July 20 marks the 49th anniversary of the Apollo 11 moon landing. Only a dozen astronauts have walked on the surface of the moon, all Americans. Of those 12, only four remain. The surviving moonwalkers from the Apollo program are Buzz Aldrin, Harrison Schmitt, David Scott and Charles Duke. Several of the astronauts have died recently, including John Young and Alan Bean. No one has traveled further from the Earth. And many wonder when, if ever, people will travel so far again.