NASA Working On Nuclear Thermal Rocket Engines:

NASA is experimenting with the Nuclear Thermal Rocket Engines to propel the next generation of space explorers beyond the boundaries of the gravitational force of the Earth. These rocket engines will use a nuclear reactor to heat hydrogen to super high temperatures which will then escape through the nozzle with an exhaust velocity of about 10km/s to create the required thrust. The performance of such rocket engines is said to be limited by the temperature withstanding capacity of the material used for the body of the engine.

The advance propulsion researchers at NASA believe that this technology will be a significant step in future man-missions to the Mars. An important point to note is that these rockets are twice as efficient as the current generation of chemical rockets. Several tests were recently conducted at the Marshall’s Nuclear Thermal Rocket Element Environmental Simulator, or NTREES where the researchers were able to create hot hydrogen (approximately 5,000 degrees Fahrenheit) at a pressure of about 1000 psi. The data obtained from the tests will help the engineers improve their rocket design further.
http://www.youtube.com/watch?feature=player_embedded&v=j6gKFvPjGpQ