Running ahead of schedule, astronauts John Grunsfeld and Andrew Feustel are suiting up for the Atlantis crew's fifth and final spacewalk today, a six-hour excursion to equip the Hubble Space Telescope with a second three-battery power pack, a refurbished fine guidance sensor and fresh equipment bay insulation panels. One panel was deferred from a spacewalk Sunday when the crew ran out of time. Grunsfeld and Feustel plan to install it today if time is available.

Today's EVA is the fifth and final for the Atlantis crew. Credit: NASA

Barring a future mission to drive Hubble safely out of orbit at the end of its life, this is the last time any astronauts will touch the space telescope and the end of today's spacewalk promises to be an emotional moment for Grunsfeld, an astronomer-astronaut and self-described "Hubble hugger" making his third visit to the iconic observatory.

In a news conference before launch, asked he he planned to "leave his mark" on Hubble, Grunsfeld said yes, "I think we all plan to leave our mark. And it's Wide Field Camera 3, it's Cosmic Origins Spectrograph, RSUs, batteries, fine guidance sensor." Then he joked that robot arm operator Megan McArthur "hopes to leave no marks at all on the telescope with the arm."

But given Grunsfeld's long history with Hubble, his role as NASA's chief scientist when he had to support former Administrator Sean O'Keefe's controversial decision to cancel this final servicing mission and his subsequent assignment to the crew when the flight was reinstated, NASA's most experienced Hubble astronaut almost certainly will have something memorable to say before returning to Atlantis' airlock.

Today's spacewalk is scheduled to begin around 9:16 a.m., although the crew was running well ahead of schedule and hoped to get out the door early. For identification, Grunsfeld, call sign EV-1, will be wearing a spacesuit with red stripes around the legs. Feustel, call sign EV-2, will be wearing an unmarked suit. In a reversal of roles from their earlier spacewalks, Grunsfeld will ride the shuttle's robot arm while Feustel will free float.

One of the top priorities of Atlantis' servicing mission was to install six fresh nickel-hydrogen batteries to replace a degraded set that has been in place since launch in 1990. The first three-battery power module was installed in equipment bay 2 during the crew's second spacewalk Friday.

Today, the final power module will be installed in bay 3. Feustel will prepare the new battery pack for removal from its transport container, removing 12 bolts holding it in place, while Grunsfeld, on the end of the shuttle's robot arm, opens the bay 3 door on Hubble and removes the old batteries, releasing 14 bolts and six electrical cables.

McArthur, operating the robot arm, will move Grunsfeld back down to the cargo bay to swap battery packs with Feustel, who will store the old set for return to Earth while Grunsfeld installs the new set on the telescope.

The next item on the agenda is installation of a refurbished fine guidance sensor, one of three that provides data to the telescope's computer system to help lock onto and track its astronomical targets.

"There are lots of ways that the pointing system on Hubble acquires targets, locks in on those targets, and then allows the science to be produced," Grunsfeld said in a NASA interview. "It has some eyeballs on the outside of the telescope, which are the fixed head star trackers, and they kind of give Hubble that 'Snoopy' look. They acquire the general orientation of the telescope just as the ancient mariners did by looking up at the sky as they were crossing the ocean.

"Once you get into the vicinity of where you want to look the fixed head star trackers' job is done and the fine guidance sensors take over. And what they do is they steal a little bit of light from the primary mirror. The light goes up from the primary mirror to the secondary and through a hole in the middle of the primary mirror, and the fine guidance sensors take that light, look at stars, identify them, lock onto them, and that's how the telescope points in the right place in the sky. And then the rate sensor units take care of all the little oscillations and disturbances. All these parts work together."

Grunsfeld and Feustel first will remove the old FGS 2, opening an access door, attaching a handling fixture, disconnecting electrical cables and attachment clamps and pulling the 900-pound unit out along its guide rails.

