Launch of the shuttle Columbia on a medical research flight has been postponed until at least Wednesday because of balky navigation gear. Carrying seven astronauts, 30 rats, and some 2,500 baby jellyfish, Columbia had been scheduled to take off Saturday on a flight to study the effects of weightlessness.

But engineers ran into problems with a balky inertial measurement unit, one of three such devices that feed critical navigation data to the shuttle's four primary computers during flight. After three attempts to calibrate the device, launch director Robert Sieck reluctantly canceled the countdown.

Columbia originally was scheduled for launch May 22, but the flight was delayed 10 days after a computer failed, an electronic control device malfunctioned, and tests disclosed cracks in critical fuel-line temperature sensors.

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