Fresh out of a lengthy overhaul, the shuttle Endeavour is poised for blastoff Wednesday on a space station assembly mission, the orbiter's first flight since the 2003 Columbia disaster. The high-profile multi-spacewalk mission features Barbara Morgan, a 55-year-old elementary school teacher-turned-astronaut who has waited 21 years for a chance to fulfill Christa McAuliffe's legacy as the original Teacher in Space.

"I am going up doing the job of an astronaut, the work of an astronaut, but I'm going up with a teacher's eyes, ears, heart and mind," Morgan, McAuliffe's backup, said in an interview. "And so I look very much forward to doing that with an open mind and being able to come back and ... translate that into how can we best provide wonderful opportunities for our colleagues and our students."

NASA managers are especially eager to get Endeavour back into orbit after a four-and-a-half-year hiatus. Now sporting state-of-the-art satellite navigation gear and converters to tap into the space station's solar power grid, Endeavour should be able to reduce the load on its own generators and stay docked at the lab complex longer than any previous flight.

"The return of Endeavour to flight status is personally an emotional milestone for me," said Program Manager Wayne Hale, who served as ascent/entry flight director during Endeavour's last mission in November 2002. "It's like a new space shuttle. It's been completely inspected from stem to stern for any defects in the wiring, any structural corrosion and it's come out clean. It's like driving a new car off the showroom floor."

The goals of shuttle mission STS-118 include installation of a 5,000-pound solar array spacer segment; replacement of a critical stabilizing gyroscope; installation of a 7,000-pound external equipment storage platform; and delivery of fresh water and some 5,000 pounds of needed hardware and supplies.

While the shuttle astronauts are plowing through their busy schedule, the space station crew - Expedition 15 Commander Fyodor Yurchikhin, Oleg Kotov and Clay Anderson - will carry out a long, complex repair job to replace a critical component in the Russian computer system. The unit is mounted near an air conditioner in the Zvezda command module and engineers believe corrosion found on cables leading to and from the box may have played a role in widespread computer failures during a June shuttle visit.

The new hardware was delivered to the station Aug. 5 aboard an unmanned Russian Progress supply ship. It will take the station crew four days to complete the computer overhaul and test the wiring.

"It's got a little bit of everything," lead shuttle Flight Director Matt Abbott said of Endeavour's mission. "We've got some assembly operations with the S5 truss installation, some repair operations with the control moment gyroscope replacement, a lot of resupply - the Spacehab module has about 5,000 pounds of cargo going up in it and we'll bring back about 4,000 pounds of cargo. We've also got some external spares to be installed on the outside of the station, we've got some science going on, several middeck payloads and really a very, very busy timeline."

Endeavour's flight will clear the way for a dramatic shuttle mission in October to move a huge set of stowed solar arrays to the far left end of the station's main power truss and attach a multi-hatch node called Harmony. The new module, temporarily mounted on the left side of the central Unity compartment, will be moved to the front of the station after the shuttle departs, providing docking ports for European and Japanese research modules scheduled for launch in December and early next year.

"This flight really sets the stage for the remainder of the year for the ISS," said Kirk Shireman, deputy manager of the space station program at the Johnson Space Center in Houston. "In October, we're going to launch node 2, the Harmony module, which is the place where we'll berth all the international partner modules. In December, we're going to launch the first international partner module, the Columbus (laboratory, which) will be berthed to node 2. Early next year in February, we're going to launch the first piece of the Japanese research module Kibo. So we're looking forward, not only to this flight, but we're looking forward to the remainder of the year."

On a human level, at least, space station hardware, no matter how important, cannot compete with Morgan and the compelling drama of an elementary school teacher's journey from an Idaho classroom to the space shuttle.

Morgan was selected on July 19, 1985, to train as backup to Christa McAuliffe in NASA's original Teacher in Space program. McAuliffe and seven crewmates died in the Jan. 28, 1986, Challenger disaster but Morgan never gave up her dream of carrying education to the final frontier.

Now, more than two decades after Challenger and nine years after she was accepted as a full-fledged NASA astronaut, Morgan is finally ready to continue McAuliffe's legacy.

"Christa was and is and always will be a great representative of the teaching profession," said Morgan, now 55. "And we are really, really proud of her. She was, is, and always will be our Teacher in Space.

