Engineers restarted the repaired shuttle Discovery's countdown Monday, setting the stage for launch Thursday on a space station resupply mission that's been grounded since November because of cracks in the ship's external tank.

Credit: NASA

The countdown began at 3 p.m. EST and if all goes well, Discovery will blast off on its 39th and final flight at 4:50:24 p.m. Thursday, roughly the moment Earth's rotation carries the launch pad into the plane of the International Space Station's orbit.

"The people who work on this team have loved Discovery for many, many years," said NASA Test Director Jeffrey Spaulding. "And so it's a bittersweet thing for them to say goodbye to her. ... I think everybody's proud of what they've done and they're really happy to see her go off on this last mission."

Spaulding said there were no major technical problems going into the countdown and weather officer Kathy Winters said forecasters expect an 80 percent chance of acceptable conditions Thursday. Higher-than-allowable winds are expected at NASA's backup emergency landing sites in California and New Mexico, but with good weather expected in Florida and at two emergency runways in Europe, there are no constraints to launch.

The forecast deteriorates somewhat after Thursday, however, dropping to 70 percent "go" on Friday and 60 percent Saturday. After that, NASA managers will have to work around two upcoming unmanned launches that use the same Air Force tracking systems as the shuttle. Discovery's launch window closes March 6 to avoid conflicts with an upcoming Russian Soyuz mission.

NASA originally hoped to launch Discovery in November, but the flight was delayed by technical problems, bad weather and, ultimately, by cracks in structural rib-like "stringers" in the shuttle's external tank. The cracks have been repaired and the tank was modified to prevent the formation of any additional stress-related cracks.

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Additional coverage for subscribers:
VIDEO: LAUNCH COUNTDOWN PREVIEW AND WEATHER FORECAST PLAY
VIDEO: ASTRONAUTS ARRIVE AT LAUNCH SITE AGAIN PLAY | HI-DEF
VIDEO: DISCOVERY CLEARED FOR LAUNCH ON FEB. 24 PLAY

VIDEO: STEVE BOWEN BEGINS CREW TRAINING PLAY | HI-DEF

VIDEO: SUN RISES OVER SPACEPORT AFTER ROLLOUT PLAY | HI-DEF
VIDEO: SHUTTLE ARRIVES ATOP LAUNCH PAD 39A AGAIN PLAY | HI-DEF
VIDEO: CROWDS CHEER ON DISCOVERY'S ROLLOUT PLAY | HI-DEF
VIDEO: DISCOVERY RE-EMERGES FROM THE VAB PLAY | HI-DEF
VIDEO: LAUNCH TEAM PRACTICES PROCEDURES PLAY | HI-DEF

VIDEO: FRESH FOAM APPLIED TO EXTERNAL TANK PLAY | HI-DEF

VIDEO: ROOT CAUSE OF SHUTTLE TANK CRACKS FOUND PLAY

VIDEO: REPAIRS AND STRENGTHENING TO TANK PLAY | HI-DEF
VIDEO: DIGITAL X-RAY INSPECTIONS IN THE VAB PLAY | HI-DEF
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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