Engineers are troubleshooting a stuck valve in a nitrogen tank assembly that's needed to pressurize a new ammonia coolant tank installed on the space station by the Discovery astronauts.

If ground commanding does not resolve the problem, a spacewalk by the station astronauts could be required to install a spare nitrogen tank assembly to avoid a major powerdown due to higher temperatures caused by the changing orientation of the station's orbit with respect to the sun.

Engineers are hopeful it won't come to that.

"The valve that we had a problem with is in the nitrogen tank assembly, which feeds nitrogen to pressurize the ammonia system, not in the ammonia tank assembly itself," said space station Flight Director Ron Spencer. "We do have two spare nitrogen tank assemblies on orbit if we have to swap those out. And if we did do that, it would be an EVA-required task.

"The engineers right now are looking at what else can be done to troubleshoot prior to going to that option. We do have a couple of things we might be able to try, ground commanding wise, to try to get this valve to be successful prior to going to that option."

The space station features two independent coolant loops that use ammonia circulating through huge radiators to dissipate the heat generated by the station's electronic systems. Each loop is fed by a large tank of ammonia that includes an internal bellows that is pressurized by nitrogen.

As the ammonia expands and contracts due to orbital temperature swings, the pressurized bellows compensates.

The two coolant loops were launched dry and then filled with ammonia in orbit when the station's power truss was assembled. Both loops were then pressurized with independent nitrogen tank assemblies. One of the two depleted ammonia tanks was replaced during an earlier shuttle mission. The tank used by the right-side coolant loop was replaced by the Discovery astronauts.

The new tank was put in place during a spacewalk Sunday and connected to the loop's nitrogen and ammonia lines earlier Tuesday. But commands to open the nitrogen valve were not successful.

Spencer said the problem is not an immediate crisis, but engineers want to resolve the issue as soon as possible because of the always-changing nature of the space station's orbit.

The orbit is tilted 51.6 degrees to the equator and the angle between the sun and the plane of the orbit - the beta angle - periodically increases and decreases. The beta angle currently is increasing, meaning higher temperatures can be expected.

"What happens in high beta periods is the sun will be off to one side of station during the course of an entire daylight pass instead of overhead with a more regular cycling of temperatures," Spencer said. "So whatever side it's on will tend to heat up during the course of all those daylight passes and the other side will tend to cool down."

Ammonia readily expands due to temperature increases, Spencer said, "and so the concern is once you get to a certain beta angle, the expansion of the ammonia will exceed the capability of the radiators and without the nitrogen in there pressurizing the system, activating the bellows, you can't take as much swing in the total volume of ammonia that you have. So we'd have to shut down the ammonia system prior to that. And folks are going to be taking a harder look at when that is."

Engineers hope to resolve the valve problem, one way or the other, before high temperatures force flight controllers to shut the coolant loop down and along with it, half the station's systems.

"We do get to a higher beta region in April and folks are going to have to be looking really hard to see if we can make it through that period or not," Spencer said. "And I don't know yet whether we can make it."

While a spacewalk by the station crew is always an option, it is a last resort because of the planning required and the impact on other activity. Complicating the near-term picture is the planned launch of the shuttle Atlantis on May 14.

Spencer said it may be possible to coax the stuck valve open by cycling a nitrogen isolation valve that was closed earlier in the mission in preparation for the ammonia tank swap out.

"Going EVA to fix this problem is not the first option," Spencer said. "We do have engineers looking at it right now and there's going to be a couple of other things we can try ... to see if that'll help free this valve. EVA is the last option."

Additional coverage for subscribers:
VIDEO: MONDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY

VIDEO: FLIGHT DAY 8 HIGHLIGHTS MOVIE PLAY
VIDEO: MONDAY AFTERNOON'S MISSION STATUS UPDATE PLAY
VIDEO: RUSSIAN PRESIDENT CALLS THE SPACE STATION PLAY
VIDEO: SPACE STATION CREW MARKS COSMONAUTICS DAY PLAY
VIDEO: ABC, MSNBC, FOX NEWS AND KUSA-TV INTERVIEWS PLAY
VIDEO: JAXA EVENT WITH JAPANESE ASTRONAUTS PLAY
VIDEO: SUNDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY

