With repairs to its external tank complete, engineers began hauling the shuttle Discovery back to launch pad 39A Monday evening for work to ready the ship for blastoff Feb. 24 on a flight to deliver critical spare parts, supplies and a final U.S. module to the International Space Station.

Discovery heads to launch pad 39A. Credit: Justin Ray/Spaceflight Now

Carried by a powerful crawler-transporter, Discovery and its mobile launch platform began slowly rolling out of NASA's Vehicle Assembly Building at 7:58 p.m. EST, witnessed by scores of Kennedy Space Center workers who gathered nearby to watch what is expected to be Discovery's final trip to the pad. The shuttle reached the launch complex and was secured at 2:53 a.m. Tuesday.

"I have full confidence we've done everything we need to do on that tank," Stephanie Stilson, the engineer in charge of Discovery's ground processing, told CBS News. "Seeing those guys work and knowing how hard they worked to do the modifications, I have confidence in the way the shuttle program works and the fact that they're always investigating and asking more questions, more so than I've ever heard in the past."

Shuttle program managers plan to conduct a flight readiness review Feb. 10, followed by an executive-level review by senior NASA managers Feb. 18. Assuming no additional problems develop, the agency hopes to restart Discovery's countdown at 3 p.m. Feb. 21, setting the stage for launch Feb. 24 at 4:50:19 p.m.

NASA originally hoped to launch Discovery on its 39th and final flight Nov. 1, but the mission was repeatedly delayed, first by relatively minor technical problems that pushed launch to Nov. 5 and then by a gaseous hydrogen leak in a vent arm attached to the external tank.

The hydrogen leak was quickly resolved, but engineers also discovered cracks in structural rib-like "stringers" in the shuttle's external tank that triggered what turned into a nearly four-month delay.

The stringers are used in the ribbed "intertank" compartment of the external tank that separates the liquid oxygen and hydrogen sections. Engineers initially found four stress-relief fractures in two adjacent stringers on the side of the tank facing the shuttle that apparently were the result of exposure to the extreme low temperature -- minus 423 degrees Fahrenheit -- of liquid oxygen.

Engineers repaired those cracks at the pad by splicing in pristine stringer segments and attaching "doublers" to provide additional strength.

After an instrumented fueling test Dec. 18, Discovery was hauled back to the Vehicle Assembly Building for X-ray inspections of the backside of the tank, which engineers could not carry out at the pad. Additional cracks were found in three stringers, prompting NASA managers to order installation of "radius-block" doublers on most of the remaining intertank stringers.

A detailed analysis of the cracks found to that point indicated manufacturing tolerance issues and problems with a specific batch of aluminum-lithium alloy used in most of the 108 intertank stringers that left them more brittle than usual and more susceptible to stress-relief fractures.

Testing showed the radius-block modifications would provide the required margin of safety, easing concerns about the tank's structural integrity and the possible loss of foam insulation during the climb out of the dense lower atmosphere that could pose a threat to the shuttle's fragile heat shield.

When all was said and done, the five cracked stringers were repaired using a combination of doublers and radius blocks and 94 were modified with radius blocks alone. Nine stringers were not modified, one because of access issues and eight others because they were made from a different lot of aluminum-lithium alloy.

As if the crack problem wasn't enough of a headache for NASA, one of Discovery's crew members -- flight engineer Timothy Kopra -- was injured in a bicycle mishap in Houston Jan. 15. He was replaced by veteran astronaut Stephen Bowen, who will take Kopra's place in two planned spacewalks.

Bowen and his crewmates -- commander Steven Lindsey, pilot Eric Boe, Nicole Stott, Michael Barratt and spacewalker Alvin Drew -- plan to fly to the Kennedy Space Center on Feb. 20 for the start of their countdown to launch.

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Additional coverage for subscribers:
VIDEO: ROOT CAUSE OF SHUTTLE TANK CRACKS FOUND PLAY

VIDEO: WORK UNDERWAY TO REMOVE FOAM AND SENSORS PLAY | HI-DEF
VIDEO: SETTING UP ACCESS PLATFORMS AROUND TANK PLAY | HI-DEF

VIDEO: SHUTTLE ROLLED BACK TO ASSEMBLY BUILDING PLAY | HI-DEF
VIDEO: DISCOVERY DEPARTS LAUNCH PAD 39A PLAY | HI-DEF
VIDEO: LUNAR ECLIPSE OVER DISCOVERY PLAY | HI-DEF

VIDEO: HIGHLIGHTS FROM THE FUELING TEST PLAY | HI-DEF
VIDEO: RECAP OF THE FUELING EXERCISE PLAY
VIDEO: FUELING TEST COUNTDOWN UPDATE PLAY
VIDEO: COMMENTS FROM SHUTTLE LAUNCH DIRECTOR PLAY
VIDEO: GROUND UMBILICAL CARRIER PLATE CHECKED PLAY
VIDEO: INFO ON EXTERNAL TANK INSTRUMENTATION PLAY
VIDEO: DETAILED INFORMATION ON SPECIAL FUELING TEST PLAY

