Phase Space by Stephen Baxter

wasn’t going to hit Venus, or be thrown out of the system to the stars. Ra was going to hit Earth.
    Ra was the next dinosaur killer.
    It was a long time ahead: all of a million years from now. But it worried him that right now, nobody seemed to know how to deflect this damn rock.
    Whenever he got the chance, he sounded off about the dinosaur killer problem. The Weissmans told him they had plans to deal with it, when the time came. Greenberg wasn’t sure whether he believed that.
    And he wasn’t sure he wanted to be around to see this chewed-up rock auger in on the surface of the planet where he was born. But he couldn’t turn his back.
    Within the confines of the tiny world, civilizations fell and rose; by turns, the refugee race fell to barbarism, or dreamed of the stars. The guardians had planned for this.
    But the little world was not stable. This they had not anticipated.
    Its orbit was close to a resonance with that of Jupiter: it circled the sun three times in each of Jupiter’s stately years. The powerful tug of Jupiter worked on the asteroid’s trajectory, millennium after millennium.
    Quite suddenly, the orbit’s ellipticity increased. The asteroid started to swing deep into the warm heart of the solar system.
    There was nothing the inhabitants could do to steer their rock. Some adapted. Many died. Superstitions raged.
    For the first time, the asteroid dipped within the orbit of Earth.
Scale: Exp 5
    … Crossing time in unimaginable jumps, drifting between sleeping and waking, eroding towards maximum entropy like some piece of lunar rock …
    He never knew, he didn’t understand, he couldn’t believe how much time had passed. A hundred thousand years? It was a joke.
    But even the sky was changing.
    The nearby stars, for instance: Alpha Centauri and Barnard’s Star and Sirius and Procyon and Tau Ceti, names from the science fiction of his youth. You could see the changes in the light, the stain of oxygen and carbon, chlorophyll green. Even from here you could see how humans, or post-humans anyhow, had changed the stars themselves.
    And to think he used to be awed by the Vehicle Assembly Building at Canaveral.
    And the expansion must be continuing, further out, inexorably. On it would go, he thought dimly, a growing mass of humanity filling up the sphere centred on Sol, chewing up stars and planets and asteroids, until the outer edge of the inhabited sphere had to move at the speed of light to keep up, and then what would happen, he wanted to know?
    But none of that made a difference here, in the ancient system of Sol, the dead heart of human expansion. It was hard for him to trace the passing of the years because so little changed any more, even on the heroic timescales of his intervals of consciousness.
    Conditions in a lot of the inhabited rocks had converged, in fact, so that the worlds came to resemble each other. Most of them finished up with the kind of simple, robust ecosystem that sustained Ra, even though their starting points might have been very different. It was like the way a lot of diverse habitats on Earth – forests and jungles and marshes – would, with the passage of time, converge into a peat bog, the same the world over, as if they were drawn to an attractor in some ecological phase-space.
    And most of the rocks, drifting between uninhabited gravity wells, were about as interesting as peat bogs, as far as Greenberg was concerned.
    Meanwhile, slowly but inexorably, life was dying back, here in the solar system. which had once hosted billions of jewel-like miniature worlds.
    There were a lot of ways for a transformed asteroid to be destroyed: for instance, a chance collision with another object. Even a small impact on a fragile bubble-world like Ra could puncture it fatally. But nobody around seemed capable of pushing rocks aside any more.
    But the main cause of the die-back was simple ecological failure.
    An asteroid wasn’t a planet; it didn’t have the huge buffers of mass and energy