If you believe the Pentagon and Nasa, Janis Sharp is the mother of an evil genius. Her son is Gary McKinnon, the Briton accused of the biggest hacking operation in US military history, and whose $1 billion wrecking spree through cyberspace – he says it was a quest to uncover evidence of alien life forms – has left his family in their own twilight zone.

Next week Mr McKinnon, 42, will have one last, desperate throw of the dice in the British courts to halt his extradition to America, where he faces up to 70 years in a high-security prison.

Having lost every appeal since his arrest six years ago, he has all but run out of lifelines. His mother is also running out of time to convince the authorities that far from trying to terrorise the US, the biggest computer hack of recent times was the result of her son’s Asperger’s syndrome.

“It doesn’t excuse it, because if you commit a crime, you commit a crime,” she toldThe Times, in her first major interview. “I would say to Gordon Brown and David Cameron, you both have vulnerable young sons. Should we really be extraditing our vulnerable adults and letting them serve 70 years abroad? Seventy years for looking for UFOs?”

Mrs Sharp argues that Asperger’s – a form of autism that causes obsessive or repetitive behaviour and impairs social skills, renders her son dangerously unready for life in an American penitentiary – Also, for all his snooping around cyberspace in search of extraterrestial life, McKinnon has never even left the UK “He won’t survive. He’s absolutely petrified. We’ve already received taunting messages about male rape, stun guns,” she says. “We worry that we won’t be alive to see him free.”

Mr McKinnon admits tapping into the highly sensitive networks, but denies any malicious intent. He also admits leaving messages such as, “Your security is crap”. He was caught in 2002 when investigators traced software that he had used to his girlfriend’s e-mail account. He was trying to download Nasa photographs that he believed had been airbrushed to conceal evidence of alien life. He admits that he was a regular cannabis user at the time.

However, US prosecutors argue that the hacking, which “significantly disrupted governmental function” after the September 11 terrorist attacks until February 2002, should not simply be excused as the harmless actions of a conspiracy theorist lost in cyberspace. Consider the targets: the US Department of Defence; the Johnson Space Centre; the Air Force Office of Special Investigations; the US Army; even the Earle Naval Weapons Station, which supplies munitions for the Atlantic fleet.

His virtual fingerprints were found across the entire matrix of American military might. That they were left there by a stoned loner in his dressing gown in a North London bedroom is not the point, say US prosecutors. The havoc wreaked by the hacker known as Solo presented an “intentional and calculated [attempt] to influence and affect the US Government by intimidation and coercion”.

Mrs Sharp responds: “He’s not a genius. He good, but he’s not the best. They had no passwords, no firewalls, and that’s the problem. Gary embarrassed them. They wanted to make an example of somebody for computer crime, so they thought Gary was a soft touch. I think governments try to frighten people. They like to create villains and threats to justify some of their actions.”

Mr McKinnon was born in Glasgow, but moved to London aged 6, after his parents split up. He remains close to his father, Charlie, a scaffolder, and stepfather, Wilson Sharp, his mother’s husband of more than 30 years, and the man who bought him his first computer – an Atari 400 console.

He was an introverted and sensitive child, who, though not exceptionally academic, taught himself to play piano by the age of 7. “One day, we came in to find him playing Moonlight Sonata,” Mrs Sharp, 60, recalls. A fear of travelling also developed at an early age. “As a toddler, he would scream every time he had to go on a bus,” she says.

According to his mother, his social skills short-circuited at a very early age. “Gary was always unusual,” admits Mrs Sharp, a Glaswegian film-maker who now lives in Enfield, North London. “He got a lot of teasing at school because people thought of him as different. He didn’t have many friends.”

She recalls a party that his girlfriend held at their home. The unemployed systems analyst and former hairdresser couldn’t understand why it was impolite to sit in the middle of the floor, tapping away on his laptop, oblivious to the guests around him.

They were all classic signs of Asperger’s. But the condition was not diagnosed until last year, when an interview on ITV prompted a medical expert to get in touch.

He has since undergone sessions with Simon Baron-Cohen, Professor of Developmental Psychopathology at the University of Cambridge and a leading international expert in autism and Asperger’s. Mrs Sharp said: “He always had obsessions. When he was a toddler, he used to talk about stars and galaxies and planets. Then he went through a stage of obsessing about people dying.” But it would be his lifelong obsession with UFOs that would lead Mr McKinnon to hack into 97 top- level Nasa computers, steal computer passwords, delete files and allegedly shut down the entire military network protecting Washington.

He got his interest in extra-terrestrial activity from his stepfather (Mr Sharp hails from a part of central Scotland that science-fiction fans like to call the UFO capital of the world, thanks to the high concentration of strange sightings across the night skies of Bonnybridge).

By 10, Mr McKinnon had joined the British UFO Research Association. Soon he was teaching himself programming language on his new Atari. After hearing his stepfather’s account of a dream involving “huge space-ships”, he was hooked.

Mrs Sharp said: “Gary always had an ‘interest’. But the problems started when he began looking for proof. He felt he was on some kind of moral crusade, uncovering evidence that should not be suppressed.”

What angers Mrs Sharp most are the controversial terms of the extradition treaty between Britain and the US. American prosecutors do not need to show evidence to secure an extradition. Britain, however, cannot force America to hand over its citizens. “Why our Government ever signed such a thing is beyond me. We should have equal rights with America. Why on earth would our Government sign our rights away like that?” she asked. “Gary is not trying to get off with it. He simply wants the right to be tried in his own country.”

Yesterday he moved one step closer to realising that hope, after offering to plead guilty to an offence under the Computer Misuse Act. His family are now appealing to the Director of Public Prosecutions to accept the plea, and allow Mr McKinnon to face proceedings in the UK.

Since losing his appeal at the House of Lords last year, Mr McKinnon has been living a half-life with his girlfriend. Forbidden from accessing a computer, “the gentle musician”, as his mother describes him, spends his time writing “moody” songs. “He is extremely depressed,” she said. “He bitterly regrets ruining his life.”

By next week the family of Britain’s most notorious hacker will find out whether their legal options are finally exhausted. “Now we’re at the end of the road,” she concedes. “But we’ve been there many times before.”

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