Scientists unveil plan designed to cut cost of space travel

Gravitational corridors could help spacecraft ply the solar system like ships borne on ocean currents, it has been disclosed by scientists investigating space travel.

Scientists in the United States are trying to map the twisting ''tubes'' so they can be used to cut the cost of journeys in space.

Each one acts like a gravitational Gulf Stream, created from the complex interplay of attractive forces between planets and moons.

Depicted by computer graphics, the pathways look like strands of spaghetti that wrap around planetary bodies and snake between them.

The pathways connect sites called Lagrange points where gravitational forces balance out.

Professor Shane Ross, from Virginia Tech in the US, said: ''Basically the idea is there are low energy pathways winding between planets and moons that would slash the amount of fuel needed to explore the solar system.

''These are freefall pathways in space around and between gravitational bodies. Instead of falling down, like you do on Earth, you fall along these tubes.

''Each of the tubes starts off narrow and small and as it gets further out it gets wider and might also split.

''I like to think of them as being similar to ocean currents, but they are gravitational currents.

''If you're in a parking orbit round the Earth, and one of them intersects your trajectory, you just need enough fuel to change your velocity and now you're on a new trajectory that is free.''

Riding one of the gravitational currents was unlike exploiting the ''slingshot'' effect of a planet or moon's gravity, a routine space travel technique, he explained.

''It's not the same as a slingshot,'' said Prof Ross. ''Slingshots don't put you in orbit round a moon, whereas this does.''

Just one US mission so far has made use of the concept. The Genesis spacecraft was launched in 2004 to capture solar wind particles and return them to Earth. Following the gravitational pathways allowed the amount of fuel carried by the probe to be cut 10-fold.

The mission ended in failure, but only because a parachute failed on landing.

The corridors were especially useful for voyaging between a planet's moons, said Prof Ross, speaking at the British Science Festival at the University of Surrey in Guildford.

''Once you get to another planet that has its own tubes you can use them to explore its moons,'' he added. ''You could travel between the moons of Jupiter essentially for free. All you need is a little bit of fuel to do course corrections.''

The trade off was time, he said. It would take a few months to get round the Jovian moon system.

However, interplanetary travel would always require some fuel, Prof Ross pointed out. Attempting to get a free tube ride from Earth to Mars would take thousands of years.