Nemesis by Isaac Asimov

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 a 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 significantpart 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.”
14.
    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 ageor 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