With the shuttle Discovery's countdown on hold, engineers reviewed historical data and refined a "fault tree" to understand what caused irregularities in a critical circuit Tuesday, delaying launch to at least Thursday.

Credit: Justin Ray/Spaceflight Now

NASA's mission management team plans to meet at 2 p.m. EDT to review overnight progress and to make a decision on whether to launch Discovery as is Thursday or to stand down for repairs.

While the weather forecast does not normally play a roll in launch decisions, it could be a factor in this case. Meteorologists are predicting dismal conditions Thursday with high winds expected Friday and Saturday.

With a frontal system moving through the area, forecasters are predicting an 80 percent chance of low clouds and rain that would prohibit a launching Thursday. High winds are expected Friday and Saturday, with forecasters putting the odds of acceptable weather at 60 percent and 40 percent respectively.

The shuttle's current launch window runs through Sunday, with a possible extension to Monday. If Discovery isn't off the ground by then, launch would slip to Dec. 1, the opening of the year's final shuttle launch window.

If the MMT decides to proceed with a Thursday launch attempt, engineers will roll a protective gantry away from Discovery around 7 p.m. Wednesday, exposing the orbiter to view and setting the stage for the start of fueling at 6:04 a.m. Thursday.

Discovery's six-member crew -- commander Steven Lindsey, pilot Eric Boe, Michael Barratt, Nicole Stott and spacewalkers Timothy Kopra Alvin Drew -- would begin strapping in around 12:09 p.m. to await liftoff at 3:29:43 p.m.

Assuming an on-time liftoff, Lindsey would guide Discovery to a docking with the International Space Station around 11:29 a.m. on Nov. 6. Spacewalks by Kopra and Drew would be scheduled for Nov. 8 and 10. If all goes well, Discovery would undock from the lab complex around 5:27 a.m. on Nov. 13, setting up a landing back at the Kennedy Space Center at 10:24 a.m. on Nov. 15.

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Launch was delayed 24 hours to Thursday after engineers ran into problems with a circuit associated with a control computer mounted on one of Discovery's three hydrogen-fueled main engines. One of the controller's two channels did not power on normally. Later, after it was operating, telemetry showed a brief, unexpected voltage drop.

Each main engine is equipped with a controller that monitors engine operation -- valve positions, temperatures, pressure, vibration and other factors -- 50 times per second. Those data are fed to the shuttle's flight computers and if a problem develops, an engine can be safely shut down before a catastrophic failure occurs.

Engineers suspect the electrical anomalies in the backup channel of main engine No. 3's controller were the result of "transient contamination" in a circuit breaker. The breaker in question was cycled five times overnight and the circuit was used to power up the shuttle's master events controllers. The engine controller has operated normally ever since it was activated.

But Mike Moses, chairman of the mission management team, said Tuesday that engineers needed to come up with an explanation of what went wrong in the first place and develop a solid flight rationale before Discovery will be cleared for launch.

"Does it make sense that a circuit breaker with a little bit of a bad contact explains both of these signatures that we saw? The community feels pretty confident that that is the case, but they do need time to polish that story," Moses said Tuesday.

He emphasized that NASA would not let the relatively short launch window drive a decision to launch Discovery if the engineering data do not support show it is safe to do so.

"If tomorrow we come in and go we either still don't understand it or we understand it and we need to fix it, then we're going to do that," he said. "We're not just going to fly as is."

Additional coverage for subscribers:
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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