with anyone else. After the Leaving,however, when the secret was revealed, they had grown apart again.
“Janus,” she said, “there is something eating away at me and I had to come to you with it. It’s Nemesis.”
“Is there anything new? You can’t say you’ve found out it isn’t where you thought it was. It’s right out there, less than sixteen billion kilometers away. We can see it.”
“Yes, I know. But when I first found it, at a distance of two-plus light-years, I took it for granted that it was a companion star, that Nemesis and the Sun were circling a common center of gravity. Something that close would almost have to be. It would be so dramatic.”
“All right. Why shouldn’t things be dramatic now and then?”
“Because as close as it is, it is clearly too far away to be a companion star. The gravitational attraction between Nemesis and the Sun is terribly weak, so weak that the gravitational perturbations of nearby stars would make the orbit unstable.”
“But Nemesis
is
there.”
“Yes, and more or less between ourselves and Alpha Centauri.”
“What has Alpha Centauri got to do with it?”
“The fact is that Nemesis is not much farther from Alpha Centauri than it is from the Sun. It’s just as likely to be a companion star of Alpha Centauri. Or, more likely, whichever system it belongs to, the presence of the other star is now disrupting it, or has already disrupted it.”
Pitt looked at Insigna thoughtfully and tapped his fingers lightly on the arm of his chair. “How long does it take Nemesis to go around the Sun—assuming it’s the Sun’s companion?”
“I don’t know. I’d have to work out its orbit. That’s something I should have done before the Leaving, but there were so many other things occupying me then, and now, too—but that’s no excuse.”
“Well, make a guess.”
Insigna said, “If it’s a circular orbit, it would take Nemesis just over fifty million years to make a circuit about the Sun, or, more properly, about the center of gravity of the system, with the Sun making a similar circuit. The line between the two, as they moved, would always pass through that center. On the other hand, if Nemesis isfollowing a highly elliptical orbit and is now at its farthest—as it must be, for if it ventured farther still, it would certainly not be a companion star—then perhaps as little as twenty-five million years.”
“Last time, then, that Nemesis was in this position, more or less between Alpha Centauri and the Sun, Alpha Centauri must have been in a much different position than it is now. Twenty-five to fifty million years would move Alpha Centauri, wouldn’t it? How much?”
“A good fraction of a light-year.”
“Would that mean that this is the first time Nemesis is being fought over by the two stars? Till now, would it have been circling peacefully?”
“Not a chance, Janus. Even if you count out Alpha Centauri, there are other stars. One star may have arrived now, but there had to be another star in interfering distance at some other part of its orbit in the past. The orbit just isn’t stable.”
“What’s it doing here in our neighborhood, then, if it isn’t orbiting the Sun?”
“Exactly,” said Insigna.
“What do you mean, ‘exactly’?”
“If it were orbiting the Sun, it would be moving at a speed, relative to the Sun, of somewhere between eighty and one hundred meters a second, depending on Nemesis’ mass. That’s very slow motion for a star, so it would seem to stay in the same place for a long time. It would therefore remain behind the cloud for a long time, especially if the cloud is moving in the same direction relative to the Sun. With such a slow motion and its light dimmed, it’s no wonder it’s never been noticed till now. However—” She paused.
Pitt, who made no effort to seem devouringly interested, sighed and said, “Well? Can you get to the point?”
“Well, if it’s
not
in orbit about the Sun, then it is
“Janus,” she said, “there is something eating away at me and I had to come to you with it. It’s Nemesis.”
“Is there anything new? You can’t say you’ve found out it isn’t where you thought it was. It’s right out there, less than sixteen billion kilometers away. We can see it.”
“Yes, I know. But when I first found it, at a distance of two-plus light-years, I took it for granted that it was a companion star, that Nemesis and the Sun were circling a common center of gravity. Something that close would almost have to be. It would be so dramatic.”
“All right. Why shouldn’t things be dramatic now and then?”
“Because as close as it is, it is clearly too far away to be a companion star. The gravitational attraction between Nemesis and the Sun is terribly weak, so weak that the gravitational perturbations of nearby stars would make the orbit unstable.”
“But Nemesis
is
there.”
“Yes, and more or less between ourselves and Alpha Centauri.”
“What has Alpha Centauri got to do with it?”
“The fact is that Nemesis is not much farther from Alpha Centauri than it is from the Sun. It’s just as likely to be a companion star of Alpha Centauri. Or, more likely, whichever system it belongs to, the presence of the other star is now disrupting it, or has already disrupted it.”
Pitt looked at Insigna thoughtfully and tapped his fingers lightly on the arm of his chair. “How long does it take Nemesis to go around the Sun—assuming it’s the Sun’s companion?”
“I don’t know. I’d have to work out its orbit. That’s something I should have done before the Leaving, but there were so many other things occupying me then, and now, too—but that’s no excuse.”
“Well, make a guess.”
Insigna said, “If it’s a circular orbit, it would take Nemesis just over fifty million years to make a circuit about the Sun, or, more properly, about the center of gravity of the system, with the Sun making a similar circuit. The line between the two, as they moved, would always pass through that center. On the other hand, if Nemesis isfollowing a highly elliptical orbit and is now at its farthest—as it must be, for if it ventured farther still, it would certainly not be a companion star—then perhaps as little as twenty-five million years.”
“Last time, then, that Nemesis was in this position, more or less between Alpha Centauri and the Sun, Alpha Centauri must have been in a much different position than it is now. Twenty-five to fifty million years would move Alpha Centauri, wouldn’t it? How much?”
“A good fraction of a light-year.”
“Would that mean that this is the first time Nemesis is being fought over by the two stars? Till now, would it have been circling peacefully?”
“Not a chance, Janus. Even if you count out Alpha Centauri, there are other stars. One star may have arrived now, but there had to be another star in interfering distance at some other part of its orbit in the past. The orbit just isn’t stable.”
“What’s it doing here in our neighborhood, then, if it isn’t orbiting the Sun?”
“Exactly,” said Insigna.
“What do you mean, ‘exactly’?”
“If it were orbiting the Sun, it would be moving at a speed, relative to the Sun, of somewhere between eighty and one hundred meters a second, depending on Nemesis’ mass. That’s very slow motion for a star, so it would seem to stay in the same place for a long time. It would therefore remain behind the cloud for a long time, especially if the cloud is moving in the same direction relative to the Sun. With such a slow motion and its light dimmed, it’s no wonder it’s never been noticed till now. However—” She paused.
Pitt, who made no effort to seem devouringly interested, sighed and said, “Well? Can you get to the point?”
“Well, if it’s
not
in orbit about the Sun, then it is
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