The old FGS will be temporarily mounted on an access platform along the left wall of the payload bay. The replacement FGS then will be removed from its transport container and moved to Hubble for installation. After bolting it in place and making the required electrical connections, Grunsfeld and Feustel will retrieve the old FGS and mount it in the transport container for return to Earth.

The flight plan originally called for installation of a new outer blanket layer - NOBL - insulation panel on equipment bay 8 Sunday, during the crew's fourth spacewalk, followed by a NOBL panel for bay 5 today. But the NOBL 8 panel was deferred Sunday when work to repair Hubble's Space Telescope Imaging Spectrograph ran longer than expected.

The spacewalkers hope to install both NOBLs today, on equipment bays 5 and 8, if time is available.

Grunsfeld described the NOBL panels as "big pizza sheets that we're going to put on the outside of the telescope where the insulation has become damaged and is peeling up and the stuff inside is either getting too hot or too cold. So we're going to fix those items."

Because this is the crew's fifth spacewalk in a row, flight controllers do not want it to run long because of concern about crew fatigue. Grunsfeld said Sunday, however, that the crew is in good shape and eager to complete the work.

Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision F of the NASA television schedule):

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

05:31 AM...06...15...30...Crew wakeup
06:46 AM...06...16...45...EVA-5: Preparations begin
08:16 AM...06...18...15...EVA-5: Spacesuit purge
08:26 AM...06...18...25...EVA-5: Spacesuit pre-breathe
09:06 AM...06...19...05...EVA-5: Airlock depressurization
09:16 AM...06...19...15...EVA-5: Spacesuits to battery power
09:21 AM...06...19...20...EVA-5: Airlock egress and setup
09:46 AM...06...19...45...EVA-5: Bay 3 battery R&R
11:06 AM...06...21...05...HST: Battery aliveness test
11:16 AM...06...21...15...EVA-5: FGS-2 replacement
12:41 PM...06...22...40...HST: FGS-2 aliveness test
01:16 PM...06...23...15...EVA-5: NOBL 5 (NOBL8 if time is available)
01:46 PM...06...23...45...EVA-5: Cleanup and airlock ingress
02:46 PM...07...00...45...HST high gain antenna deploy (1)
03:01 PM...07...01...00...EVA-5: Airlock repressurization
03:11 PM...07...00...10...Spacesuit servicing
03:21 PM...07...01...20...HST: Solar arrays slewed to 90 degrees
03:41 PM...07...01...40...HST: Battery functional test
04:00 PM...07...01...59...Mission status briefing on NTV
04:16 PM...07...02...15...LIOH and battery config
04:26 PM...07...02...25...Spacesuit swap
05:11 PM...07...03...10...HST high gain antenna deploy (2)
05:11 PM...07...03...10...Rendezvous tools checkout
05:56 PM...07...03...55...HST: FGS-2 functional test
06:16 PM...07...04...15...HD downlink opportunithy
08:31 PM...07...06...30...Crew sleep begins
08:45 PM...07...06...44...HST update on NTV
09:00 PM...07...06...59...Daily highlights reel on NTV
10:01 PM...07...08...00...HST: SSR engineering playback

Additional coverage for subscribers:
VIDEO: NARRATED PREVIEW OF SPACEWALK NO. 5 PLAY

VIDEO: FLIGHT DAY 7 HIGHLIGHTS MOVIE PLAY
VIDEO: SUNDAY NIGHT UPDATE FROM HST CONTROL CENTER PLAY
VIDEO: SUNDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: SPACE TELESCOPE IMAGING SPECTROGRAPH WORK DONE PLAY
VIDEO: OPENING COVER ON THE ELECTRONICS MODULE ON STIS PLAY
VIDEO: SPACEWALKER MIKE MASSIMINO REMOVES TINY SCREWS PLAY
VIDEO: TRIP FROM AIRLOCK BACK WITH SPARE POWER TOOL PLAY
VIDEO: CAPTURE PLATE MOUNTED ONTO STIS POWER MODULE PLAY
VIDEO: MASSIMINO FIGHTS WITH STUCK BOLT ON HANDRAIL PLAY
VIDEO: SPACEWALKERS GET ORGANIZED FOR STIS REPAIR PLAY
VIDEO: NARRATED PREVIEW OF SPACEWALK NO. 4 PLAY
VIDEO: FLIGHT DIRECTOR'S PREVIEW OF MISSION'S DAY 7 PLAY

