Yet Mr Armstrong, 78, has always been embarrassed by his place in history, shunning the spotlight and preferring to play down his contribution even as the United States revelled in the glory of its greatest scientific accomplishment.

This weekend, as Nasa celebrates the 40th anniversary of the first moon landing, Mr Armstrong will again be cast in the role of reluctant hero. Apart from one high-profile appearance with his two Apollo 11 crewmates Edwin 'Buzz' Aldrin and Michael Collins at a lecture in Washington tomorrow night, the jamboree will go on without him.

There will be no interviews with the media, no autograph sessions and no other public appearances connected with the occasion except a brief trip to Houston next Friday for the anniversary of his space capsule's splashdown.

'If there were no anniversary at all, if there were no celebrations, he would be fine with it,' said James Hansen, Mr Armstrong's friend and official biographer.

'It has never been his personality to look for the limelight. He has never done anything for the purpose of celebrity or fame.'

By contrast, Mr Aldrin, the second man to set foot on the lunar surface, has been more than busy, recording rap songs with Snoop Dogg, promoting his third autobiography on a nationwide book tour and plugging all manner of anniversary souvenirs, from shortwave radios to expensive designer luggage.

'Compared [with] Aldrin, Neil's going to look reclusive, because Buzz is ubiquitous, he does everything. They're pretty much polar opposites in many respects,' said Professor Hansen, who lectures in history at Auburn University in Alabama and is a former Nasa historian. 'Those who really know Neil and know what he does on a daily basis, that he is out in the world doing things, reject the term 'reclusive'. It's kind of a Howard Hughes description, someone who's holed up in the castle tower. Neil's very engaged with the world and does a lot of events.'

Mr Armstrong's sense of modesty, and his composed and measured demeanour, made him an ideal candidate when Nasa was picking its second class of nine astronauts in September 1962 to join America's original space pioneers, the 'Mercury Seven'.

Psychological stability was as much a requirement as any physical aptitude and experience, given that the men were voyaging into the unknown, and the aerospace-engineering graduate passed the evaluations with flying colours.

Even so, there was some debate among Nasa's hierarchy about whether the reserved pilot was the right man to handle the inevitable scrutiny and media pressure that would follow his return to earth.

'He was not pre-designated to be the commander of the first moon-landing mission,' Professor Hansen said. 'The crew assignments and missions lined up in such a way that it just turned out that Apollo 11 was going to attempt the first landing, and Neil was its commander.

'There needed to be a decision as to which of the two astronauts inside the lunar module would step out first, and that raises some interesting questions about the choice of Armstrong.

'But Neil was not really looking for the first-man role in terms of the icon that he now is to all of us. He was an engineer first and foremost and he was a pilot, someone who was thrilled by the idea of being the first of the astronauts to land this brand-new and very different type of flying machine on another world.

'The landing challenges were what really captured his mind and heart and soul, and what he did when he landed he was trained to do. But to be so remembered just for stepping down off a ladder ... he didn't even think what he said then was going to be as memorable as what he said when they actually landed.'

Even on the moon, Mr Armstrong barely showed his face. As the main operator of the still camera that the crew took with them, he snapped photographs of Mr Aldrin that have become legendary, while Mr Aldrin's shots of Mr Armstrong largely showed him at a distance, in shadow, or with his back turned.

Now, for the first time, a still image has emerged that shows Mr Armstrong's face in clear view as he bounces across the lunar surface (see page A9). The image was shot by a movie camera mounted on the lunar lander but lasts for only a fleeting moment.

Life after Apollo, and the ticker-tape parades in Washington and elsewhere that followed, saw the three trailblazers heading their separate ways. They were never the best of friends - 'amicable strangers' was Mr Collins' description of their relationship - and Mr Armstrong left Nasa in 1971 to teach aerospace engineering at the University of Cincinnati in his home state of Ohio. He was 41.

He also took directorships at several companies. 'He accepted offers only where he had an expertise that would allow him to make a contribution in a technical sense,' Professor Hansen said.

In retirement, Mr Armstrong leads a quiet life with his second wife Carol, whom he married in 1994, in an upscale neighbourhood of Cincinnati. He remains in good health despite a minor heart attack in 1991. Although his public persona is of a man detached from his space-flight background, friends say he is engaged in the debate over the direction of US space policy, and expect him to use the Washington lecture to encourage Nasa to pursue its plans to return to the moon.

Mr Aldrin, meanwhile, has already stated his preference for working towards a mission to Mars by the middle of the century.

'Buzz has done a number of books that talk candidly about his career and his personal life, and that's fine, that's Buzz,' Professor Hansen said. 'But as a result of that, if there's a story today about Buzz in the news, people will pay some attention, but it's just another story about Buzz.

'If Armstrong comes out with something, it's much bigger news because he just isn't there all the time. His voice means something because it's used so rarely.'

As for the next generation of space travellers, after next year's retirement of the space shuttle, Mr Armstrong retains a detached view.

'When he talks about exploration, he's much more excited in a piece of flight machinery or flying sailplanes or something like that than he is about Mars, about moon colonies,' Professor Hansen said. 'Neil is very pragmatic in his approach to most everything. I never really see a gleam in his eye or hear electricity in his voice.'