'This mission is deeply significant, not only symbolically but in absolute terms,' says John Logsdon, director of the Space Policy Institute at George Washington University. 'The future of the space programme rests upon it.'

Discovery, one of three remaining space shuttles from Nasa's original fleet of five, is scheduled to blast off from Cape Canaveral in Florida on Wednesday.

At the controls will be Commander Collins, 48, who as a teenage pizza waitress saved her tip money for three years then marched to the local airfield to ask how many flying lessons it would buy her. So began an odyssey from humble working-class roots to a career as one of the US Air Force's first woman pilots and, ultimately, status as a national heroine in Nasa's elite astronaut corps.

Born in 1956, her father James was a postman and her mother Rose Marie was a prison secretary.

Home was a housing authority property in Elmira, a town of 30,000 in upstate New York, and the couple - who separated when Eileen was 13, but remained cordial with her father living in a nearby apartment - often relied on welfare handouts to buy food for their four children.

'You could ask anybody that knew me in Elmira, New York, and they would say, 'Never thought she would be an astronaut someday',' the former air force colonel admits. 'I am quite amazed I'm here.'

As a secondary student in a private Catholic school, she cleaned classrooms every day in exchange for her tuition.

In 1972, the family home was flooded after a hurricane and their possessions were destroyed. A federal grant helped them get back on their feet and the children were given three new shirts and three pairs of trousers each.

Money was so tight that her dreams of going to community college seemed doomed. So she flitted between part-time jobs at a pizza parlour, a catalogue showroom, a doughnut shop and a miniature golf course to pay her way. By the end of college, she had enough left for flying lessons and earned her private pilot's licence.

After winning a scholarship to Syracuse University - from where she graduated in 1978 with a bachelor's degree in maths and economics - she tried to join the US Air Force's Reserve Officers' Training Corps, a gateway into the military. Her dream was nearly thwarted, however, when she narrowly failed an eye test. Believing in her potential, however, the corps examiner advised her to eat carrots for two weeks, then come back for a second try, which she passed. 'I ate so many carrots that the tips of my fingers turned orange,' she says.

It was in 1990, while serving as an air force flight instructor, that she was selected for the astronaut programme, setting a milestone by becoming the first woman to pilot a shuttle when she blasted into space aboard Discovery in 1995.

In 1997, she headed to the now-defunct Mir space station aboard Atlantis and, two years later, became the first female shuttle commander aboard Columbia.

On February 1, 2003, seven of her friends perished when Columbia broke up as it re-entered the Earth's atmosphere. It emerged that a chunk of cladding from the fuel tank had detached during liftoff 16 days earlier and knocked a hole in the left wing, allowing super-hot gases to burn into the vehicle and blow it apart.

As America reeled, the shuttle programme was grounded. Shocking revelations ensued as the Columbia Accident Investigation Board pinpointed a culture of 'flawed decision-making, self-deception and introversion' at Nasa, which it said had contributed to the tragedy.

Nasa has reshuffled management and spent US$1.1 billion fixing technical flaws. Some remain sceptical that Discovery is fit to fly, but Commander Collins and her crewmates are satisfied. She has reassured her husband and two young children that she expects it to be the 'safest mission ever'. 'The return-to-flight effort has not been easy, but because of the work we have done, we are stronger, we are smarter and we are more humble - and we're safer,' she says. 'It's time to fly.'