Floating in the space station's Quest airlock module, astronauts Richard Mastracchio and Clayton Anderson switched their spacesuits to battery power at 1:31 a.m. EDT Friday to officially kick off a planned six-and-a-half-hour spacewalk.

"Gentlemen, the weather's beautiful outside, it's a great day for an EVA," pilot James Dutton radioed from outside the airlock. "Have a great time, guys."

This is the 141st EVA devoted to station assembly and maintenance since construction began in 1998, the fifth so far this year and the first of three planned by the Discovery astronauts. It is the fourth spacewalk overall for both Mastracchio and Anderson, who carried out an EVA together in August 2007.

The primary goal of today's spacewalk is to move a new ammonia coolant tank from the shuttle Discovery's cargo bay to a temporary storage location on the space station's power truss and to prepare a depleted tank for removal.

The astronauts also will replace a rate gyro assembly, a device that helps the station's computers determine the lab's orientation in space, along with performing a few "get-ahead" tasks for upcoming assembly work.

Additional coverage for subscribers:
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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