"This mission is symbolic and I know that people will be thinking about not just Christa, but the Challenger crew and the Challenger mission. And that's a good thing. And I know they will be thinking about so many people over the years, the families, friends, colleagues and people the Challenger crew never ever even knew ... who for so many years have been working so hard at continuing on their work and their dreams."

As a full-fledged astronaut, Morgan will not teach any lessons from space as McAuliffe once planned. A few modest educational events are planned, but her focus will be on mission-critical work, operating the shuttle's robot arm and overseeing logistics transfer activity.

"I know people are going to think about Challenger, and they should," Morgan said. "And I want people to remember what great folks they were and that what happened with Challenger was wrong, but what the crew and NASA was trying to do was absolutely right. I'm grateful that we are continuing that."

Joining Morgan aboard Endeavour will be commander Scott Kelly, pilot Charles Hobaugh, Tracy Caldwell, Rick Mastracchio, Canadian astronaut Dafyyd "Dave" Williams and Benjamin "Al" Drew. Kelly, Hobaugh, Mastracchio and Williams each have one previous flight to their credit. Caldwell, Morgan and Drew are rookies.

Liftoff from pad 39A at the Kennedy Space Center is targeted for 6:37 p.m. on Aug. 8, roughly the moment Earth's rotation carries the shuttle into the plane of the station's orbit.

The astronauts initially hoped to take off Aug. 7, but the flight was delayed one day because of time lost replacing a leaky positive pressure relief valve located behind the orbiter's toilet. Engineers replaced the valve with one borrowed from the shuttle Atlantis and a subsequent pressure check revealed no problems.

"We found a little tiny piece of debris in the sealing surface, which was causing that slight leak rate," said NASA Test Director Steve Payne. "Once we removed it and retested the valve, it was tight as can be. So there's nothing wrong systemically with any of the valves."

Morgan will ride into space strapped into the center seat on the shuttle's lower deck, the same seat position used by McAuliffe aboard Challenger.

"A lot of people, maybe from outside NASA, look at Barbara Morgan as the 'teacher-in-space' type of thing," Mastracchio said in an interview. "We don't look at Barb as a teacher, we look at Barb as another astronaut. She's a crew member on this mission. And she's got a lot of responsibilities on the mission and oh yeah, by the way, she's going to do some public affair events where she's going to do some question and answers with some students to help promote education.

"NASA's been promoting education since day one, this is just another step in maybe trying to reach into the classroom, trying to get kids interested in space, science, math, any way to motivate kids to learn. Barb is a very hard working person and she really believes in what she's doing. I think she'll do a really good job and we're happy to have her."

Said Kelly: "She's got an incredible amount of willpower, I think. She's been involved in this program for a long time, a lot longer than any of us on the crew. She's just kind of pressed ahead through a lot of adversity so you've got to really respect her for that."

Appropriately enough, Endeavour was built to replace Challenger and it was named by school kids in a nationwide competition. Fresh from its final orbiter maintenance and down period, or OMDP, Endeavour will fly seven more times before closing out the shuttle program with a final flight in July 2010. "This will be our 22nd shuttle flight to the international space station and we're really excited to have Endeavour ready to fly again," said Kim Doering, deputy shuttle program manager. "It's been almost five years since Endeavour last landed in December of 2002 and we've made good use of that time."

During the 1,665 days Endeavour spent in the Orbiter Processing Facility at the Kennedy Space Center, NASA and its contractors delivered 1,657,173 parts, replaced 2,045 heat-shield tiles and completed 13,156 checks to make sure the flight hardware met specifications.

In a first for the shuttle program, Endeavour's three main engines are equipped with new vibration sensors and control software to safely - and automatically - shut an engine down before a problem can trigger a catastrophic failure.

NASA also upgraded the solid-fuel booster self-destruct system to lower weight and reduce power consumption while improving reliability. An improved power system was added to the wing leading edge impact sensor system, a post-Columbia upgrade designed to record impact-related forces that might occur during launch or because of micrometeoroid strikes in orbit."

A major upgrade was installation of Global Positioning System satellite navigation system technology, replacing the orbiter's original Tactical Air Navigation - TACAN - system that used UHF radio signals from a transmitter at the landing site to determine distance and bearing information during return to Earth.

"GPS has been a long time coming," Hale said. "GPS on board the space shuttle is not like the little GPS's that you stick on the window of your car that tell you where to turn to get to the restaurant off the next exit on the interstate. This is completely integrated into the navigation system on board the space shuttle.

