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  Sandy Coleman, NASA's external tank project manager, said improvements made on the fuel tank ``gives us confidence that problems like what happened on Columbia will not happen again.

``This is the safest, most reliable tank NASA has ever produced,'' Coleman said Tuesday in a telephone news conference from the Marshall Space Flight Center in Huntsville, Ala.

The changes in the external tank add less than 150 pounds in weight. The total cost of the new tank, including tests and redesign, is still being calculated, but it will be more expensive than the $40 million cost of the old-style tank, said Coleman.

Coleman said the tank was expected to start on Friday a barge trip from Michoud Assembly Facility near New Orleans to Kennedy Space Center. The barge journey, which crosses the Gulf of Mexico, rounds the tip of Florida and then up the east coast, takes five to six days.

NASA plans a launch in May or June of space shuttle Discovery. The space shuttle fleet has been grounded since the Columbia accident as NASA scrambled to make changes in hardware, procedures and personnel to comply with recommendations from the Columbia Accident Investigation Board.

Fixing the external tank was a key part of the NASA's recovery, officials said.

The tank holds the liquid hydrogen and oxygen which are the propellants for the shuttle's main rocket engines during launch. The supercooled chemicals cause the formation of ice on the outside of the tank as the shuttle is prepared for launch.

Insulation, applied as a foam, reduces the amount of ice. But investigators believe it was chunks of foam insulation that peeled off the external tank during launch which led to the destruction of Columbia. The debris, moving at a high relative speed, ripped a hole in the left wing of the space shuttle. On Feb. 1, 2003, as the spacecraft re-entered Earth's atmosphere, superheated gas penetrated the wing through the hole and melted metal struts. The craft shattered, showering east Texas with flaming debris. Seven astronauts were killed.

The Columbia Accident Investigation panel conducted tests to prove that chunks of the light weight foam insulation could cause the damage that was fatal to Columbia. The tests included using an air canon to fire foam insulation chunks at test wing panels.

To correct the problem, engineers from NASA and Lockheed Martin Space Systems, manufacturer of the fuel tanks, conducted extensive tests to find out why the foam insulation broke loose during launch.

This led to several changes including new ways of applying the foam insulation, the addition of heaters at key points to prevent the formation of ice before launch, and adding cameras that can monitor the outside of the tank during launch.

``We can never completely eliminate foam coming off the tank,'' Coleman said Tuesday. But she said tests suggested that any debris that did fly free would not cause damage like that which destroyed Columbia.

Redesign of the external tank was considered to be a key and critical part of NASA's effort to return the shuttle to space, but is only one of a long list of recommendations from the Columbia Accident Investigation Board. NASA also is designing ways to check for launch damage to the space shuttle after the vehicle is in orbit. The agency is also developing ways for spacewalking astronauts to fix damage to wing panels like that which destroyed Columbia.

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