HIGH DEFINITION TV DAY 7: MEAL TIME IN ZVEZDA PLAY
HIGH DEFINITION TV DAY 7: GRUELING 7.5-HOUR EVA FINSHES PLAY
HIGH DEFINITION TV DAY 7: VENTURING OUTSIDE THE STATION PLAY
HIGH DEFINITION TV DAY 7: GETTING READY FOR ANOTHER EVA PLAY
HIGH DEFINITION TV DAY 7: OLD CREW SLEEP BUNK REMOVED PLAY
HIGH DEFINITION TV DAY 7: TRANSFERS AND MORE TRANSFERS PLAY

VIDEO: FLIGHT DAY 7 HIGHLIGHTS MOVIE PLAY
VIDEO: CREW'S HOME MOVIES FOR FLIGHT DAY 7 PLAY
VIDEO: SUNDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: FINALLY WINNING THE BATTLE WITH TANK BOLTS PLAY
VIDEO: NEW COOLANT TANK MANEUVERED INTO POSITION PLAY
VIDEO: DEPLETED AMMONIA TANK REMOVED FROM STATION PLAY
VIDEO: STEP-BY-STEP WALKTHROUGH OF EVA NO. 2 PLAY
VIDEO: PREVIEW OF FLIGHT DAY 7 ACTIVITIES PLAY
VIDEO: SATURDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY

HIGH DEFINITION TV DAY 6: LIFE OF A SHUTTLE ASTRONAUT PLAY
HIGH DEFINITION TV DAY 6: "BUCKET BRIGADE" ON THE STATION PLAY
HIGH DEFINITION TV DAY 6: WORF INSTALLED IN DESTINY LAB PLAY

VIDEO: FLIGHT DAY 6 HIGHLIGHTS MOVIE PLAY
VIDEO: CREW'S HOME MOVIES FOR FLIGHT DAY 6 PLAY
VIDEO: SATURDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: SCIENCE RACK INSTALLED BY JAPANESE ASTRONAUTS PLAY
VIDEO: NAVAL POSTGRADUATE SCHOOL EDUCATIONAL EVENT PLAY
VIDEO: CBS AND NEBRASKA MEDIA INTERVIEWS WITH CREW PLAY
VIDEO: SMOKE ALARM IN THE ZVEZDA SERVICE MODULE PLAY
VIDEO: PREVIEW OF FLIGHT DAY 6 ACTIVITIES PLAY

VIDEO: MISSION EXTENSION NEWS FROM FLIGHT DIRECTOR PLAY

HIGH DEFINITION TV DAY 5: SPACEWALKERS RETURN TO AIRLOCK PLAY
HIGH DEFINITION TV DAY 5: UNLOADING THE LEONARDO MODULE PLAY
HIGH DEFINITION TV DAY 5: STATION'S NEW EXERCISE MACHINE PLAY
HIGH DEFINITION TV DAY 5: OPERATORS OF THE ROBOTIC ARM PLAY
HIGH DEFINITION TV DAY 5: SPACEWALKERS FREE AMMONIA TANK PLAY

VIDEO: FLIGHT DAY 5 HIGHLIGHTS MOVIE PLAY
VIDEO: CREW'S HOME MOVIES FOR FLIGHT DAY 5 PLAY
VIDEO: STOWAGE RACKS MOVED INTO STATION PLAY
VIDEO: NEW LABORATORY FREEZER FOR KIBO PLAY
VIDEO: FRIDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: RATE GYRO ASSEMBLY REMOVED FROM STATION PLAY
VIDEO: NEW AMMONIA TANK STOWED IN TEMPORARY SPOT PLAY
VIDEO: ANCHORING FIXTURE ATTACHED TO AMMONIA TANK PLAY
VIDEO: EXPERIMENT RETRIEVED FROM JAPAN'S SCIENCE DECK PLAY
VIDEO: STATION'S ROBOTIC ARM LIFTS TANK FROM SHUTTLE PLAY
VIDEO: NEW AMMONIA COOLANT TANK UNBOLTED FROM CARRIER PLAY
VIDEO: SPACEWALKER PREPS AMMONIA TANK IN SHUTTLE BAY PLAY
VIDEO: THE START OF MISSION'S FIRST SPACEWALK PLAY
VIDEO: STEP-BY-STEP WALKTHROUGH OF EVA NO. 1 PLAY
VIDEO: PREVIEW OF FLIGHT DAY 5 ACTIVITIES PLAY
VIDEO: THURSDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY
VIDEO: THURSDAY AFTERNOON'S MANAGEMENT TEAM UPDATE PLAY