"We have taken the 1950s-era technology of TACANs, which have been used for a long time in aviation, they're obsolete and being phased out world wide, and we've replaced them with the new technology, global positioning satellite technology, and we've got a far superior system, far safer, far more accurate to fly our big glider back home with."

One of the most noticeable upgrades is the new station-to-shuttle power transfer system, or SSPTS. By tapping into the station's solar power grid, a visiting SSPTS-equipped shuttle can minimize the use of limited supplies of hydrogen and oxygen to power its own fuel cells and remain docked longer.

"It basically takes the power on the space station, which is 120 volts, steps it down to 28 volts for the shuttle to use, allows the shuttle to power down and actually stay on board the ISS," said Shireman.

"It's important for this flight, it's really important for continued assembly of the international space station. Because having an additional six to seven crew members on board ISS for a few extra days allows a tremendous amount of work in terms of assembly, in terms of preparing the ISS to maximize the use of these remaining shuttle flights. It's really important to us and we're very much looking forward to the successful activation of SSPTS."

Said Mastracchio: "It's a very simple concept. The space station has these large solar arrays, it's creating a lot of power. The shuttle has very limited power, because it runs on cryogenics, hydrogen, and oxygen, and we run out of those things after a certain number of days. So if we could utilize the solar arrays of the space station to power the space shuttle, the space shuttle can stay docked to the station a longer period of time. That's what the station-to-shuttle power transfer system is. We dock with the station. We then utilize their power to maintain our time on orbit, and we reduce the amount of cryogenics we need to stay docked to them. And that allows us to stay docked to the station two or three days longer. It's a simple concept, but it's a good idea."

Playing it safe, however, NASA is going into STS-118 with a flight plan that assumes SSPTS does not work properly. Based on fuel cell hydrogen and oxygen consumption alone, Endeavour's mission is baselined to last 11 days and include just three spacewalks.

SSPTS will be automatically plugged in when Endeavour docks with the space station. The system will be turned on later that day, allowing electricity to flow from the station into the shuttle. The system will be powered down the day after docking for a spacewalk to attach a short spacer segment to the right side of the station's main solar power truss. Because SSPTS draws its power from arrays on that side of the truss - and because the normally rotating panels must be locked in place for the truss work - SSPTS must be powered down.

But the system will be reactivated after the spacewalk and if it continues to work normally, NASA's Mission Management Team will extend the flight by three days and give the crew permission to press ahead with a fourth spacewalk.

"We intend to hook up the power transfer system shortly after docking to the space station," Shireman said. "And we'll take a couple of days to look at the performance of that, transferring power. And at a certain point in the mission, if the mission operations director and flight director feel that system is performing as expected, they'll come to the Mission Management Team with a recommendation that we extend the flight. So we're going to make sure we see everything we need to see up there and then we'll make that decision."

Assuming a launch on Aug. 8, Endeavour will dock with the space station around 1:53 p.m. on Aug. 10. Mastracchio and Williams would conduct spacewalks on Aug. 11 and 13 while Mastracchio and Anderson would perform spacewalk No. 3 on Aug. 15. Assuming the flight is extended, the fourth and final spacewalk, by Anderson and Williams, would follow on Aug. 17.

Under the extended mission scenario, Endeavour would undock from the lab complex around 8 a.m. on Aug. 20 and land back at the Kennedy Space Center around 12:49 p.m. on Aug. 22.

"STS-118 is going to be a really exciting mission," Williams said in a NASA interview. Along with installing the S5 solar array truss segment and delivering other needed supplies and equipment, "the third objective of the spaceflight, which is really exciting, is the first flight of the educator astronaut.

"Barbara Morgan will be flying with us as a mission specialist-educator astronaut, trying to captivate the imagination of the youth of America, looking at that next generation of space exploration, what we're going to need in terms of technology, to live and work on the surface of the moon or send humans to Mars."

Continue to Part 2 -->

Additional coverage for subscribers:
VIDEO: STS-118 PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: MONDAY MORNING'S STATUS BRIEFING PLAY
VIDEO: SUNDAY COUNTDOWN AND WEATHER UPDATE PLAY
VIDEO: CREW ARRIVES AT KENNEDY SPACE CENTER PLAY
VIDEO: COMMENTS FROM EACH OF THE ASTRONAUTS PLAY
MORE: STS-118 VIDEO COVERAGE
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