And he says extra-terrestrials have visited Earth on several occasions - but the alien contact has been repeatedly covered up by governments for six decades.
Dr Mitchell, 77, said during a radio interview that sources at the space agency who had had contact with aliens described the beings as 'little people who look strange to us.'
He said supposedly real-life ET's were similar to the traditional image of a small frame, large eyes and head.
NASA denies any knowledge but Mitchell seemed serious:
Mr Margerrison said: "I thought I'd stumbled on some sort of astronaut humour but he was absolutely serious that aliens are definitely out there and there's no debating it."DKK
Officials from NASA, however, were quick to play the comments down.
In a statement, a spokesman said: "NASA does not track UFOs. NASA is not involved in any sort of cover up about alien life on this planet or anywhere in the universe.
'Dr Mitchell is a great American, but we do not share his opinions on this issue.'