I'm not in danger here. Not at all. I know my mind. I've lived with it all my life. I understand it. It's not in danger.'
Genarr said, 'Be reasonable, Marlene. However stable you feel your mind to be, it's subject to disease and deterioration. You might get meningitis, epileptic symptoms, a brain tumor, or, eventually, senescence. Can you hold any of those things at bay just by being sure none of it will happen to you?'
'I'm not talking about any of those things. I'm talking about the Plague. That won't happen to me.'
'You can't possibly be sure, dear. We don't even know what the Plague is.'
'Whatever it is, it won't happen to me.'
'How can you tell, Marlene?' asked Genarr.
'I just know,'
Insigna felt her patience break. She caught Marlene by both elbows. 'Marlene, you must do as you're told.'
'No, Mother. You don't understand. On Rotor, I've felt a pull toward Erythro. It pulls me more strongly than ever, now that I'm on it. I want to stay on it. I'll be safe here. I don't want to go back to Rotor. I'll be less safe there.'
Genarr raised his hand, stopping whatever it was that Insigna was about to say. 'I suggest a compromise, Marlene. Your mother is here to make certain astronomical observations. It will take her some time. Promise that, while she is busy at it, you will be content to stay inside the Dome and take such precautions as I think will make sense, and that you submit to periodic tests. If we detect no change in your mental functioning, you can wait here in the Dome till your mother is done and then we can discuss it again. Agreed?'
Marlene bent her head in thought. Then she said, 'All right. But, Mother, don't think of pretending to be finished when you're not finished. I will know. And don't think of doing a quick job instead of a good one. I will know that, too.'
Insigna frowned and said, 'I won't play games, Marlene, and don't think I will ever deliberately do bad science - even for your sake.'
Marlene said, 'I'm sorry, Mother. I know that you find me irritating.'
Insigna sighed heavily. 'I don't deny that, but irritating or not, Marlene, you are my daughter. I love you, and I want to keep you safe. As far as that goes, am I lying?'
'No, Mother, you are not lying, but please believe me when I say I am safe. Since I've been on Erythro, I've been happy. I never was happy on Rotor.'
Genarr said, 'And why are you happy?'
'I don't know, Uncle Siever. But being happy is enough, even when you don't know why, isn't it?'
36
'You look tired, Eugenia,' said Genarr.
'Not physically, Siever. Just tired inside after two months of calculations. I don't know how it was possible for astronomers in prespatial times to do what they did with nothing more than primitive computers. For that matter, Kepler worked out the laws of planetary motion with nothing more than logarithms, and had to consider himself fortunate that they had just been invented.'
'Pardon a nonastronomer, but I thought that these days, astronomers simply gave their instruments their directions, then went to sleep and, after a few hours, woke up and found everything printed up neatly and waiting at the desk.'
'I wish. But this job was different. Do you know how precisely I had to calculate the actual velocity of Nemesis and the Sun relative to each other, so that I could know exactly where and when the two made their closest approach? Do you know how tiny an error would be sufficient to make it seem that Nemesis would do Earth no harm when it would really destroy it - and vice versa?'
'It would be bad enough,' Insigna went on intensely, 'if Nemesis and the Sun were the only two bodies in the Universe, but there are nearby stars, all of them moving. At least a dozen of them are massive enough to have a tiny effect on Nemesis or the Sun or both. Tiny, but large enough to mount up to an error of millions of kilometers one way or another, if ignored. And in order to get it right, you have to know the mass of each star with considerable precision, and its position, and its velocity.'
'It's a fifteen-body problem, Siever, enormously complicated. Nemesis will go right through the Solar System and have a perceptible effect on several of the planets. A lot depends on the actual position of each planet