A New Stellar Drive

Engineers at Pioneer Astronautics believe it's possible to propel a craft using a solar sail and achieve a speed close to 1% the speed of light. An ultra-lightweight sail, made of metal coated carbon nanotubes, could pass close to the Sun. The intense illumination and the gravity slingshot could enable speeds over 3000 kilometres per second, bringing Gliese 581c within a 2000 year trip.

However, as Robert Zubrin, president of Pioneer Astronautics points out the construction of such a sail is unlikely to be possible for at least 30 years. Also, unfortunately a sail tens of thousands of kilometres across would be required to transport a craft, crew and provisions.

In the 1980s, Robert Forward suggested a Solar Sail could reach higher speeds if it were illuminated by a laser beam. A space based laser focused onto a 1000 kilometre sail could get a craft to Gliese581c in about 40 years. However such a laser beam would require a focusing lens about 1000 kilometres across, and require power of several million gigawatts - more than the world's electricity output.

These mind blowing energy requirements are not easy to come by. It would appear that interstellar travel would require a new fuel source. Consider antimatter: identical to conventional matter but having the opposite electrical charge. When antimatter and matter collide, they annihilate each other releasing all the energy associated with their mass.
So a kilogram of antimatter could provide about 100,000 billion billion joules of energy - about 10 billion times as much as a kilogram of TNT. Antimatter could provide 1000 times more energy than nuclear fission and 100 times more than nuclear fusion.

But antimatter is hard to come by. Virtually unknown on Earth, we would have to make our own supply. Antiparticles can be created in powerful particles accelerators. Physicists have even worked out how to trap and store antiparticles, using powerful magnetic and electric fields in a device called a Penning trap.
A Penning trap would be the fuel tank for an antimatter powered spacecraft. To use the energy liberated from antimatter would require a technique known as the ‘beamed core engine'. Antiprotons and protons would trickle into a reaction chamber where they would annihilate each other, releasing high energy gamma rays. These gamma rays would be focused using magnets and beamed out of the back of the craft. The gamma rays would be travelling close to the speed of light and provide a powerful motive force. A craft with a payload of 100 tonnes powered by antimatter could reach 100,000 kilometres per second. Which would get us to Gliese581c in about 60 years.

Unfortunately at present antimatter can only be made at a rate of 10 nanograms per year at a cost of $600,000. So we might have a gram in a billion years time. But our mission to Gliese581c would need 1000 tonnes of antimatter!

But antimatter occurs naturally in space when cosmic rays collide with gas clouds. This happens in our Solar System as cosmic rays crash into the Earth's atmosphere as well as Jupiter and Saturn. These antimatter particles are attracted by planetary magnetic fields form radiation belts akin to the Earth's Van Allen belts.

James Bickford, a physicist in Cambridge Massachusetts believes these belts could be harvested. A craft equipped with a powerful superconducting magnet in orbit around Jupiter could scoop up and trap antiprotons, which could then be used to propel spacecraft.

However, there are problems: superconductor technology is not up to spec yet and even then it might take a few thousand years to gather a gram of antimatter.

Beamed-core engines would use large amounts of antimatter, but other designs would require smaller amounts. Antimatter Initiated Microfusion, uses antiprotons to initiate nuclear fission in tiny droplets of deuterium and tritium. This then triggers a fusion reaction releasing particles which can be directed out of the craft to create thrust. Bickford says, "just micrograms of antimatter enable some very exotic missions that we can't tackle with any other technology. I suspect the first unmanned interstellar missions will be flown in this way."

We have been exploring space for 50 years, but it may be another decade before we are ready to launch a probe to the Oort cloud - only 0.1 light years away. For Gliese581c our spacecraft must survive a journey 200 times as long, carry its occupants and provisions and shield them from cosmic radiation. And bring then home again!

Presently a trip to Gliese581c is impossible, our technology just isn't up to the job. But progress and technology march on relentlessly, who knows what the next breakthrough may be?