Washington ? NASA takes a major step toward returning astronauts to space when engineers this week ship an improved rocket fuel tank that has been refitted to avoid the falling debris that caused the destruction of Columbia and the death of seven astronauts.

National Aeronautics and Space Administration officials said that the redesigned fuel tank, a massive vessel that supplies propellant for the launch of the space shuttle, would start a barge trip Friday from a Mississippi assembly plant to the launch site on Florida’s east coast.

Sandy Coleman, NASA’s external tank project manager, said improvements made on the fuel tank “(give) 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, still is being calculated, but it will be more expensive than the $40 million cost of the old-style tank, Coleman said.

NASA plans a May or June launch of space shuttle Discovery.