‘Well, will it settle down again in quite the same place? That's the problem. If Earth's equilibrium position is a little different - a little farther from the Sun, a little nearer, if its orbit is a little more eccentric or its axis a little more tilted, or less - how will that affect Earth's climate? Even a small change might make it an uninhabitable world.’

‘Can't you calculate it out in advance?’

‘No. Rotor isn't a good place to calculate from. It quivers, too, and a great deal. It would take considerable time and considerable calculation to deduce from my observations here exactly what path Nemesis is taking - and we just won't be sure till it gets considerably closer to the Solar System, long after I am dead.’

‘So you can't tell exactly just how closely Nemesis will pass the Solar System.’

‘It is almost impossible to calculate. The gravitational field of every nearby star within a dozen light-years has to be taken into account. After all, the tiniest uncalculated effect may build up to such a deviation in over two light-years as to make a passage that is calculated as a near-hit come out, actually, to be a total miss. Or vice versa.’

‘Commissioner Pitt said everyone in the Solar System will be able to leave if they want to by the time Nemesis arrives. Is he right?’

‘He might be. But how can one tell what will happen in five thousand years? What historical twists will take place and how that will affect matters. We can hope everyone will get off safely.’

‘Even if they're not warned,’ said Marlene, feeling rather diffident at pointing out an astronomical truism to her mother, ‘they'll find out for themselves. They've got to. Nemesis will come closer and closer and it will be unmistakable after a while and they can calculate its path much more accurately as it comes closer.’

‘But they will have that much less time to make their escape - if one is necessary.’

Marlene stared at her toes. She said, ‘Mother, don't be angry with me. It seems to me as though you'd be unhappy even if everyone got away from the Solar System safely. Something else is wrong. Please tell me.’

Insigna said, ‘I don't like the thought of everyone leaving Earth. Even if it is done in orderly fashion, with plenty of time and with no casualties to speak of, I still don't like the thought. I don't want Earth to be abandoned.’

‘Suppose it must be.’

‘Then it will be. I can bow to the inevitable, but I don't have to like it.’

‘Are you sentimental about Earth? You studied there, didn't you?’

‘I did my graduate work in astronomy there. I didn't like Earth, but that doesn't matter. It's the place where human beings originated. Do you know what I mean, Marlene? Even if I didn't think much of it when I was there, it's still the world where life developed over the eons. To me it's not only a world but an idea, an abstraction. I want it to exist for the sake of the past. I don't know if I can make that clear.’

Marlene said, ‘Father was an Earthman.’

Insigna's lips tightened a bit. ‘Yes, he was.’

‘And he went back to Earth.’

‘The records say he did. I suppose he did.’

‘I'm half an Earthperson, then. Isn't that so?’

Insigna frowned. ‘We're all Earthpeople, Marlene. My great-great grandparents lived on Earth all their lives. My great-grandmother was born on Earth. Everyone, without exception, is descended from Earthpeople. And not just human beings. Every speck of life on every Settlement, from a virus to a tree, is descended from Earth life.’

Marlene said, ‘But only human beings know it. And some are closer than others. Do you think about Father, sometimes, even now?’ Marlene looked up briefly at her mother's face and winced. ‘It's none of my business. That's what you're going to tell me.’

That's the feeling I just had, but I don't have to be guided by my feelings. After all, you're his daughter. Yes, I think about him now and then.' She shrugged her shoulders slightly.

Insigna said, ‘Do you think about him, Marlene?’

‘I have nothing to think of. I don't remember him. I've never seen any holograms, or anything.’

‘No, there was no point in-’ Her voice trailed off.

‘But when I was littler, I used to wonder why some fathers stayed with their children when the Leaving happened, and some fathers didn't. I thought that maybe the ones who left didn't like their children, and that Father didn't like me.’

Insigna stared at her daughter. ‘You never told me that.’

‘It was a private thought when I was little. When I got older, I knew that it was more complicated than that.’

‘You should never have had to think so. It's not true. I would have assured you of that, if I had had the slightest idea-’

‘You don't like to talk about those times, Mother. I understand.’

‘I would have anyway, if I had known about that thought of yours; if I could read your face as you read mine. He did love you. He would have taken you with him if I had allowed it. It's my fault, really, that you two are separated.’

‘His, too. He might have stayed with us.’

‘Well, he might have, but now that the years have passed, I can see and understand his problems a little better than I could then. After all, I wasn't leaving home; my world was coming with me. I may be over two light-years from Earth, but I'm still at home on Rotor where I was born. Your father was different. He was born on Earth and not on Rotor, and I suppose he couldn't bear the thought of leaving Earth altogether, and for ever. I think about that now and then, also. I hate the thought of Earth being deserted. There must be several billion people there whose hearts would break to leave it.’

There was silence between them for a moment, then Marlene said, ‘I wonder what Father is doing back on Earth right now.’

‘How can we possibly tell, Marlene? Twenty trillion kilometers is a long, long way, and fourteen years is a long, long time.’

‘Do you suppose he's still alive?’

‘We can't even know that,’ said Insigna. ‘Life can be very short on Earth.’ Then, as though suddenly aware she wasn't talking to herself, she said, ‘I'm sure he's alive, Marlene. He was in excellent health when he left, and he's only just approaching fifty now.’ Then softly, ‘Do you miss him, Marlene?’

Marlene shook her head. ‘You can't miss what you've never had.’

(But you had him, Mother, she thought. And you miss him.)