Oliver Morton in More Intelligent Life:

GravityIt will be the most far-flung rendezvous in historyand the end of the most taxing uphill trip ever made. On Bastille Day 2015, NASA’s New Horizons spaceprobe will reach Pluto after a slog of more than 5 billion kilometres, with the sun’s gravity pulling against it every step of the way. That such a trip is possible at all is remarkable. That it could be managed in less than a decade is a tribute both to the most brute force and the most subtle calculation. First the force. As interplanetary missions go, this is a small one, weighing half a tonne. But when it was launched in January 2006, it was sitting on top of one of America’s largest rockets, an Atlas V. The launcher burned more than a tonne of rocket fuel and oxygen for every kilo of the craft’s mass. As a result New Horizons headed off to Pluto faster than any previous space mission: 45km a second. Puck boasted that he could put a girdle round the Earth in 40 minutes. At that rate New Horizons could have done it in 14. The need for speed was simple; as the probe headed to the solar system’s outer edge, the centring sun pulled it back. Its gravity was not so strong as to bring the spacecraft to a haltNew Horizons will be the fifth human spacecraft to leave the system entirelybut it was enough to slow it down, draining away the kinetic energy the mighty Atlas had given it at lift-off day by day. By the time it reached Jupiter, about a year later, New Horizons had lost more than half its initial speed.

This is where the cleverness came in. Jupiter did not just provide a target on which New Horizons could test its cameras and other instruments. It also sped it back up. This pick-me-up, known as a gravity-assist manoeuvre, knocked five years off the time taken to get to Pluto. And unlike the Earth-shaking, sky-splitting $200m-or-more expense of an Atlas launch, it needed no fuel and no money. Gravity assists are a beautiful example of the conservation of momentum, one of the most fundamental ideas of Newtonian mechanics: for every action, there is an equal and opposite reaction. When a spacecraft swings past a planet, the pull of the planet’s gravity changes its momentum so that it ends up moving not just in a different direction, but also faster than before.

More here.