Countdown clocks began Saturday for the shuttle Columbia's blastoff Wednesday on a medical research flight featuring seven astronauts, 30 rats, and 2,700 jellyfish. Columbia's four-man, three-woman crew was scheduled to take off at 8 a.m. EDT Wednesday to kick off a planned nine-day Spacelab mission to find out how weightlessness affects human physiology.

Scientists hope to gain insights into how weightlessness affects the human ear by studying how it affects similar gravity receptors used by the jellyfish. The rats will undergo tests upon return to Earth.

Columbia's mission marks the first shuttle flight dedicated to life-science research.

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