Apollo 17 astronaut Gene Cernan, who died on Monday, snaps a picture of crewmate Harrison "Jack" Schmitt in the Taurus-Littrow valley on the moon.

The generation of astronauts who inspired a world with the first and, so far, only journeys beyond Earth's orbit is dwindling, with just six of the 12 Apollo moonwalkers still alive.

The last astronaut to walk on the lunar surface was Apollo 17 astronaut Gene Cernan, who died on Monday.

Still living are Harrison "Jack" Schmitt, who accompanied Cernan to the lunar surface in December 1972; Apollo 11's Buzz Aldrin; Apollo 12's Alan Bean; Apollo 15's David Scott; and Apollo 16's John Young and Charles Duke.

All of the men are now in their 80s.

"It appears we are condemned to forego the human exploration of the solar system until the full measure of the first generation of space explorers has passed," former space shuttle program manager Wayne Hale, now a consultant, wrote on Facebook.

In all, 12 astronauts walked on the lunar surface during six Apollo missions to the moon between July 1969 and December 1972.

RELATED: Eugene Cernan, Last Man to Walk on the Moon, Has Died at 82

Each Apollo mission also included a pilot who remained in orbit around the moon while a two-man landing crew flew to the surface. Four of those pilots — Apollo 11's Michael Collins, Apollo 12's Dick Gordon, Apollo 15's Al Worden and Apollo 16's Ken Mattingly — are still alive.

In addition to the six moon landings, NASA flew two precursor orbital missions, Apollo 8 in December 1968 and Apollo 10 in May 1969.

Another intended moon landing during the Apollo 13 mission turned into a lunar flyby after an oxygen tank explosion crippled the spacecraft.

In all, only 24 people have ever ventured beyond Earth's orbit and of those 15 are still living.

NASA is hoping to fly a crew around the moon during a test flight of its Orion spacecraft in 2021.

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Originally published on Seeker.