
http://www.nasa.gov/centers/goddard/news/topstory/2006/antimatter_spaceship.html
Most self-respecting starships in science fiction stories use antimatter as fuel for a good reason – it’s the most potent fuel known. While tons of chemical fuel are needed to propel a human mission to Mars, just tens of milligrams of antimatter will do (a milligram is about one-thousandth the weight of a piece of the original M&M candy).
Image right: A spacecraft powered by a positron reactor would resemble this artist's concept of the Mars Reference Mission spacecraft. Credit: NASA
However, in reality this power comes with a price. Some antimatter reactions produce blasts of high energy gamma rays. Gamma rays are like X-rays on steroids. They penetrate matter and break apart molecules in cells, so they are not healthy to be around. High-energy gamma rays can also make the engines radioactive by fragmenting atoms of the engine material.
The NASA Institute for Advanced Concepts (NIAC) is funding a team of researchers working on a new design for an antimatter-powered spaceship that avoids this nasty side effect by producing gamma rays with much lower energy.