On eve of launch, 'make or break' flight for shuttle

The countdown to Saturday afternoon's scheduled launch of the space shuttle Discovery also triggers the countdown to the end of an era in US manned spaceflight.

The orbiters are due to be retired in 2010. Whether any of the remaining spacecraft will actually see four more years of service, however, is subject to a calculated risk by the National Aeronautics and Space Administration. Despite the continued risks of damage to the shuttles from falling foam, the agency is not delaying launches while it works on the problem. But the loss of a single shuttle would end the 25-year program, NASA's chief has said.

That pronouncement is unprecedented, analysts say.

"Now you've got make-or-break missions on your hands," says Roger Launius, a former NASA historian who chairs the division of space history at the Smithsonian Institution's Air and Space Museum in Washington.

"There's no mission I'm aware of where anyone in a position of authority has said: 'If we fail here, that's the end of it,' and said so publicly. That puts a fair amount of pressure on the teams working the missions," he says.

They must try to keep the orbiters operating amid continuing concerns that bits of foam insulation the external fuel tank sheds during launch could cost the fleet another ship and, perhaps, a crew. Such concerns prompted two of the agency's top engineers to recommend a "no-go" for this flight during two days of intense flight-readiness discussions earlier this month. And the craft continue to face a range of "aging" issues, from corrosion to dealing with old electronic gear. Indeed, the agency reportedly plans to retire Atlantis in 2008 to avoid a costly overhaul.

"These vehicles are getting a little long in the tooth," says Wayne Hale, the shuttle program manager at NASA.

For its part, Discovery's seven-member international crew has two broad assignments that will involve two or three spacewalks, depending on available time.

One task involves resupplying the space station. Discovery is launching with "Leonardo," an Italian-built cargo module carrying more than 2-1/2 tons of equipment, including a new, more powerful oxygen generator, as well as food and clothing. The new oxygen generator will allow the station to host six crew members when construction is complete. Indeed, this mission will see the station crew expand from two to three members, with the addition of European Space Agency astronaut Thomas Reiter. Following the Columbia accident in 2003 and the grounding of the remaining shuttles, mission managers intentionally short-staffed the station because shuttles no longer were available to resupply the facility. And one of the spacewalks aims to replace some damaged equipment on the orbiting lab's exterior.

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