Motion | Pages 56-57
Gravity Rules

Like a traffic cop in the middle of a busy intersection, Jupiter directs the comings and goings of the asteroids and comets that whiz around the solar system. Or rather, Jupiter's gravity directs traffic. According to the field of science known as chaos theory, asteroids in particular orbits around the Sun tend to have chaotic gravitational interactions with the giant planet. Their relationships are so sensitive that the slightest push or pull or gravitational brush with another interplanetary

In some senses, Jupiter's massive presence protects Earth from what would probably be much more frequent bombardment by wandering comets and by the asteroids that inhabit the zone between Mars and Jupiter. However, over the course of millions of years, Jupiter's relentless gravitational tug has yanked some of those asteroids out of the equatorial plane, sending them into highly elliptical paths that cross the orbit of Mars. Gravitational interactions with the Red Planet further perturb the asteroids into even more eccentric orbits that cross paths with Earth and Venus. Indeed, astronomical observations reveal that Earth is surrounded by a swarm of Earth-crossing asteroids and comets (opposite). Bombardment by these missiles was and continues to be part of Earth's geological process.

Even Jupiter is not immune. As the photos on the following pages show, the tidal forces exerted by the gas giant might shatter an incoming asteroid or comet into a score of pieces, but the planet itself cannot escape the inexorable rule of gravity.