Humankind wanted to go to the Moon, and so the National Aeronautics and Space Administration (NASA) built a Saturn rocket. People wanted to live in space, and so an army of astronauts and engineers assembled a space station. Now humankind wants to travel to Mars and send robots to explore other galaxies. Thus, it is necessary to design propulsion systems that go faster and farther than ever before. From solar sails, to nuclear rockets, to propulsion with hot gases, to wild ideas that rival Star Trek' s concept of "warp speed," scientists have embarked on an adventure that goes beyond the works of the most creative science fiction writers.

The first logical step in this process is to improve conventional rockets by packing more energy into a smaller volume. Instead of burning liquid oxygen and hydrogen, as the space shuttle's main engines do, a future rocket might burn...