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NASA hopes to try launching shuttle again on Monday

NASA mission managers say they'll try again to launch the space shuttle Endeavour as early as Monday, once they figure out the cause of a puzzling problem with heaters in one of the craft's auxiliary power units.

Friday's high-profile countdown was called off even as Endeavour's six-man crew was heading for Launch Pad 39A. Liftoff would be delayed for at least three days, and possibly longer, NASA launch director Mike Leinbach said.

"We will not fly this machine until it's ready, and today it was not ready to go," he told journalists.

The shuttle's three auxiliary power units provide hydraulic pressure to operate the control system during atmospheric flight. Although the shuttle can operate with only one of the units working, NASA's launch rules state that all three have to be functional for liftoff.

One of the thermostats in the heating system apparently suffered a hard failure, and another was exhibiting "funny behavior," Leinbach reported. He said engineers suspect that there may be a short circuit either in a switchbox or in an electrical line leading to the box.

"If we can go down the easy path, we're still on track for Monday morning," he said. But Leinbach said NASA might have to delay the next launch attempt for more than a week if the problem turns out to be complicated. He expected a weekend round of troubleshooting to tell the tale.

The delay meant that hundreds of thousands of spectators would miss out on seeing Endeavour's final liftoff on Friday. Would-be witnesses included President Barack Obama and his family; and U.S. Rep. Gabrielle Giffords, who was critically wounded during a January mass shooting.

Giffords — who is married to Endeavour's commander, Mark Kelly — was flown to Florida from her Houston rehabilitation center this week to watch the launch. "Bummed about the scrub!!" the congresswoman's staff wrote in a Twitter update. "But important to make sure everything on shuttle is working properly."

"We're all bummed," NASA astronaut Ron Garan tweeted back from the International Space Station, Endeavour's intended destination.

Thousands go home disappointed
Giffords and the Obamas were to watch the liftoff from secure locations here at Kennedy Space Center in Florida, while NASA expected as many as 750,000 other spectators to gather at VIP areas and public viewing spots. Thousands of people had already taken up places along the roads surrounding the launch site and had to go home disappointed.

Despite the launch scrub, Obama and his entourage were flown in to Kennedy Space Center on military helicopters for a quick tour of NASA facilities, allowing the president to meet with Kelly and Giffords as well as other members of Endeavour's crew and their families.

The launch was called off less than four hours before the scheduled 3:47 p.m. ET liftoff time, just as Kelly and his crew were heading out toward the launch pad. The van carrying the crew stopped at NASA's launch control center, then turned around to return to crew quarters as word of the postponement got out.

Before the postponement, forecasters said there was a 70 percent chance of acceptable weather for launch. When the appointed time for liftoff came, they said Endeavour would have been "go" for launch if weather were the only determining factor. Mike Moses, who chairs NASA's pre-launch mission management team, said that fact served to rub "a little salt in the wound."

Endeavour's mission to the space station, known as STS-134, represents the shuttle's last scheduled flight before retirement and the second-last flight of the 30-year space shuttle program.

15 tons of supplies
Endeavour is loaded up with nearly 15 tons of supplies and equipment, highlighted by the 7.5-ton Alpha Magnetic Spectrometer, which will be attached to the space station's exterior during a robotic arm operation. Once it's installed, the $2 billion particle detector will almost immediately start sending down data about cosmic rays.

Physicists from 16 nations have worked for 17 years to build the AMS, and for a while it looked as if all that effort would be wasted. Fortunately, Congress and NASA worked together to get the van-sized device on this mission. Over the next decade or longer, physicists hope that findings from the AMS will shed light on the mysteries surrounding the nature of dark matter, and the balance of matter and antimatter in the universe.

Endeavour's payload also includes a platform that will be used to store spare parts at the station, communication equipment and a wide range of scientific experiments.

Shuttle program's last spacewalks
Four spacewalks are to be conducted during the shuttle's visit, marking the last outings by shuttle astronauts at the space station. The spacewalkers are to install equipment, do electrical work and conduct maintenance on the station's cooling system and robotic arm system.

In addition to Kelly, the all-male, all-veteran crew includes pilot Greg Johnson, spacewalkers Mike Fincke, Drew Feustel and Greg Chamitoff, and Italian astronaut Roberto Vittori. The mission is currently scheduled to last 14 days, but NASA could extend it another day or two.

When Endeavour's mission is over, it will be prepared as a museum piece for the California Science Center in Los Angeles. NASA has scheduled only one more shuttle flight to the station, to be taken on by Atlantis this summer.

After the shuttles retire, NASA will have to depend on Russian, European and Japanese transports to get supplies to the space station — at least until next year, when U.S. commercial cargo flights are due to begin. NASA hopes that commercial crew transports will become available starting in the middle of this decade.

Earlier this month, NASA said it would pay up to $269.3 million to four companies — Blue Origin, the Boeing Co., Sierra Nevada Corp. and SpaceX — to work on those future spaceships. Mark Sirangelo, Sierra Nevada's executive vice president, said he saw the program as a "bridge" between the shuttle era and the next stage of human spaceflight.

"I don't see it as an end," he told reporters. "I see it as the beginning of the next step."

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