Now, thanks to high-resolution cameras orbiting the moon, the mystery is solved: All the flags but one are still standing. The exception is the flag for Apollo 11, the historic first human moon landing in 1969, said professor Mark Robinson of Arizona State University, lead scientist for the cameras aboard the Lunar Reconnaissance Orbiter.
The lack of an Apollo 11 flag is consistent with astronautBuzz Aldrin's memory of the famous mission. Aldrin said the flag blew over from the rocket blast when astronauts left the surface.
Robinson previously had doubted whether any flags would be visible.
"Personally, I was a bit surprised that the flags survived the harsh ultraviolet light and temperatures of the lunar surface, but they did. What they look like is another question (badly faded?)," Robinson wrote in a recent blog on the orbiter's website.
Images taken by the orbiter show the flags and their shadows but aren't detailed enough to reveal whether the Stars and Stripes are still visible.
The Lunar Reconnaissance Orbiter is an unmanned spacecraft that has been circling the moon for more than three years. The minivan-size orbiter has equipment that is photographing the moon's surface, recording temperatures and measuring radiation.
The photographs are being used to map the surface and could be used to identify future landing sites, although the United States now has no plans to send humans back to the moon.
NASA's Apollo program included multiple launches in the late 1960s and early 1970s and was designed mainly to land humans on the moon and bring them back to Earth safely.
Six missions accomplished this. Others were not as successful.
A cabin fire on Apollo 1 killed three astronauts during a launch-pad test in 1967. Apollo 13 was unable to land after an oxygen tank exploded and forced the crew to return to Earth in 1970.
Even with the mishaps, the Apollo program is viewed as widely successful and credited with igniting a generation's interest in space. The last manned mission to the moon wasApollo 17 in December 1972.
Signs of the missions are still visible on the moon's surface. Photos taken by the lunar orbiter show tracks made by lunar rovers and equipment left behind, including backpacks jettisoned by astronauts. Images taken of the Apollo 17 site show the astronauts' foot trails.
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