(05/19/1998) --- Preliminary reports indicate the Space Shuttle's first super lightweight external tank (SLWT) is in excellent condition following the completion of a tanking test yesterday during a simulated launch countdown at Launch Pad 39A. The pad's Rotating Service Structure will be closed around Discovery later today as preparations for the STS-91 launch on June 2 continue. The primary objectives of the test were to evaluate the strut loads between the tank and the solid rocket boosters and to verify the integrity of the new components of the tank. The SLWT is 7,500 pounds lighter than its predecessors and was developed to increase the Shuttle payload capacity on International Space Station assembly flights. Major changes to the lighter tank include the use of new materials and a revised internal design. The new liquid oxygen and liquid hydrogen tanks are constructed of aluminum lithium a lighter, stronger material than the metal alloy currently used. The redesigned walls of the liquid hydrogen tank were machined to provide additional strength and stability, as well. The STS-91 mission will also feature the ninth Shuttle docking with the Russian Space Station Mir, the first Mir docking for Discovery, and the conclusion of Phase I of the joint U.S.-Russian International Space Station Program