The Answer:

Twelve astronauts have walked on the moon, the last in 1972.

Here are the names of those astronauts listed chronologically by the date of their walk.

July 20, 1969: Neil Armstrong and Edwin "Buzz" Aldrin

Nov. 19, 1969: Charles (Pete) Conrad and Alan Bean

Feb. 5, 1971: Alan Shepard and Edgar Mitchell

July 30, 1971: James Irwin and David Scott

Apr. 21-23, 1972: Charles Duke and John Young

Dec. 11-13, 1972: Harrison Schmitt and Eugene Cernan

—The Fact Monster