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The day 20 July 1969, 40 years ago, marked what many see as the apogee of human achievement, the most remarkable moment in humankind's existence, when history and science collided in one unique, spellbinding moment.

Across the globe, 500 million people watched in agonised expectation as Neil Armstrong slowly descended the steps of the lunar module and spoke his immortal line.

Among those waking up in the UK to watch this historic event unfold on the morning of 21 July was a six-year-old James May. Forty years later, the TV presenter can remember the moment with the kind of clarity that only the most remarkable moments of childhood leave imprinted on our memory.

"I was actually at my grandparents' house in the West Country. My dad came to get me up. My mum and dad were there, and my grandparents, and my sister, and some aunts and uncles and so on, and you could just tell that it was something very tense and important.

"I can see quite vividly the black-and-white footage – obviously I've seen it hundreds of times since, but I can remember seeing it at the time, and Armstrong saying: 'I'm at the foot of the ladder, and I'm now stepping off' and then he did his 'one small step' speech.

"When I watch that bit of footage now, I'm taken right back to that moment when I was six years old, and I can see the old wallpaper and the old-fashioned telly and everything that was in the room."

The moment marked the stirring of a fascination with space travel in the six-year-old May, and the fascination endures to this day.

May has made a one-hour film for ´óÏó´«Ã½ Two, James May On The Moon, and James May At The Edge Of Space – a 30-minute film for ´óÏó´«Ã½ Four – to coincide with the 40th anniversary of the Moon landing.

The ´óÏó´«Ã½ Two film features a fascinating plethora of archive footage, dazzling science and interviews with some remarkable individuals, culminating in the most remarkable journey James took 70,000 feet high, to the edge of the Earth.

Meanwhile, the ´óÏó´«Ã½ Four film follows James's three days of intensive training at an Air Force base, learning to wear a space suit in preparation for the flight. In the midst of it all there is the figure of May, having the time of his life and clearly unable to believe his luck.

He admits that he didn't have to embark on the most arduous programme of research. "I think it's one of those things that I already knew, but I felt I'd better check everything and make sure I had it all absolutely right."

The films feature May experiencing zero gravity in the so-called 'Vomit Comet', undergoing serious G-forces in a US Air Force centrifuge, and flying at 70,000ft in a U-2 spy plane.

But, strangely, the aspect he was most nervous about was interviewing three of the men who have walked on the moon, for James May On The Moon.

"You should never meet your heroes – they have the potential to be very disappointing," he explains. "It's why I've never tried to meet [former children's TV presenter] Brian Cant."

Not that they turned out to be disappointing – far from it. But anyone expecting lantern-jawed supermen will be in for a surprise.

"It's weird, because they're old men now, they're in their seventies, and you know they're going to be old, but when you meet them, you see Alan Bean, and he sort of looks like Alan Bean in all the famous pictures of him, but he's an old bloke, and he paints pictures and wears little glasses.

"And Charlie Duke wears a cardigan and lives with his wife, Dottie. They're fantastically ordinary, in many ways. They've just been to the Moon, and have these incredible stories to tell."

Their experiences are indeed remarkable – which perhaps explains why many astronauts are said to struggle with a civilian life thereafter.

"I know that a lot of them have problems coming to terms with normal life, and there were quite a few divorces and all that sort of thing," says May.

"Talking to Alan Bean, I got him talking about the technicalities of landing the lunar module and so on, and he does sort of light up, slightly, and you get the feeling that this is still where he lives in his heart. But then I suppose you would if you'd done it."

Neil Armstrong, though, is one who is notoriously unwilling to discuss his remarkable legacy. May is quick to sympathise.

"I can sort of understand why, if you'd been the first man on the Moon, you wouldn't really want to talk about it. I'm not sure you're ever going to really add anything that people don't already know. You'd probably only diminish it.

"I can't remember who said that 'in a thousand years time, Neil Armstrong is the only name that will be remembered from the 20th century'. That may be true."

One thing that certainly linked all of the NASA astronauts was their undoubted bravery. But even they were said to fear the arduous training programme and endless testing they were forced to undergo at places like Brooks Air Force Base.

On his own visit to the base, May submitted himself to the centrifuge test. "I found it quite interesting, actually. But I wouldn't say it was particularly enjoyable. It gave me backache - I'm a bit feeble these days. The G forces you're under as an astronaut are incredible."

The "Vomit Comet", though – upon which trainees experience weightlessness for the first time – was a different matter.

