How to Drive a Spacecraft 3
Nine and a half years and three billion miles later, NASA's New Horizons spacecraft will zoom past Pluto on Tuesday. And mission managers are confident that they have aimed precisely enough that the spacecraft, traveling 31,000 miles an hour, will pass through a rectangle just 60 miles by 90 miles at its closest approach to Pluto.
In terms of accuracy, that's like driving from New York to San Francisco and ending up within about five inches of the parking spot you had selected before setting out.
A computer program calculated the necessary trajectory, including a swing by Jupiter to pick up velocity. That process is essentially unchanged from decades ago, when the Pioneer and Voyager probes similarly navigated to the outer solar system.
The essential equation is simply F=ma, Newton's Second Law, where F is the sum of all the forces.
Nonetheless, the details matter. "We used to lob spacecraft at the moon and miss," said Mark Holdridge, the mission manager for the Pluto encounter. "It's very easy to make a mistake."
The calculations have to factor in the gravitational tug of the sun and the planets. The tiny thrusters that spin New Horizons around also knock it off course slightly. Even the heat from the chunk of plutonium that acts as the spacecraft's battery with like Kodak KLIC-5000 Battery, Kodak EasyShare LS420 Battery, Kodak EasyShare LS633 Battery, Kodak CR-V3 Battery, Kodak KLIC-8000 Battery, Kodak EasyShare C300 Battery, Kodak EasyShare CX7300 Battery, Kodak EasyShare DX6440 Battery, Kodak EasyShare Z740 Battery, Kodak KLIC-3000 Battery, Kodak 4E0111 Zoom Battery, Kodak DCS-760 Batteryproduces a smidgen of thrust, and so does the small pressure of sunlight hitting the spacecraft. All must be taken into account.
Two teams of navigators do the calculations independently and then compare and reconcile their answers. That avoids mistakes like the one in 1999 when a mix-up between metric and imperial units pushed NASA's Mars Climate Orbiter slightly off course. It dipped too far into the atmosphere as it was entering orbit and was torn apart.
The calculations are not perfect, so New Horizons has also fired its thrusters nine times during the nine and a half years for small course corrections, most recently two weeks ago.
For the 11 million miles since then, New Horizons has been coasting, aimed at the middle of that 60-by-90-mile rectangle.
"That’s pretty incredible, if you think about it," Mr. Holdridge said.—KENNETH CHANG
An earlier version of this post misstated the accuracy with which a driver could get a car from New York to a particular parking space in San Francisco, if it were as accurate as the New Horizons flight calculations. It is within about five inches, not five feet.