VIDEO: FLIGHT DAY 4 HIGHLIGHTS MOVIE PLAY
VIDEO: RADIO AND TV INTERVIEWS WITH CREW PLAY
VIDEO: THURSDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: LEONARDO MODULE ATTACHED TO SPACE STATION PLAY
VIDEO: STATION'S ARM GRAPPLES THE LEONARDO MODULE PLAY
VIDEO: NARRATED PREVIEW OF LEONARDO'S INSTALLATION PLAY
VIDEO: WEDNESDAY EVENING'S FLIGHT DIRECTOR INTERVIEW PLAY
VIDEO: WEDNESDAY AFTERNOON'S MANAGEMENT TEAM UPDATE PLAY

VIDEO: FLIGHT DAY 3 HIGHLIGHTS MOVIE PLAY
VIDEO: WEDNESDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: SHUTTLE CREW WELCOMED ABOARD THE STATION PLAY
VIDEO: POST-DOCKING OF THE SHUTTLE BAY AND TAIL PLAY
VIDEO: DISCOVERY DOCKS TO THE SPACE STATION PLAY
VIDEO: SHUTTLE PERFORMS 360-DEGREE BACKFLIP PLAY
VIDEO: PREVIEW OF RENDEZVOUS AND DOCKING ACTIVITIES PLAY
VIDEO: OBJECT LOST FROM SHUTTLE TAIL DURING LAUNCH PLAY
VIDEO: TUESDAY AFTERNOON'S MANAGEMENT TEAM UPDATE PLAY
VIDEO: THE FULL STS-131 LAUNCH EXPERIENCE PLAY

VIDEO: FLIGHT DAY 2 HIGHLIGHTS MOVIE PLAY
VIDEO: TUESDAY MORNING'S MISSION STATUS BRIEFING PLAY
VIDEO: PREVIEW OF FLIGHT DAY 2 ACTIVITIES PLAY
VIDEO: NARRATED TOUR OF DISCOVERY'S PAYLOAD BAY PLAY
VIDEO: DESCRIPTION OF KU-BAND ANTENNA PROBLEM PLAY

VIDEO: FLIGHT DAY 1 HIGHLIGHTS MOVIE PLAY
VIDEO: SHUTTLE DISCOVERY BLASTS OFF! PLAY | HI-DEF
VIDEO: GO INSIDE MISSION CONTROL DURING LAUNCH PLAY
VIDEO: POST-LAUNCH NEWS CONFERENCE PLAY

VIDEO: LAUNCH REPLAY: VAB ROOF PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PRESS SITE PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PATRICK AFB PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD PERIMETER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: BEACH TRACKER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD CAMERA 070 PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD CAMERA 071 PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PLAYALINDA BEACH PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD FRONT CAMERA PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: KSC WEST TOWER PLAY | HI-DEF

VIDEO: ASTRONAUTS LEAVE CREW QUARTERS PLAY | HI-DEF
VIDEO: CREW FINISHES GETTING SUITED UP PLAY | HI-DEF

VIDEO: NARRATED REVIEW OF SHUTTLE'S PREPARATIONS PLAY | HI-DEF
VIDEO: NARRATED REVIEW OF PAYLOADS' PREPARATIONS PLAY | HI-DEF
VIDEO: PREPARING AN EXTERNAL TANK FOR LAUNCH PLAY | HI-DEF

VIDEO: TIME-LAPSE OF GANTRY ROLLING BACK FOR LAUNCH PLAY
VIDEO: LAUNCH PAD'S SERVICE TOWER RETRACTED PLAY | HI-DEF
VIDEO: PAYLOAD BAY DOORS CLOSED FOR LAUNCH PLAY | HI-DEF
VIDEO: ASTRONAUTS INSPECT THE PAYLOAD BAY PLAY | HI-DEF

VIDEO: STS-131 MISSION PREVIEW MOVIE PLAY | HI-DEF
VIDEO: PRE-LAUNCH INTERVIEW WITH COMMANDER POINDEXTER PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH PILOT JIM DUTTON PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH RICK MASTRACCHIOPLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH METCALF-LINDENBURGER PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH STEPHANIE WILSON PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH NAOKO YAMAZAKI PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH CLAY ANDERSON PLAY

VIDEO: PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: COUNTDOWN STATUS AND WEATHER OUTLOOK PLAY
VIDEO: PREVIEW OF DISCOVERY'S LAUNCH COUNTDOWN PLAY
VIDEO: ASTRONAUTS ARRIVE FOR LAUNCH PLAY | HI-DEF

