Sorry, you need to enable JavaScript to visit this website.

Oliver Morton

The Music of Science

The magic of the gravity assist: a spaceprobe passing a planet gets the cheapest of all possible lunches

Oliver Morton | March/April 2015

It will be the most far-flung rendezvous in history—and 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 halt—New Horizons will be the fifth human spacecraft to leave the system entirely—but 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. Because Newton, God and all other relevant authorities insist there should be no such thing as a free lunch, the speeding up of the spacecraft is balanced by a slowing down of the planet. But because momentum depends on mass as well as velocity, the slowing of the planet is smaller than the quickening of the spacecraft to the exact extent that the one outweighs the other, and that makes the lunch pretty remarkably cheap. Since New Horizons weighs less than a car and Jupiter weighs 300 times as much as the Earth, the planet’s lost momentum is as close to imperceptible as you can get. Fly a momentum-pinching New Horizons past Jupiter every day for a billion years and you would only slow the planet down by two millimetres a second.

The brute force of rockets has been known for centuries, and the idea of using them to visit other worlds has a heritage that stretches back at least as far as Cyrano de Bergerac. The subtlety of gravity assists is a more recent romance. It was discussed by a few enthusiasts early in the 20th century, but the first person to become deeply enthused by it was Michael Minovitch, a physicist at JPL, the Californian lab that has handled most of NASA’s planetary exploration, in the 1960s. At a time when most in the fledgling field of astrogation treated the gravitational influences of other planets as a problem to be minimised, Minovitch saw that, with the right geometry, you could use such perturbations to find quicker or more fuel-efficient routes to any of the planets bar Venus (too close). Working obsessively at night, in little contact with his colleagues, he came up with trajectory after trajectory, including the “grand-tour” approaches that used a gravity assist from Jupiter to get to Saturn, one from Saturn to get to Uranus and one from Uranus to get to Neptune.

Minovitch saw himself as inventing a whole new type of space travel. He didn’t see that less zealous colleagues could come up with the same solutions even if ignorant of all that he had done (the grand-tour orbits were discovered by someone else independently). He felt he was being written out of history; he left JPL and threatened to sue it.

To Minovitch the gravity assist was an invention to be owned. To most people it was a discovery which, once made, was just a fact about the world. It is not an uncommon tension in science. There is a time when only one mind has had an idea, at least as far as that mind knows; then there is a time when everyone thinks it. There is something wonderful about both, but the path between them is all too often strewn with arguments about priority and plagiarism, its trajectories and turnabouts far less satisfactory than the routes around the planets.

Although gravity assists are now standard, finding a truly clever new trajectory, or one that lets you use a smaller rocket for the same mission, remains a creative and satisfying act whether an invention or a discovery. Some astrogators will see their trajectories as cunning ways of outsmarting the limits that the sheer scale of space missions would otherwise impose, others as nature itself offering a helping hand. Either way, their existence makes the vast mechanism of the solar system just that bit more accessible to explorers—a little more quirky, a tad more user-friendly, a touch more like home.

Readers' comments

Sign in or Create your account to join the discussion.