"It was good fun, actually," – a testimony borne out by the footage of him giggling manically in the film as he bounces off the walls, floor and ceiling.

But for all his glee at getting to try the centrifuge or experience zero gravity, it is May's unbridled enthusiasm for the technical achievements involved in the moon landings that is most infectious.

"They say the best part of 400,000 people were employed in one way or another sending Apollo 11 to the moon," he says with wonderment.

"It's just amazing, when you think of the standard of communications back in those days, and the means of storing and retrieving data, the fact that the drawings for the components would have filled a room rather than a computer file. And you've got all the people at all the different locations, the manufacturers involved in the different bits of it, having to make it all match up.

"I just can't work out where they started. They worked out the principles, and they decided that it would have to be a spacecraft with a separate lunar module, that was all a matter of working out the weight and size of the rocket they could produce, and how much payload they could have, but then having worked out those principles, what did they do? Did they sit down and draw a line and say 'It must be this tall?' I don't know. How the hell do you start?

"It is still, I think, the world's most complicated machine. There must have been a point where they picked up a rivet and said: 'That's the beginning'."

In one eye-popping sequence in the film, May reveals that NASA expected around 0.1 per cent of the components on the spacecraft to fail. That doesn't sound like much, until you realise there were 6 million components involved.

Suddenly, you're talking about sitting on top of what is effectively an enormous fuel bomb, proposing to ride it into space, and it is expected to have 6,000 faulty components on board. It's no wonder May is in awe of the astronauts.

James May On The Moon does not actually put James May on the Moon. Indeed, nobody's been there since 1972.

"Once they'd been, photographed the Earth from space, gone and landed on the Moon and collected some samples and so on, the returns began to diminish," says May with a hint of regret. "And, to be honest, so did public interest."

In the absence of a moon mission, though, May did manage to get a good deal closer to space travel than most, hitching a lift on a U-2 spy plane to a remarkable 70,000ft.

The trip, though, wasn't a straightforward matter of strapping himself into a seat and choosing an in-flight meal.

"We had to do three days of training and preparation to make sure I was healthy enough, and to ensure I could get out if there was an emergency.

"One of the problems with a U-2 is that it flies so high that even if you are directly above the air force base, if you bail out at 70,000 feet you could land in the Pacific or in the desert – you can end up such a long way away.

"So I had to learn a bit about water survival, how to put up a shelter in the desert, they take you through all this Ray Mears-type stuff."

So, if the TV work starts to dry up, does he fancy himself as the next Ray Mears? "No," comes the firm reply. "He eats funny stuff."

Back to the U-2 flight, then. Unexpectedly, at least to those more used to May's jokey, blokey persona in Top Gear, the moment when May looks down and sees the curvature of the Earth is a genuinely moving one – both for the viewer and for May.

"I found it amazing. I knew what I would see, but it was still amazing when I saw it. The atmosphere is so very thin, in terms of its depth over the surface of the earth. At 70,000ft, almost 95 per cent of the atmosphere, by mass, was below us.

"And that's all that makes the difference between the world being a 'beautiful, brightly coloured pea,' as Armstrong put it, and the grey and completely barren Moon. It's incredible to see it, and see that it really is just a little mist over the surface, that's all…

"Then you look up and you think: 'Well that just goes on forever, and we are just a speck of dust in it, that's all it is.'"

At that moment, May and his pilot were higher than any human beings apart from the two astronauts in the Space Station.

"There were no other U-2s up that day, so we were the second highest pair of blokes in the Universe, as far as we know."

For May, though, the high went on long after he returned to Earth. "You expect people to say: 'Oh, that really changed my life', but I think it did a little bit."

Soon, the age will be upon us when commercial flights will take paying customers into orbit. His appetite whetted, May says he will be among them. But for him, the real excitement comes with the pushing of boundaries, the dawn of new eras of achievement.

He quotes Armstrong's fellow astronaut Michael Collins: "To explore isn't a choice, it's an imperative."

The next big mission, he says, will be taking people to Mars.

"A manned mission to another true planet will be the thing that next really grabs the world. The impression I get from talking to people is that it's a possibility now. The technology exists, it's a matter of funding it, sorting out the details, making the kit, finding the people and so on. But it's perfectly possible, it's just a matter of doing it."

So, 40-odd years from now, could we look forward to an 86-year-old James May presenting James May On Mars? "I'd very much like to think so, yes."

Just try stopping him.