Members of the space shuttle Discovery crew leave the Operations and Checkout building during a count down dress rehearsal for flight STS-121 at the Kennedy Space Center in Cape Canaveral, Flarida on June 15, 2006

The U.S. space shuttle Discovery is set to launch to the International Space Station July 1, a decision that has caused controversy. The space agency, NASA, is sending the ship up for the first time in nearly one year, despite objections from two top agency officials that engineers have not done everything possible to ensure the orbiter's safety after the loss of the shuttle Columbia three years ago. 

After exhaustive modifications to the shuttle's external fuel tank, NASA wants to see how the unit flies. The work redesigned several areas of the hard insulating foam that covers the tank. The goal was to ensure that a chunk large enough to destroy a shuttle never breaks away during launch again, as it did with Columbia in 2003.

"We have made the largest aerodynamic change to this vehicle that has been made since we started flying 25 years ago," said Wayne Hale, the shuttle program manager.

Technicians stripped 16 kilograms of foam from the shuttle's fuel tank from a total of more than 1,800 kilograms. Hale says they will try to reduce foam shedding even more for later flights, but insists that the newest change must be flight tested first.

"When you have made a major aerodynamic change, and you have studied it in the wind tunnel, and you have studied it with the computer simulations and you think it's good to go fly with, you go fly with that one change, because all those wind tunnels and all that analysis and all those computer models are never quite as good as real life," continued Hale.

But NASA's chief safety officer and chief engineer disagreed with the decision in a shuttle management meeting. They argued against launching Discovery, because they believe foam debris is still a threat.

Hale admits that all shedding cannot be eliminated under the enormous stresses of liftoff. But agency technical analysis shows that debris heavier than 113 grams is unlikely. That is less than one-seventh the weight of the piece that punched a big hole in Columbia's wing during liftoff.

The two dissenting shuttle officials say they accept NASA's decision to launch because expected foam loss would not endanger the crew, only the orbiter. They note that since the Columbia accident, NASA has devised procedures for astronauts in orbit to inspect and repair launch damage. If there is an unfixable problem that would cause a dangerous re-entry through the atmosphere, the crew can remain aboard the space station, until a rescue shuttle arrives.

During the coming mission, astronaut Piers Sellers will be one of two crewmen to

Astronaut Piers Sellers

practice inspection and repair techniques during a spacewalk. He says Discovery is ready to fly.

"We have now come to the point, I think, that the major villains out there in terms of threat have been dealt with, or at least thoroughly addressed, and we have driven those risks down as far as we can, which leaves the things you don't [one doesn't] know about," noted Sellers.

Wayne Hale frequently reminds listeners that shuttles have about two million parts, any one of which can break down. The shuttle is extremely complicated because it was designed for many types of missions, according to a former U.S. intelligence official, who was a member of the panel that investigated the shuttle Challenger explosion in 1986. Albert Wheelon says shuttles were built for human space flight, to support scientific research and to deploy satellites for U.S. military, intelligence and commercial clients. He points out that each use had specific design requirements.

"Following the Challenger accident, all but the manned space program essentially abandoned the shuttle as a launch vehicle," he explained. "It was left to serve only the space station and the shuttle itself. But the requirements of all the users remained in the design, and it had made it a great deal more complicated than it need to have been, and I think that just adds to the burden of the NASA team trying to make each mission perfect."

On the coming mission, in addition to the spacewalk, the shuttle team will deliver German astronaut Thomas Reiter to expand the station crew to three for the first time since the Columbia accident. It will also bring up supplies and replacement parts to keep the outpost operating.