The interplanetary superhighway

By Fred Watson 30 July 2010
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Scientists consider making fuel-efficient low gravity tracks to link the stars and planets.

NAVIGATORS HAVE USED THE stars to find their way around the world since time immemorial. In fact, some say that once astronomers had ironed out the bugs in navigation towards the end of the 18th century, they stopped being useful and became merely an expensive luxury.

Today, finding your way around our planet is as easy as switching on your GPS. While there are hiccups – my GPS recently tried to send me from Coonabarabran to Sydney via Marrakesh – the technical challenge of navigating the Earth’s surface has been largely eliminated. Today’s navigational problems lie in space.

For the most part, navigating a spacecraft is a matter of pointing it in the right direction, giving it a push with its rockets and letting gravity do the rest. Even in Earth orbit your height above the surface is precisely determined by your speed and the Earth’s gravitational pull.

When there’s more than one celestial object providing the pull of gravity, the situation gets more complicated. But through mathematics, today’s space navigators have discovered a new trick for finding their way among the planets.

Celestial objects have points between them where their gravity cancels out. Here a spacecraft will feel no gravitational pull, but a slight push one way or the other will tip it into a desired orbit. In that case, a small expenditure of energy produces a large difference in motion.

Gravity voids marks possible interplanetary superhighway

The Earth-Moon system has five such gravity voids, known as Lagrange points, and if you introduce other bodies into the calculation (the Sun and other planets, for example), those Lagrange points connect with similar ones relating to the other objects to define subtle pathways between them.

Travel along these pathways requires a minimum expenditure of energy, and hence is very inexpensive in fuel. The best way to imagine them is as a network of invisible tubes linking the planets and their various Lagrange points, snaking through space in constant motion as the planets progress in their orbits.

This exciting concept has the planners of future space missions poring over their supercomputers to look for new low-energy routes. Even though such fuel-efficient trajectories tend to be slower than more direct routes, the potential benefits to unmanned deep-space missions are enormous, and it’s no accident that scientists now describe this network as the interplanetary superhighway.

With a well-defined selection of Lagrange-point connections between one route and another, interplanetary navigation will soon be almost as easy as turning on your GPS.

Source: Australian Geographic (91) Jul – Sep 2008
Fred Watson is Astronomer-in-Charge of the Australian Astronomical Observatory at Coonabarabran in north-western NSW, and well-known to ABC radio listeners. Visit his website at