VIDEO: FLIGHT DAY 6 HIGHLIGHTS MOVIE PLAY
VIDEO: SATURDAY NIGHT UPDATE FROM HST CONTROL CENTER PLAY
VIDEO: SATURDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: REPAIR OF ADVANCED CAMERA FOR SURVEYS FINISHED PLAY
VIDEO: WATCH AS GRUNSFELD REPAIRS HUBBLE CAMERA PLAY
VIDEO: COSTAR STOWED IN ATLANTIS' PAYLOAD BAY PLAY
VIDEO: UMBILICALS HOOKED UP TO NEW INSTRUMENT PLAY
VIDEO: COSMIC ORIGINS SPECTROGRAPH INSTALLED PLAY
VIDEO: SPECTROGRAPH PREPPED IN LAUNCH CARRIER PLAY
VIDEO: COSTAR OPTICS PACKAGE REMOVED FROM HUBBLE PLAY
VIDEO: SPACEWALKERS OPEN SHROUD DOORS ON TELESCOPE PLAY
VIDEO: NARRATED PREVIEW OF SPACEWALK NO. 3 PLAY
VIDEO: FLIGHT DIRECTOR'S PREVIEW OF MISSION'S DAY 6 PLAY

VIDEO: FLIGHT DAY 5 HIGHLIGHTS MOVIE PLAY
VIDEO: FRIDAY NIGHT UPDATE FROM HST CONTROL CENTER PLAY
VIDEO: FRIDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: SPARE GYRO UNIT INSTALLED INTO THIRD POSITION PLAY
VIDEO: ANOTHER GYROSCOPE UNIT PUT INTO SECOND SLOT PLAY
VIDEO: TROUBLE INSTALLING SECOND GYROSCOPE PACKAGE PLAY
VIDEO: REMOVAL OF SECOND GYROSCOPE PACKAGE FROM HUBBLE PLAY
VIDEO: INSTALLATION OF FIRST GYROSCOPE INTO HUBBLE PLAY
VIDEO: REMOVAL OF FIRST GYROSCOPE PACKAGE FROM HUBBLE PLAY
VIDEO: NARRATED PREVIEW OF SPACEWALK NO. 2 PLAY
VIDEO: FLIGHT DIRECTOR'S PREVIEW OF MISSION'S DAY 5 PLAY

VIDEO: FLIGHT DAY 4 HIGHLIGHTS MOVIE PLAY
VIDEO: THURSDAY NIGHT UPDATE FROM HST CONTROL CENTER PLAY
VIDEO: THURSDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: SOFT CAPTURE MECHANISM PUT ON HUBBLE PLAY
VIDEO: SCIENCE INSTRUMENT COMMAND & DATA HANDLING UNIT PLAY
VIDEO: PARTIALLY FAILED SIC&DH UNIT REMOVED FROM HST PLAY
VIDEO: WFPC2 CAMERA STOWED IN ATLANTIS FOR RETURN PLAY
VIDEO: OVERVIEW OF WIDE FIELD CAMERA 3 INSTRUMENT PLAY
VIDEO: SPACEWALKERS INSTALL THE WIDE FIELD CAMERA 3 PLAY
VIDEO: NEW WIDE FIELD CAMERA 3 PREPPED FOR INSTALLATION PLAY
VIDEO: 16-YEAR-OLD WFPC2 CAMERA REMOVED FROM HUBBLE PLAY
VIDEO: LATCHING BOLT ON WPFC2 FINALLY RELEASED PLAY
VIDEO: FIGHTING WITH TROUBLESOME BOLT ON WPFC2 PLAY
VIDEO: FEUSTEL INSTALLS HANDLE ONTO WPFC2 FOR REMOVAL PLAY
VIDEO: HANDLING FIXTURE DEPLOYED TO HOLD OLD CAMERA PLAY
VIDEO: GRUNSFELD INSTALLS SUPPORT POSTS UNDER HST PLAY
VIDEO: FEUSTEL OPENS LATCHES ON INSTRUMENT CARRIER PLAY
VIDEO: ASTRONAUTS EXIT AIRLOCK TO START EVA NO. 1 PLAY
VIDEO: NARRATED PREVIEW OF SPACEWALK NO. 1 PLAY

