Johnson joined NASA’s jet propulsion laboratory in 1979, where he worked to develop a nuclear power source for Galileo’s mission to Jupiter and Cassini’s mission to Saturn. Johnson had the idea for the Super Soaker water gun while working at NASA, trying to come up with a heat pump that would use water instead of Freon, which is unfriendly to the environment. The Super Soaker went on to become incredibly popular, and has made sales of almost $1 billion.