Chasing New Horizons: Inside Humankind's First Mission to Pluto

by Alan Stern

Shortly after, Mariner 10 would make the first visit to Mercury, traveling there by way of Venus, where it would make the first ever use of a “gravity assist,” a nifty trick that has since become indispensable for getting around the solar system. In a gravity-assist maneuver, a spacecraft is sent on a near-miss trajectory to one planet, which pulls it in and then speeds it toward its next target. It seems too good to be true—like getting something for nothing, but it’s not—the equations of orbital mechanics do not lie. For the planet, the tiny loss of orbital speed it trades with the spacecraft has no meaningful effect, but the spacecraft gets a whopping shove in just the right direction. Pioneer 11 was slated to use this same trick during its planned flyby of Jupiter, allowing it to then go on to Saturn.

The gravity assist