24 May 2012

Now that we are discovering hundreds of extrasolar planets, the next logical jump will be to discuss finding ways to reach the star systems of some of those planets, particularly ones that are only a few light years away.  Remotely analyzing these planets with telescopes is great but it may never fully prove whether life exists there or has evolved into sentient beings.  So how do we get to the stars?  The biggest science fiction method, particularly that of Star Trek, has always involved antimatter.

Mirror Mirror

Antimatter is the mirror image of the electrical charges we see in normal matter.  It was very abundant just after the Big Bang, but once it touched normal matter there was instant annihilation in the form of gamma ray bursts.

In our universe there is an excess of normal matter to antimatter, called the CP violation which is the breakdown of the predicted symmetry between the number of regular particles and antiparticles made during the Big Bang.  Recent experiments from CERN in Switzerland have shown evidence for the subtle differences between matter and antimatter and possibly explain how the early universe survived.

The Lack of Antimatter

One issue with developing an antimatter propulsion system is the lack of antimatter.  Current estimates are that it would take 1000 years to make a single microgram of antimatter with our current crop of particle accelerators.  However, the intensity of the beams of antiprotons has been increasing by four every decade.  Likewise the growth of the production of liquid hydrogen, which propelled the Shuttle, has increased exponentially in the past few decades.  So as a result it seems likely that the supply of antihydrogen may grow exponentially to produce a microgram of fuel by the middle of the 21st century.

Unfortunately a lot more antimatter is needed for an interstellar mission.  There needs to be enough fuel for reconnaissance and landing.  The starship also has to have enough fuel to decelerate upon arrival at the system.  For example:  A starship with 100 tons of cargo that cruises at 40 percent the speed of light needs about 80 ocean supertankers full of antimatter fuel.  If we slow the rate to only 25 percent then far less is needed.

So as we are able to store larger and larger quantities, the dream of reaching the stars becomes more feasible.  Once collected the antimatter must be stored and shielded from normal matter.  In 2011 CERN's Antihydrogen Laser Physics Apparatus (ALPHA) trapped 309 atoms of antimatter for over a quarter of an hour.

Using Naturally Occurring Antimatter

To acquire such large amounts of antimatter, it may be necessary to turn to more natural ways of acquiring it.  Antiprotons have been discovered in the Earth's magnetic field recently.  Specifically about 28 antiprotons were found, about three orders of magnitude more than you'd find in the solar wind.  The Alpha Magnetic Spectrometer onboard the International Space Station should also be able to detect and identify antiparticles in Earth orbit.

There are theoretical studies that suggest the magnetospheres of larger planets like Jupiter should contain more than Earth.

From Science Fiction to Reality

Even back in 2000 NASA had drafted up early designs of an antimatter engine that would be used for missions to Mars.

More recently, physicists Ronan Keane of Western Reserve Academy and Wei-Ming Zhang of Kent State University wrote a paper and studied on antimatter propulsion.  Their latest results from computer simulators have shown that at least one key component in creating a working antimatter propulsion engine is that of highly efficient magnetic nozzles.  Their studies have shown that these nozzles need to be extremely efficient and that it is feasible to make them this way using our current technology.

The antimatter propulsion system in general is much more effective than chemical ones.  Matter/antimatter reactions produce 10 million times the energy than conventional chemical reactions such as the hydrogen and oxygen combination for the Space Shuttle.  These are reactions that are even 1,000 times more powerful than that of nuclear fission and 300 times that of fusion.

Keane and Zhang also outlined how the particles would avoid a matter/antimatter annihilation as they exit the engine.  Their technique relies on charged pions that result from proton-antiproton collisions.  A nozzle that emits a strong magnetic field could channel the emitted charged particles into a focused stream of charged pions accelerating them to make an even stronger thrust.

Past calculations have shown that the pions' initial speed would be 90 percent the speed of light, the nozzle would only be 36 percent efficient, resulting in only 1/3 the speed of light.

So Keane and Zhang have focused on the design of the nozzle to make it more efficient by using CERN software.  They now state a nozzle that is 85 percent efficient can be achieved.  However, they did find the pions speed to be about 80 percent of the speed of light rather than 90.  The net result is a 70 percent of the speed of light velocity. 

With such an antimatter starship, a trip to Earth's closest star system, Proxima Centauri would take about 6 years to travel the 4.2 light years.  Due to the relativistic effect of traveling that fast, such a trip would seem like only 4.3 years to those on board the starship and 6 years to everyone else viewing from back on Earth.

Our efforts of detecting these extrasolar planetary systems will certainly drum up the discussions of how to get there in the coming years.  And as that discussion heats up, so will research and development of technologies to get us there.  The antimatter engine could be closer at hand than we may think.  By corollary even Albert Einstein was quoted in 1932 as saying, "There is not the slightest indication that nuclear energy will be obtainable."  What may seem impossible with antimatter right now might become reality in years to come.

Source:              Discovery, TechnologyReview
Image Credits:  NASA

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