In popular usage the word librate has long passed out of style. It originally came from the Latin word librare, to balance. Librate means to vibrate slightly, as a balance or a scale does before it settles down. An object that librates is poised between two competing forces. Scientifically the word has had a longer lifetime, because the depths of outer space are sprinkled with what are called libration points: places where a satellite or a pebble or anything else that might get there would find itself perfectly balanced between competing gravitational forces. There are five of them shared by Earth and the sun, for instance, and another set of five shared by Earth and the moon. As the moon rotates around Earth, and Earth around the sun, these libration points rotate with them. Put a satellite at a libration point and it would appear motionless from Earth, hanging in space as though the laws of gravity had been suspended.
Libration points are mathematical fictions, geometric fantasies, topological flights of fancy. There is nothing visible at a libration point. No signpost says HERE ALL FORCES CANCEL. But if you arrive at one, you may stay there with extraordinarily little effort, or you could orbit around one, as though the libration point were a planet rather than a spot of nothing. You could even surf from one libration point to another, from one side of Earth to the other, or one side of the solar system to the other, while barely exerting any effort.
In the past few years the study of libration points has gone from an academic exercise to a revelation. NASA now has four space missions in the works that will use the gravitational weirdness of libration points for everything from mapping the whisper of radiation left over from the Big Bang to photographing Earth 24 hours a day. Meanwhile, researchers at Caltech and Purdue University in Indiana have applied the mathematics of libration points to the solar system at large, creating a theory of how asteroids, comets, and dust move around and how spacecraft could follow the same invisible rivers of gravity to travel from planet to planet or moon to moon with little more fuel than it would take to drive a car from New York to Los Angeles. The study of libration points has become the pursuit of free rides. If mission planners do their math right, says Purdue astronautical engineer Kathleen Howell, once a spacecraft reaches the right velocity and position above Earth’s atmosphere, “you’ll never have to turn its engines on. It will just go where it has to go.”
