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With the Rotating Service Structure (RSS) rolled back, at left, the Space Shuttle Columbia is nearly ready for launch of STS-90. Rollback of the RSS is a major preflight milestone, typically occurring during the T-11-hour hold on L-1 (the day before launch). The scheduled launch of Columbia on Apr. 16 from Launch Pad 39B was postponed 24 hours due to difficulty with network signal processor No. 2 on the orbiter. This device formats data and voice communications between the ground and the Space Shuttle. The unit, which is located in the orbiter's mid-deck, will be removed and replaced. Prior to launch, one of the final steps will be to load the external tank with approximately 500,000 gallons of liquid hydrogen and liquid oxygen for fueling the orbiters three main engines. Tanking had not yet begun when the launch scheduled for Apr. 16 was scrubbed. STS-90 is slated to be the launch of Neurolab, a nearly 17-day mission to examine the effects of spaceflight on the brain, spinal cord, peripheral nerves and sensory organs in the human body.