Sometime in the Earth's young childhood, well over 4 billion years ago when it still was mostly a molten mass of rock, the wildly chaotic gravitational pulls of the early solar system almost certainly sent a body the size of Mars or larger careening off our fledgling home, slamming material off into space. That material - or much of it - remained within the Earth's gravitational pull, and before long in cosmic years coalesced into the moon we watch circle our planet today.
Scientists think this is true because rocks brought back by astronauts match almost exactly the very makeup of the Earth's mantel.
I thought of this while reading this week's Newsweek, which uses its final page to try to graphically illustrate some issue. The current "Back Page," about dangerous asteroids, is headlined: "Is the end nigh?" Newsweek suggests not.
It does, however, point out that while an asteroid the size of a basketball crashes into the Earth's atmosphere daily, a basketball-court sized object blows into town an average of every 200 years. And a football-field sized object comes by once every 10,000 years.
The piece offers a table showing the most dangerous asteroids - that we know about - that could strike us in the next 100 years or so. The sizes range from 98 feet in diameter to 3,609 feet, more than half a mile. But the odds for all of them are low - from 1 in 770 for a cute little 121 footer to 1 in 53 million for that big guy. (And for comparison, the asteroid that put the dinosaurs and 75 percent all other species to bed 65 million years ago had to be seven to eight miles wide.)
So a really big, humankind-ending event isn't very likely anytime soon. But, as they say, you never know. It was only in 1908 that a 120-foot visitor broke up over Siberia, flattening 800 square miles of forest. I was struck by the comments of astronomer Alex Filippenko, who insisted that over the next few hundred millions of years the Earth is almost certain to be blasted by what would be the end of the world for us. He urged that we quickly begin readying the means to deflect such a monster. After all, he said, we are far from knowing about all that is out there - and an estimation of probabilities wouldn't have been much help to those dead dinosaurs.
Bodies the size of Mars no long zoom erratically throughout our solar system. We would have seen them. But who knows about those 10-milers?