A series of American flags that were planted on the moon during visits by Apollo astronauts some four decades ago are still standing, except for one, a NASA scientist has said.

The missing flag was the one planted by the pioneering astronauts on the Apollo 11 mission in 1969 when the first humans -- Americans Buzz Aldrin and Neil Armstrong -- stepped foot on the moon.

The details are contained in an online article by principal investigator Mark Robinson who analyzed data from NASA's Lunar Reconnaissance Orbiter Camera (LROC).

"From the LROC images it is now certain that the American flags are still standing and casting shadows at all of the sites, except Apollo 11," he wrote.

"Astronaut Buzz Aldrin reported that the flag was blown over by the exhaust from the ascent engine during liftoff of Apollo 11, and it looks like he was correct!"

Apollo missions ran from 1969 to 1972, shuttling American astronauts to the surface of the moon. Six flags were planted during this timespan.

The LROC orbiter was launched in 2009 to analyze new landing sites on the moon and send back visual information about the lunar environment via narrow and wide angle cameras.

"Personally I was a bit surprised that the flags survived the harsh ultraviolet light and temperatures of the lunar surface, but they did," Robinson wrote. "What they look like is another question (badly faded?)."