VIDEO: FULL FLIGHT READINESS REVIEW NEWS BRIEFING PLAY
VIDEO: RECAP OF THE FLIGHT READINESS REVIEW PLAY | HI-DEF
VIDEO: HELIUM VALVE NO CONSTRAINT TO LAUNCH PLAY | HI-DEF
VIDEO: MANAGERS ASSESS ISSUES BEFORE FLIGHT PLAY | HI-DEF
VIDEO: UPDATE ON PRE-LAUNCH PREPS AT PAD 39A PLAY | HI-DEF

VIDEO: SPACEWALKING SUITS LOADED ABOARD PLAY | HI-DEF
VIDEO: INTERVIEW WITH STS-131 PAYLOAD MANAGER PLAY | HI-DEF
VIDEO: INTERVIEW WITH BOEING PAYLOAD MANAGER PLAY | HI-DEF
VIDEO: PAYLOADS DELIVERED TO LAUNCH PAD 39A PLAY | HI-DEF
VIDEO: TRANSPORT CANISTER ROTATED VERTICALLY PLAY | HI-DEF
VIDEO: LEONARDO PLACED INTO THE TRANSPORTER PLAY | HI-DEF
VIDEO: STATION'S NEW AMMONIA COOLANT TANK PLAY | HI-DEF
VIDEO: LEONARDO HATCH CLOSED FOR FLIGHT PLAY | HI-DEF
VIDEO: FILLING UP ONE OF THE SUPPLY RACKS PLAY | HI-DEF

VIDEO: CAN THE SHUTTLE PROGRAM AVOID LOOMING RETIREMENT? PLAY
VIDEO: WHAT ABOUT ADDING ONE MORE SHUTTLE MISSION? PLAY
VIDEO: FULL BRIEFING BY SHUTTLE AND STATION OFFICIALS PLAY
VIDEO: THE STS-131 MISSION OVERVIEW PRESENTATIONS PLAY
VIDEO: PREVIEW BRIEFING ON MISSION'S SPACEWALKS PLAY
VIDEO: THE ASTRONAUTS' PRE-FLIGHT NEWS BRIEFING PLAY

VIDEO: SHUTTLE EVACUATION PRACTICE PLAY | HI-DEF
VIDEO: CREW MODULE HATCH IS CLOSED PLAY | HI-DEF
VIDEO: ASTRONAUTS BOARD DISCOVERY PLAY | HI-DEF
VIDEO: CREW BRIEFED ON EMERGENCY PROCEDURES PLAY | HI-DEF
VIDEO: TEST-DRIVING AN EMERGENCY ARMORED TANK PLAY | HI-DEF
VIDEO: NIGHTTIME APPROACHES IN TRAINING AIRCRAFT PLAY | HI-DEF
VIDEO: ASTRONAUTS CHAT WITH REPORTERS AT PAD 39A PLAY
VIDEO: CREW ARRIVES FOR PRACTICE COUNTDOWN PLAY | HI-DEF

VIDEO: GANTRY PLACED AROUND DISCOVERY PLAY | HI-DEF
VIDEO: DISCOVERY REACHES PAD 39A PLAY | HI-DEF
VIDEO: OVERNIGHT ROLLOUT BEGINS PLAY | HI-DEF
VIDEO: SHUTTLE HOISTED FOR ATTACHMENT TO TANK PLAY | HI-DEF
VIDEO: CRANE ROTATES THE ORBITER VERTICALLY PLAY | HI-DEF
VIDEO: DISCOVERY MOVES TO ASSEMBLY BUILDING PLAY | HI-DEF

VIDEO: ASTRONAUTS VISIT THEIR SPACECRAFT PLAY | HI-DEF
VIDEO: CREW GOES INSIDE LEONARDO MODULE PLAY | HI-DEF

VIDEO: EXTERNAL TANK ATTACHED TO BOOSTERS PLAY | HI-DEF
VIDEO: FUEL TANK LIFTED INTO CHECKOUT CELL PLAY | HI-DEF
VIDEO: EXTERNAL TANK ARRIVES AT SPACEPORT PLAY | HI-DEF

VIDEO: DISCOVERY'S NOSE POD ATTACHED PLAY | HI-DEF
VIDEO: INSTALLING DISCOVERY'S MAIN ENGINES PLAY | HI-DEF
VIDEO: KSC'S SHUTTLE MAIN ENGINE SHOP PLAY | HI-DEF
VIDEO: GASEOUS NITROGEN TANK REMOVED PLAY | HI-DEF
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