The astronauts - Neil Armstrong, Buzz Aldrin and Gene Cernan - made their appeal during a news conference at the Kennedy Space Centre in Florida to commemorate the 30th anniversary of the launch of Apollo 11, which took the first men to the Moon.

Neil Armstrong, who made the first historic Moon-walk, said the Apollo missions had demonstrated that "humanity was not forever chained to this planet", and that opportunity was unlimited.

Armstrong and Aldrin want a Mars voyage within 20 years

He and his fellow Apollo 11 astronaut, Edwin "Buzz" Aldrin, said they would like to see man reach Mars within 20 years.

"To put together the effort that it takes to get there once, twice, three times, and then turn our backs on it would be very sad for me and I would not like to see that happen," said Aldrin.

"We could do it in 2020 if we committed to it today, added Armstrong. "So the real question is: When are you going to commit to it?"

Relive the launch of Apollo 11

All the astronauts at the news conference said they would relish the opportunity to return to space, and made it clear they would do everything possible to emulate John Glenn, who became the oldest man in orbit with his shuttle flight last year.

'We came in peace'

The four days of celebrations are expected to culminate on Tuesday - exactly 30 years to the day after Armstrong's Moon-walk.

Buzz Aldrin recreates his 1969 moon-walk

The lift-off of Apollo 11 took place at 1332 GMT on 16 July, 1969.

After completing their work on the Moon's surface, the Apollo 11 crew left behind a plaque that read: "Here Men From Planet Earth First Set Foot Upon the Moon July 1969 AD. We Came In Peace For All Mankind."

David Baker, a space consultant, says it was a very significant event which cut mankind's umbilical cord to Earth.

"We felt at the time that mankind was being born as a true cosmic force. Around the far side of the Moon, those men were totally alone for the first time since humans evolved," Mr Baker told the BBC.

The Moonwalkers saw Earth like no-one before

That first voyage to the Moon took less than 12 minutes to reach orbit and 30 minutes for the capsule to separate from the Saturn rocket which carried it into space.

The mission was fuelled by the Cold War and American fears of Soviet domination in space exploration.

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In a BBC interview to be broadcast on Tuesday - the anniversary of the landing - Aldrin says the real reason for Apollo missions was to "bolster support for the military services which would stand as a bulwark against the increasing threat of Soviet invasion".

A silent death

The heroism of the three-man crew was highlighted this year by the release of a memo written to then-President Nixon at the time of the landings.

Under contingency plans, if Armstrong and Aldrin's lunar module had not been able to blast off from the Moon's surface to hook up with the command module on return, communications would have been cut and they would have been left either to commit suicide or die slowly.

The anniversary comes at a time when interest in space exploration is again growing.

This time businesses, not governments, are looking to space and they hope to exploit opportunities for satellite launches and space tourism.