Fission and Fusion
Between 2010 and 2020, scientists may be able to use a Nuclear Electric Propulsion (NEP) vehicle for unmanned missions beyond Jupiter, or to support manned missions to Mars and beyond. This system would use nuclear fission (the splitting of atomic nuclei) which provides much more power than chemical reactions. An example of a nuclear propulsion system is the NERVA (Nuclear Engine for Rocket Vehicle Application) nuclear fission rocket engine. This system, being developed by NASA, consists of a fission reactor core with small tubes running through it. Liquid hydrogen is passed through the tubes and heated to about 1,700 °C (3,100 °F) to yield a high-thrust engine with twice the exhaust velocity of a chemical rocket.
Between 2020 and 2050, scientists may be able to use nuclear fusion (fusing atomic nuclei to release energy) to propel spacecraft. Nuclear-fusion vehicles could accelerate to 10 percent of the speed of light—about 30,000 kilometers (18,600 miles) per second. Their high speed would make fusion vehicles better-suited for interstellar (between the stars) missions than for flights within the solar system.
However, it is possible to slow down a fusion-powered spacecraft by adding a nonreactive material to the high-energy plasma (hot gas of ions) created during the reaction. This technique could make it possible to complete a round-trip manned Mars mission in three months, or a one-way robotic mission to Pluto in three years.