did not like to talk about Earth's destruction, that there was internal friction inhibiting her discourse, that if she were left to herself, she would stop talking. Her expression - the way she pulled away a little from Marlene, as though anxious to leave; the way she licked her lips very delicately, as though she were trying to remove the taste of her words - was clarity itself to Marlene.
But she did not want her mother to stop. She had to know more.
She said gently, 'If Nemesis misses, how will it destroy the Earth?'
'Let me try to explain. The Earth goes around the Sun, just as Rotor goes around Erythro. If all there were in the Solar System were the Earth and the Sun, then the Earth would go around in the same path almost eternally. I say "almost" because, as it turns, it radiates gravitational waves that bleed the Earth's momentum, and that causes it very, very slowly to spin into the Sun, We can ignore that.
'There are other complicating factors because Earth isn't alone. The Moon, Mars, Venus, Jupiter, every object in the neighborhood pulls at it. The pulls are very minor compared to that of the Sun, so Earth remains in its orbit more or less. However, the minor pulls, which are shifting in direction and intensity in a complicated way, as the various objects themselves move, introduce minor changes in Earth's orbit. Earth moves in and out slightly, its axial tilt veers and changes its slant a bit, the eccentricity alters somewhat, and so on.
'It can be shown - it has been shown - that all these minor changes are cyclic. They don't progress in one direction, but move back and forth. What it amounts to is that the Earth, in its orbit about the Sun, quivers slightly in a dozen different ways. All the bodies in the Solar System quiver in this way. Earth's quiver doesn't prevent it from supporting life. At the worst, it may get an ice age or an ice disappearance and a rise and fall in sea level, but life has survived everything for well over three billion years.
'But now let us suppose Nemesis dashes by and misses, that it doesn't approach closer than a light-month or so. That would be less than a trillion kilometers. As it passes - and it would take a number of years to pass - it would give a gravitational push to the system. It would make the quivering worse, but then, when it was gone, the quivers would settle down again.'
Marlene said, 'You look as though you think it would be a lot worse than you make it sound. What's so bad about Nemesis giving the Solar System a little extra quiver - if it all settles down again afterward?'
'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