NASA has continued its exploration of the moon since Apollo 17, but it has been over 40 years since one of its astronauts stepped on the surface. Gene Cernan was the second American to walk in space and the last set foot on the moon during that mission. Today, the retired US Navy captain died at age 82. During his time as an astronaut, Cernan logged over 500 hours in space and he spent more than 73 of those on the surface of the moon.