From New Scientist :
In the 1980s the US aerospace engineer and visionary Robert Forward suggested that a solar sail could reach far greater speeds if it were illuminated by a powerful Earth-based laser than by relying on sunlight alone. Using a space-based lens to focus a laser beam onto a sail 1000 kilometres across would give a people-carrying interstellar craft enough of a kick to speed it past the nearest star in less than a decade and roll up at Gliese 581c about 40 years after take off. Just the ticket!
Forward’s idea does still have to leap a few technical hurdles. To keep the laser beam focused onto the sail you would need a space-based lens about 1000 kilometres across. Then there is the laser itself. According to Forward’s calculations it would need a power of many million gigawatts – far more than the world’s total electrical generating capacity.
To summon up this amount of power, perhaps we could harness the most energy-rich material we know: antimatter. This is made up of antiparticles with the same mass as conventional particles but the opposite electrical charge. When antimatter collides with matter, they annihilate each other, releasing the energy locked up in their mass.
This means that annihilating a kilogram of antimatter will give out about 1017 joules, or around 10 billion times as much energy as a kilogram of TNT releases when it explodes. Per kilogram of fuel, this is also 1000 times more energy than nuclear fission and 100 times more than nuclear fusion could generate. It sounds like an interstellar tour operator’s dream come true.
True, antimatter is tricky to get hold of. Antineutrons, antielectrons or antiprotons are virtually unknown here on Earth, so we would have to make our own, or acquire it elsewhere. Antiparticles can be created in powerful accelerators like those at CERN in Switzerland and Fermilab in the US. Physicists have even worked out how to trap and store antiparticles, using a device called a Penning trap, which uses powerful magnetic and electric fields to fence in the antiparticles.
A Penning trap or similar storage device would act as the fuel tank of any antimatter-powered spaceship. There would be various ways you might use the energy it liberates, but the technique that seems best suited to high-speed interstellar travel is the “beamed core” engine. The basic idea is to trickle antiprotons into a reaction chamber where they meet matter containing normal protons. As they annihilate each other, they release high-energy charged particles which are focused with magnets and beamed out of the back.
Anti-matter rocket propulsion has been the holy grail of rocket science since the 1960s, and well, since the U.S.S. Enterprise made the concept famous during the same time frame.
Engineering issues not withstanding, anti-matter drives (or some variety of it) will more than likely not power our first interstellar probes. Political pressures, money and the desire to weaponize anti-matter are the driving forces here. Don’t get me wrong, I would very much like to see anti-matter drives become a reality. But I don’t think it’ll be a government that will do this. It’ll be some private group or other entity that’ll find a way to make it efficient, if at all.
In my opinion, it’ll be decades, maybe even not during this century that anything resembling an interstellar exploration project will be undertaken. Nations of the world and certain private companies are only now taking serious steps to explore and exploit the Moon, a mere 230,000 miles away. And that’s only in the next twenty years!
I suspect that when the time comes for Mankind to seriously tackle interstellar travel, we’ll have already moved on to being something other than “human” in the classic sense and antimatter propulsion might not even be in the scenario at all!