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US Space Shuttle <i>Discovery</i> Poised to End Long Launch Hiatus

T he first U.S. space shuttle countdown in more than two years is under way. The orbiter Discovery is poised for a Wednesday launch, ready to be the first shuttle to fly since Columbia disintegrated upon its re-entry into the atmosphere. Discovery and the two other remaining shuttles have undergone extensive safety modifications. But officials at the U.S. space agency, NASA, warn that no upgrade can guarantee against another disaster.

When Discovery takes off to supply the International Space Station, it will be with many thousands of enhancements to upgrade its hardware and improve NASA's ability to track and inspect it in flight.

"I believe this is the safest vehicle we've ever flown," said shuttle program manager Bill Parsons. "We know a lot more about this vehicle than [when] we started two years ago after the accident. I believe that we're ready for flight," he said.

NASA's focus has been on preventing pieces of hard insulating foam covering the shuttle's huge external fuel tank from shedding during liftoff and damaging the orbiter. A piece of the falling foam damaged Columbia's wing on takeoff in 2003, allowing extremely hot atmospheric gases to penetrate and destroy the shuttle during the searing heat of re-entry.

The foam is used to prevent the frigid liquid hydrogen and oxygen fuel from causing a dangerous ice buildup on the tank. The insulation has now been removed from the location it had fallen from on Columbia's tank and replaced with heaters. Elsewhere, its application has been improved. Technicians have also reduced the chances that falling ice would threaten the orbiter, too.

Shuttle engineering manager John Muratore says his team has exhaustively simulated falling debris millions of times on supercomputers in an effort to ensure they have thought of every situation where it might occur.

"So we have a good feeling we've encompassed the range of conditions that we could potentially see against in flight. We've basically been keeping all of the supercomputers in NASA busy working this problem for the last year-and-a-half," he said.

But deputy shuttle program manager Wayne Hale says that despite the effort, NASA cannot entirely eliminate the chance a foam pellet would cause havoc.

We have done a huge amount of work to reduce the debris environment; we believe that we have taken significant risk reduction, but it's my understanding that we're still going to fly with some risk. To characterize it otherwise I think would be inappropriate," he said.

As a result, NASA has increased the number of cameras that observe shuttles at every stage of ascent. More than 100 of them will document debris shedding from positions around the launch site, on a ship at sea, on observation aircraft, and on the shuttle and the external fuel tank itself. Radar will be used for night launches. In addition, a new camera at the end of a mechanical arm can inspect the surface in orbit.

The shuttle's wings also have new sensors to measure debris impacts and any heat increase.

NASA has still not finished developing ways that spacewalking astronauts could repair the shuttle's surface with caulking or plugs. So if serious damage occurs in flight, the astronauts would be forced to move into the space station for up to 45 days while ground technicians prepare a second shuttle to rescue them. That would be a solution of last resort.

Deputy space station program manager Bill Gerstenmeier says room aboard the station would be tight and facilities strained.

"It's something that you don't want to have to execute, but in a survival sense, it's executable. It's a reasonable plan that's available if it's absolutely needed. Again, I think the conditions won't be good on the station, but it's better than the alternatives and overall I think we're prepared to go execute if we need to execute," he said.

The resumption of shuttle flights will eventually allow the size of International Space Station crew to expand to three again. NASA first wants to see how the flight of Discovery and the subsequent shuttle mission go. The third station position was eliminated during the shuttle flight moratorium because Russian supply rockets do not have enough volume for cargo needed to supply three crew members.