Space Shuttle Endeavour Launch Is Postponed

UPDATE: A malfunction in a power unit has forced NASA officials to postpone the launch of Endeavour until at last May 2.

NASA officials have postponed the final launch of the space shuttle Endeavour after discovering a problem with heaters in one of the vehicle's auxiliary power units.

Endeavour had been scheduled to launch just before 4 p.m. ET, and had been given the go-ahead earlier April 29. However, the launch was scrubbed just before 1 p.m. ET after the malfunction was discovered. NASA officials said the next opportunity to get the shuttle into space would be May 2.

According to NASA, engineers during countdown detected a failure in in one of two heater circuits in what the agency calls APU (Auxiliary Power Unit) 1. Engineers unsuccessfully tried to activate the heater. Such heaters are used to to keep the APUs' hydrazine from freezing while in orbit. The APUs reporteldy help power hydraulics for the shuttle's engines and flaps.

The malfunction made that power unit unusable. Endeavour reportedly has three APUs, but needs all three functioning before being sent into space.

According to NASA officials, engineers seem to believe that the problem has to do with either a load control assembly, which the agency described as a switchbox found at the aft end of the shuttle, or an electrical short in the wires leading into or out of the switchbox.

Engineers will drain Endeavour's external tank, NASA said. That will give them access to the ship's aft compartment overnight, and eventually they will determine whether the switchbox needs to be replaced, or faulty wiring fixed.

NASA officials are scheduled to speak at a press conference at 4 p.m. ET to discuss the issue.

The scrubbing of the launch came hours after NASA space shuttle managers gave a "go" to begin loading shuttle Endeavour's external tank with more than 500,000 gallons of liquid oxygen and liquid hydrogen, the space agency reported. The operation is set to begin at 6:22 a.m., and with a 70 percent chance of favorable weather for launch, liftoff is targeted for 3:47 p.m.

The primary weather concern was for low cloud ceilings and crosswinds at the Shuttle Landing Facility. NASA reported a front would pass through Kennedy between 7 a.m. and 8 a.m., bringing a chance for showers. After the front passes, high pressure will build into the area and winds will shift to the northeast and be near the peak limit for crosswinds. The Transatlantic Abort Landing site at Istres, France, is "go."

The crew members for space shuttle Endeavour's STS-134 mission are Commander Mark Kelly, Pilot Gregory H. Johnson and Mission Specialists Michael Fincke, Greg Chamitoff, Andrew Feustel and European Space Agency astronaut Roberto Vittori. During the 14-day mission, Endeavour and its crew will deliver the AMS (Alpha Magnetic Spectrometer) and spare parts including two S-band communications antennas, a high-pressure gas tank and additional spare parts for Dextre, a two-armed robot that is part of the Mobile Servicing System on the International Space Station, and extends the function of the system to replace some activities otherwise requiring spacewalks.

"The AMS is an amazing story all by itself," said Gary Horlacher, lead space shuttle flight director for the mission. "They've been working on this for well over 15 years now. It's bigger than a VW bug, and it will be able to look at things that the other observatories aren't even looking at. It is, without a doubt, going to continue to rewrite our future as we try to figure out our past."

The AMS is a state-of-the-art, high-energy particle-physics experiment built in Geneva by a collaboration of 16 different countries. It will search for clues on what the universe is made of and how it began, the origin of dark matter, anti-matter and strangelets, pulsars, blazers and gamma-ray bursters, and that's just what the scientists know to look for.

"I am quite confident that once we start measuring data in space, we will find things that we never anticipated," NASA's AMS project manager Trent Martin said. "And those things will lead to potential new areas of study, new areas of science and maybe even redefine some of the physics books."

After AMS is installed on the station's truss during the fourth day of the mission, a pallet of spare parts will be added to the space station on the mission's flight day five. Then there will be a string of spacewalks-the last spacewalks to be performed by a shuttle crew-dedicated to getting the station in the best-possible shape for the end of the space shuttle program.

Among Endeavour's missions was the first to include four spacewalks, and then the first to include five. Its STS-67 mission set a length record almost two full days longer than any shuttle mission before it. Its airlock is the only one to have seen three spacewalkers exit through it for a single spacewalk. And in its cargo bay, the first two pieces of the International Space Station were joined together.

NOTE: This story was updated to reflect the postponement of the space shuttle launch.