer extremes of temperature, but it would never be completely expelled from the stellar system.

But suppose you have a system with three or more stars in it? That’s not at all uncommon. Then the situation changes completely. A planet can pick up enough energy through a series of gravitational swing-bys past the stellar components to hurl it right out of the system. Once this happened it would become a sun-less world, travelling alone through the void. Even if it later encountered another star, the chance of capture was minute. The planet would be a solitary, a rogue world. Astronomers had speculated for centuries about the existence and possible numbers of such planets, but without a scrap of observational evidence.

Vandell had defined the problem: An Earth-sized planet shines only in reflected light. If it gives off radiation in the thermal infrared or microwave regions, the signal is swamped by the stellar background. Devise a technique that will permit the detection of a rogue planet as small as the Earth.

Now it seemed that Wicklund had done it, and McAndrew was happy as a pig, while everybody else at the Institute gloomed about in reaction to Tallboy’s effects on their work.

I sympathized with them. Rogue planets are fine, but I could see no way in which they could make any practical difference to me. Mac and Sven Wicklund could have my share of them. I spent a lot of time over on the Hoatzin wondering what to do next. I didn’t belong at the Penrose Institute, the only thing I offered there was the ability to pilot the long trips out. Once that was over, I might as well go back to the Titan run.

Jan’s next message back gave me mixed feelings, but at least it cheered me up.

“Not much to do out here,” she wrote — she was the only person I have ever met who could chat in a spacegram. “You were right, Jeanie. Wicklund’s as bad as McAndrew, totally wrapped up in the work he’s doing and won’t take much notice of me. And the rest of them hate company so much they run and hide when we meet in the corridors. I’ve been spending a lot of time over on Merganser. I got the impression from you that she’s an old hulk, but she’s not. She may be an antique, but everything’s still in good working shape. I’ve even been spinning-up the drive. If I can talk Wicklund into it maybe we can go off on a little bit of a trip together. He needs a rest (from physics!).”

That brought back some exciting memories. Merganser was one of the two original prototypes of the balanced drive, and McAndrew and I had ironed the bugs out of her personally. She was limited to a 50-gee acceleration, but still in good working order. I’d fly her anywhere. Mac seemed much less happy when he read the letter.

“I hope she knows what she’s doing,” he said. “That ship’s not a toy. Do you think it’s safe?”

“Safe as anything in the System. Jan won’t have any trouble. We used the Merganser for training before they mothballed her, don’t you remember?”

He didn’t, of course. He carries physics and mathematics in his head at an astonishing level of detail, but useful everyday information is another matter. He nodded at me vaguely, and wandered off to send more messages to Wicklund (who had to date provided no replies).

We heard from Jan again, just as the explicit order was coming in from Tallboy’s office to decommission Hoatzin and remove the supplies for the Alpha Centauri mission.

I screwed up Tallboy’s order into a tight ball and threw it across the room.

Then I sat down to read what Jan had to say.

No preamble this time: “Wicklund says it works! He’s already found three rogues, and expects a lot more. They must be a lot more common than anybody thought. Now sit back for the big news: there’s one only a light-year away! Isn’t it exciting?”

Well, maybe — less so to me than to Mac, I was sure of that. I assumed that solitary planets would be rather rare, so one closer than the nearest star was a bit surprising. But it was her next words that shot me bolt upright and sent a tingle through my spine.

“Merganser is working perfectly, all ready for a trip. I’ve persuaded Wicklund to take her out for a look at Vandell — that’s his name for the planet. I’m sure you don’t approve, so I won’t ask. Lots of love, and see you when we get back.”

Even as I screamed inside, I wasn’t completely surprised. She was McAndrew’s daughter all right — it was exactly the harebrained sort of thing he would have done.

Mac and I both played it very cool. That boneheaded pair, we said to each other. We might have guessed it, the follies of youth. They’ll be in trouble when they get back, even though the Merganser is an old ship that Triton Station can do what they like with.

But deep inside we both had other feelings. Wicklund had sent the coordinates of Vandell to us before they left, and as Jan said it was close, less than a light-year and a quarter away. Easily in Merganser’s range, and a lure that any scientist worth his salt would find hard to resist, even without Jan’s coaxing. Where had it come from, what was it made of, how long since it had been ejected from its parent star? — there were a hundred questions that could never be answered by remote observations, not even with the super-sensitive methods that Wicklund had developed.

But it was those same questions that made me so uneasy. If I’ve learned one thing wandering around inside and outside the Solar System, it’s this: Nature has more ways of killing you than you can imagine. When you think you’ve learned them all, another one pops up to teach you humility — if you’re lucky. If not, someone else will have to decide what did you in.

For a week after Jan’s message I monitored the messages closely that came in from the outer relay stations. And every day I would ride over to the Hoatzin and potter about there, sometimes with Mac, sometimes alone. I was supposed to be working on the decommissioning, but instead I would sit in the pilot’s chair, check all the status flags, and think my own thoughts. Until finally, ten days after Jan and Wicklund had left, I went over to visit the Hoatzin late one sleep period.