VIDEO: SHUTTLE DISCOVERY'S LAUNCH DELAYED TO FEBRUARY PLAY

VIDEO: FINAL PREPS FOR INSTRUMENTED TEST PLAY | HI-DEF
VIDEO: STRAIN GAUGES ATTACHED TO TANK PLAY | HI-DEF
VIDEO: BACKSCATTER INSPECTIONS OF TANK MIDSECTION PLAY | HI-DEF

VIDEO: INSTALLING DOUBLERS OVER STRINGERS PLAY | HI-DEF
VIDEO: GASEOUS HYDROGEN VENT ARM RETURNED PLAY | HI-DEF
VIDEO: DOUBLERS TO BEEF UP CRACKED BEAMS PLAY | HI-DEF
VIDEO: HYDROGEN FITTING IS REINSTALLED PLAY | HI-DEF
VIDEO: OPENING UP TANK'S ENTRANCE DOOR PLAY | HI-DEF
VIDEO: GUCP DETACHED AND SEALS REMOVED PLAY | HI-DEF
VIDEO: BROKEN FOAM REMOVED FROM THE TANK PLAY | HI-DEF
VIDEO: VENT ARM LETS GO FROM LEAKY GUCP PLAY | HI-DEF
VIDEO: DISCOVERY AWAITS REPAIRS TO TANK PROBLEMS PLAY | HI-DEF

VIDEO: INFORMATIVE DESCRIPTION OF TANK CRACK PROBLEM PLAY
VIDEO: DISCOVERY'S LAUNCH DELAYED TO MID-DECEMBER PLAY

VIDEO: GASEOUS HYDROGEN VENT ARM DETACHED FROM SHUTTLE PLAY

VIDEO: STS-133 MISSION PREVIEW MOVIE PLAY | HI-DEF
VIDEO: SHUTTLE DISCOVERY CREW BIOGRAPHIES PLAY | HI-DEF
VIDEO: ISS EXPEDITION 25-26 PREVIEW MOVIE PLAY | HI-DEF
VIDEO: SPACE STATION CREW BIOGRAPHIES PLAY | HI-DEF

VIDEO: POST-SCRUB NEWS BRIEFING ON LEAK PLAY
VIDEO: GASEOUS HYDROGEN LEAK SCRUBS LAUNCH PLAY | HI-DEF
VIDEO: BAD WEATHER DELAYS DISCOVERY LAUNCH PLAY
VIDEO: ELECTRICAL ISSUE NO LONGER A CONCERN FOR DISCOVERY PLAY
VIDEO: LAUNCH POSTPONED BY ELECTRICAL CONCERN PLAY
VIDEO: TUESDAY MORNING'S COUNTDOWN STATUS CHECK PLAY
VIDEO: DISCOVERY'S PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: COUNTDOWN PREVIEW BRIEFING AND WEATHER FORECAST PLAY
VIDEO: ANOTHER 24-HOUR DELAY ORDERED TO FINISH REPAIRS PLAY
VIDEO: LAUNCH DELAYED 24 HOURS BY LEAK REPAIRS PLAY

VIDEO: ASTRONAUTS ARRIVE FOR LAUNCH PLAY | HI-DEF

VIDEO: FLIGHT READINESS REVIEW SETS LAUNCH DATE PLAY

VIDEO: SHUTTLE AND STATION PROGRAM BRIEFING PLAY
VIDEO: THE STS-133 MISSION OVERVIEW PRESENTATIONS PLAY
VIDEO: PREVIEW BRIEFING ON MISSION'S SPACEWALKS PLAY
VIDEO: IN-DEPTH BACKGROUND ON ROBONAUT 2 HUMANOID PLAY
VIDEO: THE ASTRONAUTS' PRE-FLIGHT NEWS BRIEFING PLAY

VIDEO: PAYLOAD BAY DOORS CLOSED FOR LAUNCH PLAY | HI-DEF
VIDEO: CREW LEAVES KSC FOR TRIP TO HOUSTON PLAY | HI-DEF
VIDEO: ASTRONAUTS INSPECT THE PAYLOAD BAY PLAY | HI-DEF
VIDEO: SHUTTLE EVACUATION PRACTICE PLAY | HI-DEF
VIDEO: ASTRONAUTS EGRESS SHUTTLE AS SEEN LIVE PLAY
VIDEO: ASTRONAUTS BOARD DISCOVERY PLAY | HI-DEF
VIDEO: SUN RISES ON LAUNCH PAD 39A PLAY | HI-DEF
VIDEO: LAUNCH DAY REHEARSAL BEGINS PLAY | HI-DEF
VIDEO: COMMEMORATIVE WALL SIGNING IN VAB PLAY | HI-DEF
VIDEO: CREW BRIEFED ON EMERGENCY PROCEDURES PLAY | HI-DEF
VIDEO: IN SHUTTLE TRAINING AIRCRAFT'S COCKPIT PLAY | HI-DEF
VIDEO: TEST-DRIVING AN EMERGENCY ARMORED TANK PLAY | HI-DEF

