How to get to Mars? There is growing evidence that extended stays in weightless conditions can cause serious health problems. The answer, in part, would be to employ a high impulse propulsion system that could get astronauts there more quickly. Thus, it was with interest that I read the following article on the history of nuclear rockets.
The case for nuclear propulsion is based on its high “specific impulse,” which roughly equates to a rocket’s miles per gallon rating. A nuclear plant that can get a spacecraft to Mars is much lighter than a chemically-propelled one. In one common setup, hydrogen propellant is passed through a uranium nuclear reactor, it would turn the hydrogen to plasma and vent out of the nozzle, providing thrust with relatively low weight, according to David S. Portree’s history, “Humans to Mars: Fifty Years of Mission Planning, 1950-2000.”The article mentions that there had been some limited testing, but that the biggest concern with testing was safety. However, that didn't stop all testing:
An account in Annie Jacobsen’s “Area 51: An Uncensored History of America’s Top Secret Military Base,” paints an even wilder picture. On Jan. 12, 1965, Jacobsen writes, a Kiwi reactor at Los Alamos was allowed to overheat as a kind of practice drill for a nuclear accident , and eventually burst. A radioactive cloud floated west toward Los Angeles and then out to sea, according to Jacobsen’s book. The Russians argued it violated the Nuclear Test Ban Treaty. NASA’s official history confirms the test. Jacobsen asked Finger about the test in 2010. “I don’t recall the exact test,” he replied.It is also notable that the military also experimented with nuclear powered aircraft in the 1960s. (See also here).
Although testing in these programs supposedly stopped in the 1960s, there is a "UFO" encounter in 1980 that, if true, is highly suggestive of an operational test of a nuclear rocket or aircraft--perhaps experiencing mechanical difficulties.