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The Alpha Magnetic Spectrometer (AMS) experiment and four Get Away Special (GAS) payload canisters are secure in Discovery's payload bay shortly before the payload bay doors are closed for the flight of STS-91 at Launch Pad 39A. Launch is planned for June 2 with a window opening around 6:10 p.m. EDT. The AMS experiment is the first of a new generation of space-based experiments which will use particles, instead of light, to study the Universe and will search for both antimatter and "dark matter," as well as measure normal matter cosmic and gamma rays. The GAS Program, initiated to provide extremely low-cost access to space, is managed by the Shuttle Small Payloads Project at NASA's Goddard Space Flight Center. Eight GAS experiments will be conducted on STS-91. The mission will also feature the ninth Shuttle docking with the Russian Space Station Mir, the first Mir docking for Discovery, the conclusion of Phase I of the joint U.S.-Russian International Space Station Program, and the first flight of the new Space Shuttle super lightweight external tank. The STS-91 flight crew includes Commander Charles Precourt; Pilot Dominic Gorie; and Mission Specialists Wendy B. Lawrence; Franklin Chang-Diaz, Ph.D.; Janet Kavandi, Ph.D.; and Valery Ryumin, with the Russian Space Agency. Andrew Thomas, Ph.D., will be returning to Earth with the crew after living more than four months aboard Mir