Astronauts Rick Mastracchio and Clay Anderson are gearing up for a planned 6.5-hour spacewalk early Sunday, the second of three planned by the shuttle Discovery's crew. The goals of this excursion are to install a new ammonia coolant tank on the station's power truss and to move no-longer-needed debris shielding into the station for return to Earth.

Credit: NASA

This will be the 142nd spacewalk devoted to station assembly and maintenance since construction began in 1998, the sixth so far this year and the fifth overall for both Mastracchio and Anderson.

The spacewalk is scheduled to begin at 2:16 a.m. ED Sunday, when the astronauts switch their spacesuits to battery power. For identification, Mastracchio, call sign EV-1, will be wearing a suit with red stripes around the legs. Anderson, EV-2, will be wearing an unmarked spacesuit.

The International Space Station features two independent coolant loops that circulate ammonia through huge radiator panels to dissipate the heat generated by the lab's electrical systems. Each loop features a nitrogen pressurization system, an ammonia reservoir and internal bellows, or accumulators, that allow for the expansion and contraction of the coolant as the station moves through sunlight and orbital darkness.

"The coolant that we use is ammonia, so it gets circulated by a pump, it picks up heat from all our avionics and through a heat exchanger, picks up the heat from our internal(equipment) inside all the laboratories," said Flight Director Ed Van Cise. "That ammonia gets circulated out to our radiators and the radiators allow us to reject that heat from the ammonia out to the much colder space environment. That's how we keep everything cool."

When the truss segments housing the two ammonia tanks were launched, "the vast majority of the system was launched dry," Van Cise said. "It had nitrogen in it, we vented the nitrogen and then we had to fill the system with ammonia. So the original tanks that flew up full were depleted. If we had to, for some reason, refill the lines again due to a leak or something of that nature, we don't have enough ammonia to be able to do that.

"So to preposition ourselves for the long-term utilization of the space station, we're now replacing both sets of tanks so we have a full supply of ammonia so we can compensate if we need to. On a previous mission, we replaced the first ammonia tank and on this mission we're replacing the second."

Because of the distance between the tanks in the station's power truss and the shuttle's payload bay, the lab's robot arm must move to different work stations during the replacement procedure. As a result, three spacewalks are needed to remove a tank, install its replacement and then move the old tank to the shuttle's cargo bay for return to Earth.

"If you remember, on spacewalk number one we took the new tank, took it out of the (shuttle's) payload bay, we staged it on the mobile transporter," Van Cise said. "Last night on my shift, we took the space station's robot arm, we walked it off, if you will, so it's prepositioned to support the (second) spacewalk.

"So on this upcoming spacewalk, we're going to take the old tank out (of the truss), we're going to temporarily stow it on the CETA cart, a little cart that moves along the truss. They're going to tie it down to that cart. We'll go get that new tank, install the new tank into the truss and make all the connections. So by the end of the spacewalk, that new tank will be connected, our thermal operations officer can go verify everything is working.

"In the meantime, they will take that old tank and preposition it so that on the third spacewalk (Tuesday) it can then finally be put back into the payload bay."

Along with installing the new tank Sunday, Mastracchio and Anderson plan to retrieve no-longer-needed debris shields that have been temporarily stored outside the Quest airlock module. The shields will be brought inside and moved to the Leonardo cargo module for return to Earth.

While the spacewalk is going on Sunday, other astronauts will continue work to unload the supplies and equipment brought up in Leonardo and moving other no-longer-needed gear into the module to take its place.

Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision F of the NASA television schedule; best viewed with fixed-width font):

EDT........DD...HH...MM...EVENT

04/10
09:21 PM...05...15...00...Crew wakeup
10:01 PM...05...15...40...EVA-1: Airlock repress/hygiene break
10:51 PM...05...16...30...EVA-1: Airlock depress to 10.2 psi
11:51 PM...05...17...30...MPLM transfers resume

04/11
12:41 AM...05...18...20...EVA-2: Spacesuit purge
12:56 AM...05...18...35...EVA-2: Spacesuit prebreathe
01:46 AM...05...19...25...EVA-2: Crew lock depressurization
02:16 AM...05...19...55...EVA-2: Spacesuits to battery power
02:21 AM...05...20...00...EVA-2: Airlock egress
02:31 AM...05...20...10...EVA-2: S1 truss ammonia tank assembly removal
03:31 AM...05...21...10...EVA-2: ATA tiedown on CETA cart
04:16 AM...05...21...55...EVA-2: P1 radiator grapple fixture beam
04:46 AM...05...22...25...EVA-2: AGB release
05:16 AM...05...22...55...EVA-2: New ATA install on S1 truss
06:01 AM...05...23...40...EVA-2/EV-1: S1 truss ATA connectors
06:16 AM...05...23...55...EVA-2/EV-2: Foot restraint relocate
06:46 AM...06...25...00...EVA-2: CETA ATA release
07:46 AM...06...01...25...EVA-2: Airlock debris shields install
08:31 AM...06...02...10...EVA-2: Cleanup and ingress
08:46 AM...06...02...25...EVA-2: Airlock repressurization
09:01 AM...06...02...40...Spacesuit servicing
11:00 AM...06...04...39...Mission status briefing on NTV
11:06 AM...06...04...45...ISS daily planning conference
01:21 PM...06...07...00...ISS crew sleep begins
01:51 PM...06...07...30...STS crew sleep begins
02:00 PM...06...07...39...Daily highlights reel on NTV
03:00 PM...06...08...39...Shuttle booster video replay
07:00 PM...06...12...39...Flight director's update on NTV
09:51 PM...06...15...30...Crew wakeup

Additional coverage for subscribers:
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

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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