The barren, lifeless surface of the moon isn’t quite as lonely as it seems. Forty years after the first moon landing, ghostlike relics of what many call mankind’s greatest achievement still remain.

Astronauts from the United States’ Apollo missions as well as robots from many other international missions have abandoned more than 300,000 pounds of man-made equipment on the surface of the moon.

Personal items left by Apollo astronauts account for some of this mass. Alan Shepard left his golf balls on the moon after a couple practice shots, and the crew of the Apollo 15 mission left a commemorative statue known as the Fallen Astronaut in honor of those who gave their lives in the advancement of human space travel.

In the tradition of earthly explorers, Apollo astronauts also left an American flag and a plaque reading “Here Men From The Planet Earth First Set Foot Upon the Moon, July 21 1969 A.D. We Came In Peace For All Mankind.”

A variety of spacecraft also rest silently in the lunar graveyard. Many were left behind because they were too heavy to return with their crew. Others are unmanned robotic vehicles that either crashed after orbiting or went out of service. When rockets launch or return to earth, portions of the engines detach and enter freefall after their fuel is spent. The same process happens with lunar launch and departure.

The first US craft left on the moon was the Ranger 4 in 1962. Shortly after the failed Soviet Luna 2, this robotic vehicle arrived successfully in lunar orbit -- only to fall victim to computer failure and eventually crash. These are only a few of over 70 spacecraft abandoned on the moon.

Some of the scientific hardware left on the moon is still in use. The Lunar Laser Ranging Experiment uses retroflectors -- high-tech mirrors that reflect light with minimal scattering. These mirrors measure the continually changing distance between Earth and the moon. The first mirrors were installed during the 1969 Apollo 11 mission. More were later placed on the moon by astronauts from Apollo 14 and 15, as well as unmanned Soviet Lunokhod 1 and 2 rovers in 1970. All but one return signals to Earth to this day.

Until the upcoming 2009 launch of the Lunar Reconnaissance Orbiter, man-made moon debris will remain too small to be photographed from Earth. Even the powerful Hubble Space Telescope cannot capture objects on the moon’s surface less than 60 meters wide. The fact that moon debris has only been photographed from the moon itself has been a lifeline to conspiracy theorists claiming that humans have never in fact landed on the moon.

Call it a trashcan, call it a graveyard, call it a museum -- whichever way you put it, the lunar surface holds an impressive collection of objects large and small illustrating the progress of space exploration technology. The spindly-legged, gold-foiled space craft that are the work of engineers from the United States, Japan, China, Russia, and the European Space Agency will forever be preserved in the moon’s airless atmosphere.