ONCE again, the space shuttle Discovery is due to blast into space and dock with the International Space Station.
Astronauts are planning to continue the process of building the half-completed orbiting laboratory in a mission full of daunting challenges.
But the majesty of the first night-time liftoff in more than four years, rescheduled a number of times, t
he last being the early hours of today, will not dispel a question that is increasingly being asked in the United States: what is the space station for?
In 1998, when its first components were launched as a replacement for the Mir, a worn-out Soviet-era relic, the station was billed as a manned science lab of nearly unlimited potential, with promises of advances in areas such as pharmaceuticals thanks to the ultra pure crystals that could be grown in a microgravity environment.
It was to be finished by 2004, and it was to cost about $40bn, shared by 16 nations, including the US, Canada, Russia and the European Union.
Those goals are barely recognisable now. As the Columbia catastrophe forced a two-and-a-half-year delay in construction and cost overruns and changing presidential administrations forced Nasa to rethink its entire science mission, the station's price tag has ballooned to $100bn and the completion date has moved to 2010.
And questions about the station's scientific value have grown sharper than ever. David Goldston, the departing chief of staff for Congress's Science Committee, said that Nasa now seemed more motivated by the need to satisfy its commitments to international partners than by any compelling scientific objectives. "I've never heard anyone say, 'We have to do this because it's important for the future of the US space programme or science'," he said.
Nasa is now focused heavily on building a new generation of space vehicles for exploring the Moon and Mars; last week it announced plans to establish an international base camp on the Moon by 2024.
But officials insist the station is essential for researching the potential effects of prolonged weightlessness on astronauts: a round trip to Mars would take at least two years.
However, the agency has also sharply cut back plans for scientific experiments.
Nasa also faces a tight deadline: completing the station by 2010, when the agency is planning to retire the shuttle fleet. The next generation of vehicles will not be ready before 2014, leaving the station to rely on Russian and European space programmes, and potentially on entrepreneurs partly financed by Nasa.
Nasa's administrator, Michael Griffin, has said
that the need to complete the station and to develop a new space fleet has meant
science work in orbit had to be narrowed, with a tight focus on conducting scientific work that can help Nasa keep astronauts healthy on long missions to the Moon and Mars.
Experts say that in zero gravity, astronauts suffer from osteoporosis at a rate 10 times that of postmenopausal women. On a trip to Mars, 40% of them would lose more than half of the bone mineral in their hips, according to James Pawelczyk, an assistant professor of physiology at Pennsylvania State University and a former astronaut. The returning astronauts would have hips as delicate as eggshells.
Muscle mass also declines, and efforts to prevent the process have not been very successful, said Julie Robinson, the acting manager of science projects for the station programme.
Outside experts have had reservations about the shift. The National Research Council said Nasa lacked a strong plan for scientific research or for use of the space station "in support of the exploration missions".
The increasing emphasis on the next step has left many experts worried that the laboratory is being treated like an albatross, to be cast aside as quickly as possible. "Low-earth orbit" - the station orbits 220 miles above the Earth - "should not be abandoned in favour of going to the Moon and Mars," said Jeff Bingham, the staff director for the Senate Commerce subcommittee on science and space. "We're sort of saying, not too fast."
Bingham says the station is half finished and has half the six-member crew that the fully completed station is designed to support. "It's not a space station that is or should be expected to be producing anything of any significance by now," he said.
But some scientists say the new focus on the Moon and Mars has done great damage to their field.
"Since 1990, Nasa has spent literally billions of dollars building up a world-class microgravity programme that has been basically squandered," said Peter Voorhees, a professor of engineering at Northwestern University. "There's a perfect example of snatching defeat from the jaws of victory."
