As Neil Armstrong took that first step off the Lunar Module and onto the surface of the moon 40 years ago this week, he spoke one of history's most famous lines.

"That's one small step for man, one giant leap for mankind."

Or is that what he said?

As many as 500 million people - nearly half the world's population in 1969 - saw Armstrong's small step and heard his line. Still, there's controversy about what he really said.

Many people believe Armstrong flubbed the line, meaning instead to say "one small step for a man." Without that one word, Armstrong's line is a grammatical contradiction; in his usage, man means the same thing as mankind.

In his 2005 official biography, "First Man: The Life of Neil A. Armstrong" the astronaut suggests he meant to say the "a."

"I would hope that history would grant me leeway for dropping the syllable and understand that it was certainly intended, even if it was not said - although it might actually have been," Armstrong said in the book.

The grammatical inaccuracy has been debated for years by historians and others. But in 2007, an Australian computer programmer, Peter Shann Ford, said he had found the missing "a" by conducting a high-tech software analysis of NASA's official recording of the audio.

Ford said the missing "a" cannot be heard in the tape because of communications technology employed in the 1960s. But he said the signature for the missing word is visible in the graphic tracing of the audio.

Armstrong rarely grants interviews, but in his biography, he told the author, Auburn University historian James R. Hansen, that people have told him that he often drops syllables from his speech.

"It doesn't sound like there was time for the word to be there," Armstrong said in the book. "On the other hand, I didn't intentionally make an inane statement, and ... certainly the ‘a' was intended, because that's the only way the statement makes any sense."