The space shuttle Discovery repeated history Monday night, making its final trip to the launch pad — for the second time.
Discovery, NASA's oldest flying orbiter and the first one to reach its final scheduled flight, had its first launch-pad rollout last September — but unexpected damage to its external fuel tank required it to be brought back and extensively repaired.
"It's very exciting. I'm not focusing too much on it being the last one again, I'm just enjoying the fact that we've overcome a huge challenge," said Stephanie Stilson, NASA's vehicle flow manager for the shuttle Discovery. "The team has worked really hard to get us to the point where we're able to get back out to the launch pad. ... It's a beautiful sight."
Discovery began the 3.4-mile journey from the Vehicle Assembly Building here at NASA's Kennedy Space Center to Launch Pad 39A at 7:58 p.m. ET. The shuttle, mounted to its twin solid rocket boosters and newly fortified external tank, was moved atop its mobile launch platform by one of NASA's two Apollo-era crawler transporters.
Reaching a maximum speed of about 1 mph (1.6 kilometers per hour), Discovery's final trip to the pad took a little less than six hours, concluding at 2:53 a.m. ET Tuesday.
Now that it's back on the pad, Discovery will again undergo final preparations to launch on its STS-133 mission to the International Space Station.
The orbiter will deliver a new "storage closet" module for the station and carry up the first humanoid robot in space, Robonaut 2. The two-armed, legless robot will assist the ISS crew inside and eventually outside the orbiting laboratory.
NASA has targeted Feb. 24 at 4:50 p.m. ET for Discovery's 39th and final launch. The planned 11-day flight is the third-last on NASA’s manifest for its 30-year-old space shuttle program.
The mission was originally scheduled to launch last November, but after a hydrogen leak scrubbed its first attempt, shuttle technicians discovered a crack in Discovery’s external tank insulating foam. A subsequent inspection of the area revealed underlying cracks in the tank’s 21-foot (6.4-meter) U-shaped aluminum support beams, called stringers.
Although cracked stringers had been encountered during assembly of earlier tanks, NASA had never found them once the shuttle was on the launch pad. Unable to arrive at a root cause, shuttle engineers installed sensors on the tank while Discovery was still on the pad, then fueled it as they would on launch day to collect more data about what might have triggered the cracks.
As the data from the tanking test was processed, NASA rolled back Discovery to its 52-story assembly building to further X-ray the tank and perform repairs. Additional cracked stringers were found on the backside of the tank, and coupled with the results of the fueling test, NASA decided to strengthen most of the 94 beams, using support structures called radius blocks.
Technicians completed installation of the additional hardware and applied new foam to the tank this past week. Soon after, shuttle managers cleared Discovery to return to the launch pad.
"It feels really good. It feels like we're back to where we need to be," Stilson told Space.com. "It's been a challenging several months for the team, but it's also been very rewarding doing what we need to do and making sure Discovery flies safe and that we're doing everything right. We want to make sure we're able to be successful and go down in history on a positive note."
As it did in September, NASA invited about 900 employees and their families to come out and watch Discovery emerge from the Vehicle Assembly Building to begin rolling out to Pad 39A.
Exiting the voluminous hangar into the darkness of night, Discovery was bathed by xenon spotlights, illuminating the orbiter for one last photo opportunity before its final mission to space.
"When we first roll out of the Vehicle Assembly Building, we don't have the xenon lights on, but once it crosses the threshold, we then turn on the high-powered xenon lights … and that's when it glows like a candle," Stilson said. "People were cheering and clapping."
NASA plans to retire the space shuttle fleet later this year. The shuttle Endeavour is scheduled to fly its own final mission in April to deliver a billion-dollar astrophysics experiment to the International Space Station. NASA also plans to fly one last mission aboard the shuttle Atlantis to deliver extra supplies to the space station, but the final funding for that mission has not yet been approved by Congress.
NASA’s space shuttles have been launching into low-Earth orbit since 1981, and the space agency will mark the 30th anniversary of the fleet's inaugural spaceflight on April 12. Once the shuttle fleet retires, NASA will rely on Russian, European and Japanese spacecraft to ferry astronauts and supplies to the space station until commercial U.S. spacecraft become available.
Space.com staff writer Denise Chow () contributed to this story from New York City.