In the recording of Neil Armstrong's iconic phrase he says: "One small step for man. One giant leap for mankind". However, "man" and "mankind" mean much the same thing in this context.
But on returning to Earth, he explained that he thought he had said "one small step for a man".
Explanations offered for the discrepancy are that perhaps transmission static wiped out the "a" or that Commander Armstrong's Ohio accent meant that his "a's" were spoken softly.
In 2006, an analysis by an Australian entrepreneur added credence to these explanations - as it found there was a gap for the "a". However, subsequent analyses disputed this conclusion.
To settle the argument, Dr Chris Riley, author of the new Haynes book Apollo 11, An Owner's Manual, and forensic linguist John Olsson carried out the most detailed analysis yet of Neil Armstrong's speech patterns.
Mr Armstrong said he thought he had said "one small step for a man"
They are presenting the research at the Cheltenham Science Festival this week.
"For me that phrase is of great significance," said Dr Riley.
"It has been an important part of my life and those words sum up much of the optimism of the later part of the 20th Century."
Using archive material of Neil Armstrong speaking, recorded throughout and after the mission, Riley and Olsson also studied the best recordings of the Apollo 11 mission audio ever released by Nasa.
They have been taken from the original magnetic tape recordings made at Johnson Space Center, Houston, which have recently been re-digitised to make uncompressed, higher-fidelity audio recordings.
These are discernibly clearer than earlier, more heavily compressed recordings used by the Australian investigation.
These clearer recordings indicate that there was not room for an "a". A voice print spectrograph clearly shows the "r" in "for" and "m" in "man" running into each other.
The researchers say the Australian analysis may not have picked up the fact that Armstrong drawled the word "for" so that it sounded like "ferr" and mistook the softly spoken "r's" for a gap.
"It's perfectly clear that there was absolutely no room for the word 'a'," Mr Olsson explained.
The "Eagle" made its historic descent to the Moon on 20 July 1969
Riley and Olsson also concluded that Commander Armstrong and his family members do pronounce the word "a" in a discernible way.
And based on broadcasts from Neil Armstrong and Buzz Aldrin from the surface of the Moon, it is clear that the word "a" was easily transmitted to Earth without being obliterated.
But their analysis of the intonation of the phrase strongly suggests Commander Armstrong had intended to say "a man". There is a rising pitch in the word "man" and a falling pitch when he says "mankind".
According to Mr Olsson: "This indicates that hes doing what we all do in our speech, he was contrasting using speech - indicating that he knows the difference between man and mankind and that he meant man as in 'a man' not 'humanity'."
There has also been speculation that Neil Armstrong was reading from a pre-prepared script penned for him by another party. According to Mr Olsson, that is not borne out by Armstrong's body language and speech patterns.
This is one of the few images of Armstrong on the Moon
"When you look at the pictures, you see that he's moving as he is speaking. He says his first word 'that's' at the moment he puts his foot on the ground. When he says 'one giant leap for mankind', he moves his body," he said.
"As well as this, there is no linking conjunction such as 'and' or 'but' between the two parts of the sentence. So it's for all those reasons that we think this is a completely spontaneous speech."
It may well have been that spontaneity that led to Armstrong's slight mistake. But according to Mr Olsson - Armstrong may have subconsciously drawn from his poetic instincts to utter a phrase that, far from being incorrect - was perfect for the moment.
"When you look at the whole expression there's a symmetry about this. If you put the word 'a' in, it would totally alter the poetic balance of the expression," he explained.
This makes Dr Riley feel that the research has made a positive contribution to the story of the Apollo mission.
"Im pleased we've been able to contribute in this way and have hopefully drawn a line under the whole thing as a celebration of Neil and everyone involved with Apollo, rather than this constant little niggling criticism," he said.
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