It is as if the United States, humbled in the Sydney Olympics by Ian Thorpe and his size 17 feet, instructed that a prototype be produced of the world's perfect swimmer. That prototype is Michael Phelps and today, thrillingly, his Olympic launch is upon us.

To outdo the Thorpedo, Aquaman would have to be so gangly as to be uncoordinated on dry land. He would have to be hyperflexible to a rare degree, and yearn forever for the water, preferably driven there by some kind of family trauma.

He would need to be hyperactive, to help him cope with incessant training demands, and would need freakish powers of recovery setting him apart from his closest rivals. Oh, yes, and go easy on the charisma, or the demands for a slice of him would become all-consuming.

Then top it all up with the obligatory helping of all-American goo. Hype up the threat to Mark Spitz's hallowed record of seven gold medals at a single Olympics, set in Munich in 1972, and have Spitz appear at the US trials to present him with a medal - and send the crowd wild by holding his arm high to the heavens in recognition that a star is about to be born.

Phelps versus Thorpe in the 200 metres freestyle final on Monday has been billed in both the States and Australia as swimming's Race of the Century. Presumably that goes for the last century, too. But the challenge goes deeper than that. Even when they are not in direct competition, Phelps and Thorpe will vie for recognition as the star of the pool.

Their personalities, their lives, are intriguingly different. Thorpe arrived in Athens as much like a laconic rock star as a champion swimmer, hair lank, chin stubbled. He has irked the International Olympic Committee by demanding that out-of-competition drug-testing be improved and has warned Phelps, in a kindly, supportive way, that an obsession with Spitz's record may be self-destructive. Two years older, his maturity is that of a man twice his age.

Phelps, only 19, has been fondly described by a fellow swimmer as "just this big goof". His life out of the pool is that of a typical American teenager, a retreat into television, fast food and computer games. His image remains chaste; he still refuses to answer questions about whether he has had a girlfriend - as he refused in Sydney when, as a 15-year-old, he finished fifth in the final of the 200m butterfly.

Thorpe, by contrast, is Sydney's metrosexual male, responding to Australia's obsession with his sexuality since the Sydney games with beguiling ambiguity. In chiding Australia for its presumption, his response has been not as much "glad to be gay" as relaxed about being thought gay, rightly or wrongly. Australia, arguably the most macho sports society in the world, struggles to embrace new attitudes, and Thorpe remains polite, articulate, impossible to define.

If it sometimes seems that Thorpe has been involved in the education of a nation, Phelps could barely educate himself. A working-class boy raised near Baltimore, he was diagnosed at junior school with attention deficit hyperactive disorder. His parents divorced when he was nine and he spent some of his formative years on Ritalin until finding the strength to tell his mother that he could manage without it. Without that expression of self-determination, his Olympic ambitions would have been abandoned.

Phelps's physiology is intriguing. He shuffles forward shyly, his torso not designed for dry land. A recent commercial, where he had to complete a few simple dance steps, had him smiling bashfully at his gawkish attempts.

But, in the water, what majesty unfolds. His hands and feet (size 14) are like paddles, with a staggering extra flexibility of 15 degrees. His is an excessively long torso, so that between his ribs and hips he moves like a bendy bus. His gangling arms are such that his wing span is a remarkable 6ft 7in, three inches more than his body.

His level of lactic acid, which causes muscles to tire, has been measured as remarkably low; with such a heavy swimming programme, quick recovery times are essential. He should prove unsurpassable in the butterfly and individual medleys, beginning with tonight's 400m individual medley final.

By Monday's 200m freestyle we will have an inkling of Phelps' greatness. He swam five world records in six days in the US trials, whereas Thorpe has not set a world record since teaming up with a new coach, Tracey Menzies. Thorpe is desperate to win the 200m freestyle that eluded him in his own pool four years ago when he was beaten by Pieter van den Hoogenband. The Dutchman remains. Thorpedo and the Aquaman are not alone. ...