Enlarge / NASA originally studied nuclear thermal propulsion in the 1960s. Here is concept art for the Nuclear Energy for Rocket Vehicle Applications (NERVA) program. (credit: NASA)
Getting humans to Mars and back is rather hard. Insanely difficult, in fact. Many challenges confront NASA and other would-be Mars pioneers when planning missions to the red planet, but chief among them is the amount of propellant needed.
During the Apollo program 50 years ago, humans went to the Moon using chemical propulsion, which is to say rocket engines that burned liquid oxygen and hydrogen in a combustion chamber. This has its advantages, such as giving NASA the ability to start and stop an engine quickly, and the technology…