Endeavour Astronauts Board Space Shuttle Before Daybreak

Commander Mark Kelly leads his crew as they board Endeavour. Photo Credit/NASA TV

Shuttle Endeavour's six-member crew began boarding their spacecraft at the Kennedy Space Center an hour before daybreak on Monday (May 16), with a smooth countdown under way.

Favorable weather is forecast for the ship's scheduled lift off for the International Space Station at 8:56 a.m., EDT. Endeavour's payloads include the $2 billion Alpha Magnetic Spectrometer and an external platform with spare parts for the orbiting science laboratory.

More than 500,000 spectators, about 45,000 of them NASA invited guests, are gathering on Central Florida's Atlantic Coast for the launching. The crowd was predicted to eclipse the crowds that witnessed Discovery's final lift off on Feb. 24.

Endeavour's external fuel tank was successfully loaded with liquid hydrogen and oxygen propellants overnight. There was no sign of the hydraulic-system electrical problem that prompted a launch scrub on April 29. Commander Mark Kelly, pilot Greg H. Johnson and mission specialists Mike Fincke, Drew Feustel, Greg Chamitoff and European Space Agency astronaut Roberto Vittori began to crawl aboard just before 6 a.m., EDT.

Forecasters continued to point to a 70 percent chance of favorable weather conditions.  Their only reservations were the slight chance of high cross winds at the shuttle's emergency runway and low clouds.

Efforts to launch the 19-year-old orbiter's 25th and final flight on April 29 were scrubbed by the failure of a backup heater on the fuel line for Auxiliary Power Unit No. 1, one of three turbines that generate hydraulic pressure to gimbal rocket engine nozzles and move aero surfaces during the rise to orbit and descent to Earth.

During troubleshooting, the Mission Management Team traced the most likely cause to a previously undetected short in an over-temp thermostat and the subsequent short in a hybrid circuit driver in the Loads Control Assembly-2, an avionics box in the shuttle engine compartment that routes power to nine critical shuttle systems.

The thermostat, loads control assembly and external wiring were replaced and the critical circuitry was re-tested before the latest countdown began on May 13. 

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