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.
But through much of the mission, the center of attention is likely to rest on the shuttle and its external fuel tank. STS-121, as the mission is formally known, is the second of two test flights following the Columbia disaster in 2003. The aim is to put in-flight tile inspection and repair techniques through their paces and to flight-test changes made to the bullet-shaped external tank. The changes are designed to reduce the size and amount of foam insulation that shakes free during launch. The loss of Columbia and its seven-member crew during reentry was traced to foam debris that shook loose during the orbiter's climb after launch. The debris damaged tiles that are designed to protect the shuttle from the heat generated during its descent through the atmosphere.
Mission managers acknowledge they haven't solved the debris problem yet.
"We are still students at flying polyurethane foam at supersonic speeds," Mr. Hale acknowledges.
Workers have removed long foam "ramps" covering cable trays and fuel feed lines that run along the length of the tank. The shift represents the most significant change to the shuttle system's aerodynamics since the first shuttles launched, adds John Chapman, the external-tank project director.
But NASA isn't finished with foam, a touchy point with two senior engineers during a two-day flight readiness review meeting, who advised against proceeding with this mission. At issue are a series of smaller foam-covered ramps, dubbed ice/frost ramps, on various parts of the tank. The foam insulation displays internal cracks.
NASA officials say they fully expect to see debris break free from some of these, but they add that based on past experience, debris from these areas shouldn't affect the flight. And if they do, they would only affect reentry.
Given that the crew has a haven on the space station, that Atlantis could be launched on a rescue mission if needed, and NASA could draw on Russia to help, Griffin's decision to proceed with the mission was a trade-off between the risks: a little extra risk to the program now in launching while the foam problem is still being worked out versus avoiding flight until they are solved. In that case, the remaining flight schedule would be more compressed, raising its own set of risks.
"I don't want to get us into a situation where, by being more cautious than I think technically necessary today, we wind up having to execute six flights in the last two years" of the program. "That's not smart," he said during a briefing after the flight-readiness review.
In the end, given the budgetary demands, the international commitments the US still faces for finishing the space station, and the opportunities to safeguard the crew it's the right decision, notes Ray Williamson, a research professor at George Washington University's Space Policy Institute in Washington.
"These are programs that still carry a lot of risk," he says. "If we are going to continue to fly astronauts into space, we somehow as a country have got to accept the risk."