A fresh batch of 2,478 tiny jellyfish was counted, rinsed and ready to load onto space shuttle Columbia Tuesday for NASA's third attempt to launch the biomedical research mission.

Columbia is scheduled to blast off at 8 a.m. EDT Wednesday with the jellyfish as well as 30 white rats and seven astronauts. The first two tries were thwarted by equipment trouble.NASA test director Mike Leinbach said the latest countdown, which began Monday, was going well.

The countdown was halted less than an hour before scheduled liftoff Saturday because of a faulty navigational unit.

Bad fuel temperature sensors and computer components stopped the first countdown on May 21.

Dorothy Spangenberg, a biologist from Eastern Virginia Medical School in Norfolk, Va., who's in charge of the jellyfish, said the delays were frustrating but not surprising.