‘I know. And next time I'm six, you can call me Molly all you want.’
Insigna laughed. ‘Do you know what a brown dwarf is, Marlene?’
‘Yes, I do, Mother. A brown dwarf is a small starlike body, with too little mass to develop the temperatures and pressures to bring about hydrogen fusion in its interior, but enough mass to bring about secondary reactions that keep it warm.’
‘That's right. Not bad. Megas is on the borderline. It's either a very warm planet or a very dim brown dwarf. It gives off no visible light, but emits richly in the infrared. It's not quite like anything we've ever studied. It was the first extrasolar planetary body - that is, the first planet outside the Solar System - that we have been able to study in detail, and the Observatory was totally immersed in it. I wouldn't have had a chance to work on Nemesis' motion even if I had wanted to, and, to tell you the truth, I forgot about it for a time. I was as interested in Megas as everyone else was, you see?’
‘Um,’ said Marlene.
‘It turned out it was the only sizable planetary body circling Nemesis, but it was enough. It was five times the mass-’
‘I know, Mother. It's five times the mass of Jupiter, and one thirtieth the mass of Nemesis. The computer taught me that long ago.’
‘Of course, dear. And it's no more habitable than Jupiter is; less, if anything. That was disappointing at first, even though we didn't really expect to find a habitable planet circling a red dwarf star. If a planet were close enough to a star like Nemesis to keep water liquid, tidal influences would force it to face one side to Nemesis at all times.’
‘Isn't that what Megas does, Mother? I mean, one side always faces Nemesis?’
‘Yes, it does. That means it has a warm side and cold side, with the warm side quite warm. It would be at red heat, if it weren't that the circulation of its dense atmosphere tends to equalize temperatures somewhat. Because of this and because of Megas' own inner warmth even the cold side is quite warm. There are many things about Megas that were unique in astronomical experience. And then we discovered that Megas had satellite or, if you want to consider Megas a very small star, it had a planet - Erythro.’
‘Which Rotor orbits, I know. But, Mother, it's been over eleven years since there was all that fuss about Megas and Erythro. In all that time, haven't you managed to sneak a look at the spectra of Nemesis and the Sun? Haven't you done a little figuring?’
‘Well-’
Marlene said hastily, ‘I know you have.’
‘By my expression?’
‘By everything about you.’
‘You can be a very uncomfortable person to have around, Marlene. Yes, I have.’
‘And?’
‘Yes, it's heading for the Solar System.’
There was a pause. Then Marlene said in a low voice. ‘Is it going to hit?’
‘No, as far as my figures are concerned. I'm quite sure it's not going to hit the Sun, or the Earth, or any significant part of the Solar System, for that matter. But it doesn't have to, you see. Even if it misses, it will probably destroy the Earth.’
It was quite clear to Marlene that her mother 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 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.’