US astronaut Edgar Mitchell, who was the sixth man to walk on the Moon, has died aged 85.

He passed away at a hospice in West Palm Beach, Florida, his family said.

As part of the Apollo 14 mission in 1971, he spent more than nine hours on the Moon's surface.

He said he had had an "epiphany" in space and later devoted his life to studying the mind and unexplained phenomena. He said he believed that aliens had visited Earth.

"That was an overwhelming sense of oneness, of connectedness," he said in an interview years after the Apollo 14 three-man mission.

"It wasn't them and us, it was, 'That's me. That's all of it. It's one thing.'"

Mitchell left the US space agency Nasa in 1972 and set up the Institute of Noetic Sciences which aimed to support "individual and collective transformation through consciousness research"