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SIOUX FALLS, S.D. (AP) -- The telephone at the air traffic control tower at Joe Foss Field was ringing off the wall early Saturday with callers reporting a bright silver light in the predawn sky. There were so many calls that after a while, tower personnel were answering their telephone with this greeting: "UFO Watch Center." But it wasn't a UFO. It was a 340-foot-tall high-altitude balloon on a scientific experiment. The balloon, launched Friday evening from Ainsworth, Neb., was part of an experiment by the University of New Mexico and NASA to gather anti-matter particles and investigate how stars evolved and how they are changing, according to Bob Golden, an electrical engineering professor at the Albuquerque, N.M., school There hasn't been much research into anti-matter, so not much is known about the tiny particles, he said in an interview. The unmanned balloon was 340 feet in diameter when inflated and 550 feet long when stretched out on the ground uninflated, according to a worker at Raven Industries in Sioux Falls. The balloon was made in Sioux Falls, the unidentified worker said. The gondola was 15 feet high, 5 feet wide and had a 4,300-pound payload, including a computer, according to Golden. A 12-member NASA crew launched the craft. The ground team included researchers from the Goddard Space Center in Maryland. The balloon was expected to land Saturday, but scientists weren't sure where it would come to rest. People in Sheldon and Sioux City, Iowa, reported seeing the balloon, as did people in parts of Nebraska, authorities said. Golden said it will take from three months to 12 months to analyze the data gathered during the balloon flight and that the results will be published in international scientific journals. The balloon was so bright in the early morning sky because it was high enough to reflect sunlight before the sun rose, he said. The balloon, which also was visible in southwestern Minnesota, was designed to rise to 120,000 feet, according to a statement from the National Weather Service's Sioux Falls office. The balloon carried sensitive instruments to measure cosmic ray particles, the NWS said.