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A planet called Theia grew at Earth’s L5 point until it was about the size of Mars, then drifted into a collision with Earth. It hit at a forty-five-degree angle, at something less than four kilometers per second-not fast in astronomical terms. Theia’s iron core plunged in and merged with Earth’s core, and Theia’s mantle and some of Earth’s mantle were thrown into orbit. The angular momentum imparted by the hit spun Earth to a five-hour day. Two moons accreted from the ejected material rather quickly; estimates range from a month to a century. Eventually the smaller moon splatted onto the larger one, leaving behind the jagged mountains on the anti-terran side of the resulting moon, Luna.

Around the same time, a small planet about three thousand kilometers in diameter struck Mars and created the Borealis Basin, which is basically Mars’s northern hemisphere, still six kilometers lower than the southern hemisphere.

Venus was struck by a Mars-sized planet, creating a moon like Earth’s, called Neith; ten million years later, another impactor gave Venus its slow retrograde motion. This change in rotation slowed Neith and caused it to plunge back into Venus and merge with it.

Mercury was struck by a protoplanet half its size, at such a speed and angle that Mercury’s mantle was stripped off and cast throughout Mercury’s orbit. Ordinarily Mercury would have swept these pieces back up, but in the four million years this process would have taken, most of the material was pushed outward by solar radiation and thus never made it back to Mercury. About sixteen quadrillion tons of Mercury’s crust eventually ended up on Earth, and more on Venus. In the end only the heaviest 70 percent of Mercury remained, essentially the planet’s core. Thus the Mars g for a diameter smaller than Titan’s.

Somewhat later, the young Jupiter and Saturn fell into a one-to-two orbital resonance, with Jupiter spinning through two of its years for every one of Saturn’s. This created a very powerful combined gravity wave, swinging around the solar system at varying strength, depending on where the two giants were in relation to each other. This new wave at its strongest caught Neptune, which had grown just outside Saturn, and threw it away from the sun! Neptune flew out past Uranus, pulling Uranus outward too, also onto its side. Only at that point did the two smaller gas giants end up in the orbits they now occupy.

Inside Jupiter’s orbit, meanwhile, that same Jupiter-Saturn resonance wave caught asteroids and threw them like pinballs all over the system, in the period called the Late Heavy Bombardment, 3.9 billion years ago. All the inner planets and moons were pummeled with impacts, to the point where the surfaces of these planets often were seas of molten rock.

The Era of Big Hits! The Late Heavy Bombardment! Never let it be said that the great merry-go-round is entirely fixed and regular in its motion-that it doesn’t sometimes resemble more a swirl of bumper cars. Gravity, mysterious gravity, immutably following its own laws, interacts with matter, and somehow the result is complex motion. Invisible waves slinging rocks this way and that.

What if human history has such invisible waves? Because ultimately the same forces apply. What big hits made us what we are? Will some new resonance create a wave and throw us in a new direction? Are we entering our own Late Heavy Bombardment?

KIRAN AND SWAN

From the moment Kiran saw the woman his cousins had grabbed, everything changed. She was old, tall, good-looking. She moved as if she were swimming. He knew immediately that she was a spacer, and that kidnapping her was a terrible idea. After that everything went a bit too fast for him actually to be deciding what to do. This is what happened to him when he was in a pinch: he watched himself do what he did from just behind and to one side. People said he was cool, but really he was slow. And yet good things still seemed to happen.

Her hair was black; she looked Chinese or Mongol. Her eyes were brown, with a little blue patch at the bottom of one eye; and really it was her eyes that captured him. Some kind of coincidence-the girls back home had those same dark eyes with luminous whites, in a dark face-it was very compelling to him. She had looked to him the moment he had taken her arm, to show how much she wanted to be free-a very passionate look, as if she knew what being held captive was like, and was afraid. It shocked him how expressive her face was, how firmly she grabbed his attention. Her friend Zasha had called it Lima syndrome-maybe so. Maybe now he was an incompetent Peruvian.

But he was going to space. That meant leaving-but he could send money back to his relatives. They were tired of putting him up anyway. He could go and see what he had always dreamed of seeing-which was really just anywhere, but space in particular, ever since he was a small boy. Mars, the asteroids-anywhere out there. Everyone heard stories.