No spaceflight can be achieved without raw power. In the days before Plasma Impulse (Another discuss topic) Mankind rode into space on the power of chemical rockets.
For many years there were multiple studies on how to get a serious alternative to the huge weight and cost of chemical propulsion. NERVA was an early example. They were destroyed by politics and budget changes.
For many years mankind suffered from having a lack of a serious power source. The most advanced was the old fission nuclear power plants that were extremely expensive to build and maintain.
This finally changed with the advent of the "Enhanced Electrostatic Fusion Device" which is a mouthful for a small clean fusion reactor that gives hundreds of thousands of volts of pure electricity. Perfect for transforming Earth into a Hydrogen/Oxygen economy and ending many serious problems plaguing us for decades or centuries.
The most important aspect of this device was its ability and size to be fitted on large spacecraft. The match of the century was the ability to power the "Enhanced Plasma Impulse Drive" Which has revolutionized spaceflight and offers us our first manned flight to Pluto.
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--Real World--
EMC2 Fusion:
http://www.emc2fusion.org/http://video.google.com/videoplay?docid=1996321846673788606VASIMR:
http://ston.jsc.nasa.gov/collections/TRS/_techrep/TP-1995-3539.pdfhttp://www.adastrarocket.com/http://www.nasa.gov/vision/space/travelinginspace/future_propulsion.html