Tuesday's launch will be in darkness, as in May 1999

Eileen Collins, a 42-year-old Air Force colonel, will be responsible for four other astronauts, three of them older men. In 1995, Colonel Collins was the first woman to pilot a space shuttle.

She will also be responsible for one of the most expensive shuttle payloads ever - Nasa's $1.5bn Chandra X-ray Observatory, which is on a scientific par with the Hubble Space Telescope. The total price tag for the entire Chandra project, from development through five years of orbital operation will be $2.8bn.

The 25-tonne telescope is also the heaviest cargo ever lifted by the space shuttle.

Colonel Collins admits being the first woman commander does add extra pressure but says: "The extra pressure doesn't bother me - I find that I can still do my job effectively."

She does not see it as a giant leap for womankind: "It's an honour for me to be chosen, but I also see it as an evolutionary process. Eventually, having women in these roles won't be news anymore. It will be expected," she says.

"Inside of Nasa, I'm very accepted and I'm treated very much like the other astronaut Commanders. I really don't notice a difference."

Nasa's head, Daniel Goldin, obviously agrees: "Eileen Collins is the commander. So what's the news?" he said.

Astronaut Catherine Coleman tests her spacesuit for size

Outside Nasa, however, there has been great interest in her role, particularly in how she combines being an astronaut and a mother to her three-year-old daughter, Bridget. Colonel Collins says both jobs are equally difficult, yet equally rewarding.

Her fellow crew member on Tuesday's mission, Air Force Lieutenant Colonel Catherine Coleman, says the advent of the first woman commander is significant.

Columbia sits on the launchpad

"There are a lot of young girls who need to see Eileen in the commander's seat achieving her dreams, because they can realise that they can achieve their dreams if they work for them."

Colonel Collins, a former test pilot, began her flying career by taking oddjobs to pay for the flying lessons her family could not afford. She has now logged more than 5,000 hours in 30 types of aircraft and 419 hours in space.

The primary objective of the five-day mission is to deploy the Chandra X-ray Observatory, but there are many more experiments and tests to carry out including:

"Fly cast manoeuvre": This September's shuttle mission will have a 60 metre radar boom deployed when in orbit. Periodic boosting of the engines is required to stop the shuttle falling to Earth but this jolting could damage the boom. So this mission will test whether pulsing the engines with millisecond timing avoids any damage.

Treadmill test: The mission will include the last treadmill test before one is deployed on the International Space Station. Treadmills are vital for astronauts to keep healthy through exercise, but the pounding of feet can send vibrations through a spacecraft. This can disrupt experiments and so ways to isolate the vibrations are being sought.