VIDEO: FLIGHT DAY 3 HIGHLIGHTS MOVIE PLAY
VIDEO: WEDNESDAY NIGHT UPDATE FROM HST CONTROL CENTER PLAY
VIDEO: WEDNESDAY'S MISSION MANAGEMENT TEAM UPDATE PLAY
VIDEO: WEDNESDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: TIME-LAPSE OF MOVING HUBBLE INTO PAYLOAD BAY PLAY
VIDEO: SHUTTLE ATLANTIS' ROBOT ARM CAPTURES HUBBLE PLAY
VIDEO: FIRST UP CLOSE VIEWS OF HUBBLE IN 7 YEARS PLAY
VIDEO: SHUTTLE ENGINE FIRING TO CIRCULARIZE ORBIT PLAY
VIDEO: INSIGHTFUL OVERVIEW OF ATLANTIS' MISSION PLAY
VIDEO: NARRATED PREVIEW ANIMATION OF HUBBLE RENDEZVOUS PLAY

VIDEO: FLIGHT DAY 2 HIGHLIGHTS MOVIE PLAY
VIDEO: TUESDAY NIGHT UPDATE FROM HST CONTROL CENTER PLAY
VIDEO: TUESDAY'S MISSION MANAGEMENT TEAM UPDATE PLAY
VIDEO: SLOW-MOTION OF DEBRIS THAT APPARENTLY HIT TILES PLAY
VIDEO: TILE DAMAGE FOUND DURING INSPECTIONS PLAY
VIDEO: CAPCOM CALLS CREW ABOUT MINOR DAMAGE PLAY
VIDEO: TUESDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: PREVIEW ANIMATION OF HEAT SHIELD INSPECTIONS PLAY

VIDEO: FLIGHT DAY 1 HIGHLIGHTS MOVIE PLAY
VIDEO: THE FULL STS-125 LAUNCH EXPERIENCE PLAY
VIDEO: LAUNCH REPLAY: EXTERNAL TANK CAMERA PLAY
VIDEO: FOOTAGE OF FUEL TANK AFTER JETTISON PLAY

VIDEO: LAUNCH REPLAY: FRONT CAMERA PLAY
VIDEO: LAUNCH REPLAY: BEACH TRACKER PLAY
VIDEO: LAUNCH REPLAY: PAD PERIMETER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: UCS-23 TRACKER PLAY
VIDEO: LAUNCH REPLAY: CAMERA CS-1 PLAY
VIDEO: LAUNCH REPLAY: CAMERA CS-2 PLAY
VIDEO: LAUNCH REPLAY: CAMERA CS-6 PLAY
VIDEO: LAUNCH REPLAY: VAB ROOF PLAY
VIDEO: LAUNCH REPLAY: PRESS SITE PLAY
VIDEO: LAUNCH REPLAY: WEST TOWER PLAY
VIDEO: LAUNCH REPLAY: CAMERA 009 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 041 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 049 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 050 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 051 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 060 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 061 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 063 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 070 PLAY
VIDEO: LAUNCH REPLAY: CAMERA 071 PLAY

