By 2020, at least one spacecraft weighing at least 50 metric tons will be powered by using nuclear fission, nuclear fusion or anti-matter reactions as a power source for propulsion. This spacecraft will attain an orbital height of at least 2000 miles or if assembled in orbit shall attain escape velocity. It is not necessary that all of the spacecraft's propulsion power be obtained by nuclear reactions.
Background One of the most famous proposals for nuclear powered space flight was Project Orion funded by the US government. Project Orion was cancelled due to concerns about pollution and concerns about the reaction of the public. The Orion designers were most interested in using pure fusion bombs that would produce much less radioactive waste than conventional fission devices and planned to limit nuclear propulsion for use outside the atmosphere. There is still active support for the development of nuclear powered spacecraft. Advocates of nuclear powered spacecraft claim that many of the problems with nuclear powered spacecraft can be mitigated.