National Geographic : 2013 Jul
48 national geographic • JULY 2013 time they affect the paths of neighbors. As a re- sult, as Brownlee puts it, “there’s no such thing as a circular orbit.” In principle the relentless pull of gravity can amplify these small deviations until orbits migrate, cross, or otherwise go haywire. Newton concluded that God must step in from time to time to fix the clockwork. But he couldn’t say when. Even he who invented calculus was defeated by the “n-body problem”: He had no formula for calculating into the distant future the orbits of multiple bodies that were all pulling on one another. In practice no one saw evidence that plan- etary orbits had ever changed. So the clockwork solar system stuck with us—enduringly stable, it seemed, even without fixes from the Creator. But a far more dramatic view has arisen in the past decade or so. While the findings from Star- dust indicate the solar system was turned inside out during infancy, many scientists now think it also went through a raucous adolescence: Hun- dreds of millions of years after they formed, the biggest planets swept into new orbits, casting large rocks and comets every which way. In this view the scarred surface of the moon is lingering testimony to a period of epic mayhem. “Who would have thought the giant planets might move, that the entire layout of the so- lar system could change?” says Alan Stern of the Southwest Research Institute in Boulder, Colorado. Some signs were there, Stern says. But it took new telescope surveys to reveal them, along with “digital orreries”—clever algorithms that apply brute computing power to calculating comet dust in a nASA clean room a scientist examines traps containing dust snared by the Stardust probe as it flew by the comet Wild 2. Each speck left a hair-size track (right) as it plunged into the puffy aerogel at more than 13,000 miles an hour. Robert Irion directs the science writing program at the University of California, Santa Cruz. Mark Thiessen’s photographs of methane, an invisible gas, appeared in the December issue. Artist Dana Berry created the December 2009 cover and this month’s.