The truth is out there. And a former Apollo 14 astronaut wants you to know about it. Edgar Mitchell, who made the longest moonwalk in history in 1971, says alien life does exist and the US government is blocking the information from getting out.

On Monday Mitchell addressed the issue of extraterrestrial life at the National Press Club in Washington after the X-Conference, a convention of UFO researchers and activists.

"We are being visited," he said. "It is now time to put away this embargo of truth about the alien presence. I call upon our government to open up ... and become a part of this planetary community that is now trying to take our proper role as a spacefaring civilisation."

Mitchell, who has a PhD in aeronautics and astronautics from the Massachusetts Institute of Technology, said it's only a matter of time before we need to evacuate Earth and seek a new home. "The sun will burn out in due course, and we have to be off this planet if our species is to survive. At this point in human history on this planet, we're now starting, and should be, to reach out beyond our planet and then beyond our solar system to find out what is really going on out there."

Growing up in Roswell, New Mexico - where some UFO experts believe a crash took place in 1947 – Mitchell said residents of the town "had been hushed and told not to talk about their experience by military authorities" and were told they would suffer "dire consequences" if they did.

Residents relayed eyewitness accounts of alien sightings to him because they "didn't want to go to the grave with their story. They wanted to tell somebody reliable. And being a local boy and having been to the moon, they considered me reliable enough to whisper in my ear their particular story."

Mitchell's belief in the existence of aliens is well documented. In an interview with the Times in 1998, he said he is "90% sure that many of the thousands of unidentified flying objects, or UFOs, recorded since the 1940s, belong to visitors from other planets."

"A few insiders know the truth . . . and are studying the bodies that have been discovered," he told the St Petersburg Times of Florida in 2004.