What's Wrong With Space Shuttle Endeavour?

Thousands of would-be spectators, including President Obama's family and Congresswoman Gabrielle Giffords, made their way to the Kennedy Space Center in Cape Canaveral last Friday to watch the penultimate Space Shuttle flight scheduled for 3:47 p.m. Sitting on Launch Pad 39A, Endeavour looked like it had survived a violent lightning storm the previous night and was ready to carry its six astronauts, led my Giffords' husband, astronaut Mark Kelly, into space when word spread that the flight would be delayed.

The launch has since been delayed on at least one other occasion, depending on how you're counting. Top NASA officials have met several times to determine the next earliest opportunity for launch with reporters often interpreting those dates -- Sunday, May 1, and then Tuesday, May 3, and then Sunday, May 8, and then Tuesday, May 10 -- as endpoints on a new schedule. But no solid date has been set since the first failed attempt. Still, the potential launch date continues to get shifted further down the calendar. So, what's wrong with Space Shuttle Endeavour? Why can't the orbiter, which has already circled the globe more than 4,400 times on 25 missions, get off the ground?

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NASA officials called off the first launch because of a problem with the auxiliary power unit (APU), a device often found on large aircraft and land vehicles that provides energy for functions other than propulsion. During liftoff, the APU on Space Shuttle Endeavour (one of which is roughly located in the relatively tiny compartment highlighted in the image above) provides hydraulic power for the gimballing of control surfaces and engines. The Shuttle has three redundant APUs, but there's little point in arguing for a launch unless all parts, both essential and non-essential, are in working order.

Technicians believed they could get the APU up and running in less than 48 hours, and spectators expected the launch to be rescheduled for this past Sunday. But during a briefing held that day, Mike Moses, chair of the Shuttle's mission management team, dismissed speculation. "Right now we're not ready to set a launch date," he said, according to Space.com, which has closely documented all news surrounding the program. "We know right now that [May] 8th is our next available opening." Now that date, too, could pass without a launch.

During the briefing, Moses was able to give reporters a bit more information if not the launch date that they wanted. The problem with the APUs, it turned out, was that a pair of heaters were malfunctioning. The heaters keep the APUs from freezing in orbit as they're needed again upon reentry to control the Shuttle's brakes. The heaters could not just be replaced, though; technicians traced the problem all the way back to the aft load control assembly-2, a power control box. "We need to go in and change out that box," Moses said. "Once the box comes out we have to verify circuitry and prove the box itself was the failure. We still have a lot of work."

Will it ever be safe to launch Endeavour? Is the orbiter's electrical problem a sign of old age? The Shuttle, now 19 years old, has traveled more than 103 million miles. "It's a machine and occasionally machines break," Mike Leinbach, the Shuttle launch director, told Space.com, dismissing a question that implied Endeavour might be showing its years. "That's just part of the business." Leinbach is confident his team will be able to fix the problem and get the orbiter ready for launch. But will there be time? The final Space Shuttle Atlantis mission -- and the final mission of the decades-old program -- is planned for June 28 and any additional delays on Endeavour could throw off that schedule as well.

Images: NASA/Alexis Madrigal.