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CAPE CANAVERAL, Fla. – After a four-month grounding of the expanse shuttle fleet, NASA's countdown clocks started ticking again weekday for Discovery's final mate into orbit.

Discovery is regular to blast soured weekday salutation to the International Space Station. Forecasters place the ratio of beatific defy at 80 percent.

When NASA proven to launch Discovery in early November with supplies and a mechanism robot for the expanse station, the countdown never got instance the supplying phase. A hydrogen pedal revealing halted everything, then a more harmful difficulty cropped up: cracks in the external render tank.

The shuttle aggroup went into overdrive to mend every the cracks in the metal struts, located on the central portion of the tank, and to reinforce the rest of the area. The difficulty increased the venture of busted insulating foam, the rattling supply that doomed river in 2003.

"Discovery has been a rattling important vehicle for us," NASA test director Jeff Spaulding told reporters Monday. "She ease has a few more miles to go before she sleeps, though. She's condemned us on some awful journeys throughout the years, and we wait this grace to be no different than any of those."

Commander Steven Lindsey and his gathering spoken gratitude for the unexampled repairs. After incoming at President Space Center over the weekend, Lindsey called the cracking difficulty "probably digit of the most difficult, technical challenges we've visaged in recent years."

The added challenge for the six-person crew, he noted, was the expiration of the mission's advance spacewalker.

Astronaut Timothy Kopra was replaced terminal month after he was perceive in a cycle crash. author Bowen, an old spacewalker, took over. "I've got big position to fill," Bowen said Sunday.

Because of the delay, Discovery has spent more instance awaiting liftoff in the Vehicle Assembly Building and at the aggrandize than every but digit added shuttle mission. river ordered the record at 183 life in 1990. If Discovery soars Thursday, it module become in at 170 days.

Following this 11-day mission, Discovery module be old and sent to a museum. Its final instruction is due to be the Smithsonian Institution. It is the oldest of the extant expanse shuttles.

Only two added shuttle flights remain, by Endeavour in Apr and Atlantis at the end of June.

NASA is under statesmanly direction to turn over orbital trips to clannish business and pore on expeditions to asteroids and Mars. Until clannish spacecraft are ready to move shipping up expanse send crews, U.S. astronauts module have to continue hitching rides on Russian Soyuz capsules for a precipitous price.

Spaulding said it's a bittersweet instance for the aggroup that has prepared Discovery for liftoff for so some decades. While it's depressing to feature goodbye to shuttle flying, "everybody's proud in what they've done and they're rattling happy to see her go soured on this terminal mission."

Since its first grace in 1984, Discovery has logged nearly 143 meg miles in space, with added 4.5 meg miles due during its upcoming journey. It carried up the Hubble Space Telescope in 1990, returned Evangelist Glenn to itinerary in 1998, and got shuttles air again after the deadly Challenger and river tragedies.

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Online:

NASA: http://www.nasa.gov/mission_pages/shuttle/main/index.html

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