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Under the supervision of Boeing technicians, the Alpha Magnetic Spectrometer (AMS), a payload slated to fly on STS-91, is undergoing a final weight and balance check on the Launch Package Integration Stand in the Space Station Processing Facility (SSPF). Next, it will be placed in the Payload Canister and transported to Launch Complex 39A where it will be installed into Space Shuttle Discovery's payload bay. Weighing in at approximately three tons, the AMS is a major particle physics experiment that will look for cosmic antimatter originating from outside our galaxy. The data it gathers could also give clues about the mysterious "dark matter" that may make up 90 percent or more of the universe. STS-91 is scheduled to be launched on June 2 with a launch window opening around 6:10 p.m. EDT. The 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. 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