Gary McKinnon could face up to 70 years in a US prison
A British computer hacker who targeted Nasa should be tried in the UK not the US because his mental state is so fragile, the High Court has heard.
Lawyers for Gary McKinnon say there is "clear, uncontradicted expert evidence" that the stress of extradition could result in psychosis and suicide.
He has Asperger's syndrome and claims he was looking for details of UFOs.
Glasgow-born Mr McKinnon, 42, from Wood Green, north London, faces up to 70 years in prison if convicted in the US.
Mr McKinnon, who was arrested by British police in 2002, has already appealed unsuccessfully to the House of Lords and the European Court of Human Rights to avoid extradition.
He is now seeking judicial review of the then home secretary Jacqui Smith's decision last October to order his extradition.
Lord Justice Stanley Burnton and Mr Justice Wilkie, at the High Court in London, will be asked to rule on whether the health risk is too great to allow his removal.
Mr McKinnon hacked into 97 government computers belonging to organisations including the US Navy and Nasa during 2001 and 2002.
The US government says this caused damage costing $800,000 (£500,000) at a time of heightened security in the wake of the 11 September 2001 attacks.
Mr McKinnon told the BBC that he recognises that he committed a crime and said: "I am sorry for that."
But he disputes the amount of damage the US alleges he caused to its computer systems and says he did not employ any complicated techniques.
He said: "I'm not, you know, a master hacker. I didn't write my own programmes or anything. I used commercially off-the-shelf available software."
Mr McKinnon's lawyer, Edward Fitzgerald QC, told the High Court on Tuesday that the home secretary had "underestimated the gravity" of the threat to his client's mental health.
He said Mr McKinnon was "an eccentric person who has passionate views about UFOs" - not a malicious hacker - and extradition was "unnecessary, avoidable and disproportionate".
Mr McKinnon earlier told the BBC that the last seven years since his arrest had taken a considerable toll on him, both personally and financially, as he is unable to work in IT.
But he said the worst impact has been psychological.
"You're in bits every day. I mean, I wake up feeling like someone's stamping on my chest every morning."
His solicitor Karen Todner said being sent to the US would be very difficult for Mr McKinnon.
She said: "One of the problems with Asperger's is that you need to have your family and support network around you and Gary would be completely denied that."
She added: "He's absolutely terrified about going to America."
Mr McKinnon's legal case is being supported by the National Autistic Society (NAS).
The NAS said: "Asperger syndrome is part of the autism spectrum.
"It is not uncommon for people with Asperger syndrome to develop single-minded, obsessional interests, and to be unaware of the effect their actions have on others."
In February the Crown Prosecution Service (CPS) turned down a request from his lawyers for Mr McKinnon to be prosecuted in the UK on lesser charges.
Speaking at the time, the head of the CPS organised crime division, Alison Saunders, said the best place for the case to be heard was the US.
She said: "The evidence we have does not come near to reflecting the criminality that is alleged by the American authorities.
"These were not random experiments in computer hacking, but a deliberate effort to breach US defence systems at a critical time which caused well-documented damage."