VIDEO: PRE-LAUNCH INTERVIEW WITH STEVE LINDSEY PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH ERIC BOE PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH AL DREW PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH TIM KOPRA PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH MIKE BARRATT PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH NICOLE STOTT PLAY

VIDEO: PAYLOADS INSTALLED INTO DISCOVERY PLAY | HI-DEF
VIDEO: MISSION PAYLOADS ARRIVE AT LAUNCH PAD PLAY | HI-DEF
VIDEO: CANISTER HAULING PAYLOADS TURNED UPRIGHT PLAY | HI-DEF
VIDEO: MODULE HOISTED INTO SHIPPING CANISTER PLAY | HI-DEF
VIDEO: WEIGHING NEW SPACE STATION MODULE PLAY | HI-DEF

VIDEO: GANTRY PLACED AROUND DISCOVERY PLAY | HI-DEF
VIDEO: SHUTTLE ATLANTIS REACHES PAD 39A PLAY | HI-DEF
VIDEO: CROWDS WATCH DISCOVERY'S FINAL ROLLOUT PLAY | HI-DEF

VIDEO: SHUTTLE HOISTED FOR ATTACHMENT TO TANK PLAY | HI-DEF
VIDEO: CRANE ROTATES THE ORBITER VERTICALLY PLAY | HI-DEF
VIDEO: DISCOVERY DEPARTS ITS HANGAR PLAY | HI-DEF

VIDEO: TIME-LAPSE SHOWS DISCOVERY ASCENDING IN VAB PLAY
VIDEO: TIME-LAPSE SHOWS THE MOVE TO ASSEMBLY BUILDING PLAY

VIDEO: DISCOVERY'S MAIDEN FLIGHT: FIRST TRIP TO VAB PLAY
VIDEO: DISCOVERY'S MAIDEN FLIGHT: ROLLOUT TO PAD 39A PLAY
VIDEO: DISCOVERY'S MAIDEN FLIGHT: TEST-FIRING ENGINES PLAY
VIDEO: DISCOVERY'S MAIDEN FLIGHT: ASSORTED VIEWS OF FRF PLAY

VIDEO: THE HISTORY OF SHUTTLE DISCOVERY PLAY
VIDEO: THE HISTORY OF SHUTTLE ENDEAVOUR PLAY
VIDEO: THE HISTORY OF SHUTTLE ATLANTIS PLAY

VIDEO: INSPECTION OF THE MISSION PAYLOADS PLAY | HI-DEF
VIDEO: ROBONAUT ARRIVES AT KENNEDY SPACE CENTER PLAY | HI-DEF
VIDEO: SPACE STATION'S SPARE THERMAL RADIATOR PLAY | HI-DEF
VIDEO: BLANKETING LEONARDO WITH INSULATION PLAY | HI-DEF
VIDEO: RACK INSERTED INTO LEONARDO FOR LAUNCH PLAY | HI-DEF
VIDEO: LEONARDO RETURNS FROM ITS PREVIOUS FLIGHT PLAY | HI-DEF
VIDEO: STATION'S SPARE PARTS DEPOT ARRIVES PLAY | HI-DEF

VIDEO: ORBITER'S PAYLOAD BAY CLOSED FOR ROLLOUT PLAY | HI-DEF
VIDEO: ASTRONAUTS VISIT THEIR SPACECRAFT PLAY | HI-DEF
VIDEO: CREW INSPECTS LEONARDO MODULE PLAY | HI-DEF
VIDEO: DISCOVERY RECEIVES ITS MAIN ENGINES PLAY | HI-DEF
VIDEO: FUEL TANK MATED TO SOLID ROCKET BOOSTERS PLAY | HI-DEF
VIDEO: HOISTING FUEL TANK INTO CHECKOUT BAY PLAY | HI-DEF
VIDEO: EXTERNAL FUEL TANK UNLOADED FROM BARGE PLAY | HI-DEF
VIDEO: MISSION'S FUEL TANK ARRIVES AT SPACEPORT PLAY | HI-DEF

VIDEO: POST-FLIGHT DESERVICING: OMS POD PLAY | HI-DEF
VIDEO: POST-FLIGHT DESERVICING: OBSS BOOM PLAY | HI-DEF
VIDEO: POST-FLIGHT DESERVICING: ENGINES PLAY | HI-DEF
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