STS-55 (Space Transportation System 55), was the 55th overall flight of the US Space Shuttle and the 14th flight of Shuttle Columbia. This flight was a multinational Spacelab flight involving 88 experiments from eleven different nations. The experiments ranged from biology sciences to simple earth observations.
The mission surpassed the 365th day in space for the Space Shuttle fleet and the 100th day of flight time in space for Columbia, the fleet's oldest Orbiter, on its fourteenth flight.

Columbia was initially scheduled to launch in late February. However, this date slipped to early March due to concerns with the tip-seal retainers in the main engines' oxidizer turbopumps. All three turbopumps were replaced at the pad but later inspection revealed the retainers to be in good condition.
Further delays were caused by the burst of a hydraulic flex hose in the aft compartment during the Flight Readiness Test. The lines were removed and inspected and three replacements were installed.
The launch attempt on 22 March was aborted automatically at T-3 seconds when computers detected an incomplete ignition of the number three Space Shuttle Main Engine. The problem was traced to a leak in the liquid oxygen preburner check valve. All three SSMEs were replaced as a precaution.
Another launch attempt on 24 April was scrubbed due to a possible faulty reading with one of the inertial measurement units.[1]
The final launch attempt was successful with liftoff at 10:50 am EDT on 26 April 1993.

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