VIDEO: SPACE SHUTTLE ATLANTIS BLASTS OFF! PLAY | HI-DEF
VIDEO: STS-125 POST-LAUNCH NEWS CONFERENCE PLAY
VIDEO: INSIDE MISSION CONTROL DURING LAUNCH PLAY
VIDEO: NARRATED ANIMATION OF DAY 1 INSPECTIONS PLAY

VIDEO: FINAL PRE-LAUNCH READINESS POLLS CONDUCTED PLAY
VIDEO: SHUTTLE'S CREW MODULE HATCH CLOSED FOR FLIGHT PLAY
VIDEO: MISSION SPECIALIST MEGAN MCARTHUR BOARDS PLAY
VIDEO: PILOT GREG JOHNSON BOARDS ATLANTIS PLAY
VIDEO: MISSION SPECIALIST JOHN GRUNSFELD BOARDS PLAY
VIDEO: SHUTTLE COMMANDER SCOTT ALTMAN BOARDS PLAY
VIDEO: CREW DEPARTS QUARTERS FOR LAUNCH PAD PART 1 | PART 2
VIDEO: ASTRONAUTS DON SPACESUITS FOR LAUNCH PLAY
VIDEO: TIME-LAPSE OF LAUNCH PAD GANTRY ROLLBACK PLAY

VIDEO: HUBBLE'S FUTURE AFTER ATLANTIS SERVICING PLAY
VIDEO: TOP DISCOVERIES MADE BY HUBBLE TELESCOPE PLAY
VIDEO: THE ROLLERCOASTER LIFE OF HUBBLE PLAY
VIDEO: WHAT IS HUBBLE'S PLACE IN HISTORY? PLAY
VIDEO: ENTERTAINING LOOK AT ATLANTIS' CREW PLAY
VIDEO: HUBBLE SPACE TELESCOPE HISTORY MOVIE PLAY
VIDEO: A TRIBUTE TO THE CAMERA THAT SAVED HUBBLE PLAY
VIDEO: HUBBLE TELESCOPE PROGRAM BRIEFING FROM SUNDAY PLAY

VIDEO: SUNDAY'S COUNTDOWN STATUS AND WEATHER UPDATE PLAY
VIDEO: THE STS-125 PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: SATURDAY'S COUNTDOWN STATUS AND WEATHER UPDATE PLAY
VIDEO: ASTRONAUTS ARRIVE AT THE CAPE FOR LAUNCH PLAY
VIDEO: FRIDAY'S COUNTDOWN STATUS AND WEATHER UPDATE PLAY

VIDEO: NEWS BRIEFING FOLLOWING FLIGHT READINESS REVIEW PLAY

VIDEO: REFRESHER BRIEFING: THE STS-125 MISSION PLAY
VIDEO: REFRESHER BRIEFING: HUBBLE SPACE TELESCOPE PLAY
VIDEO: REFRESHER BRIEFING: THE SEVEN ASTRONAUTS PLAY

VIDEO: BIOGRAPHY MOVIE OF ATLANTIS' CREW PLAY
VIDEO: INTERVIEW WITH COMMANDER SCOTT ALTMAN PLAY | '09 UPDATE
VIDEO: INTERVIEW WITH PILOT GREG JOHNSON PLAY
VIDEO: INTERVIEW WITH MS1 MIKE GOOD PLAY
VIDEO: INTERVIEW WITH MS2 MEGAN MCARTHUR PLAY
VIDEO: INTERVIEW WITH MS3 JOHN GRUNSFELD PLAY | '09 UPDATE
VIDEO: INTERVIEW WITH MS4 MIKE MASSIMINO PLAY | '09 UPDATE
VIDEO: INTERVIEW WITH MS5 DREW FEUSTEL PLAY | '09 UPDATE

VIDEO: SHUTTLE ATLANTIS ARRIVES AT LAUNCH PAD 39A PLAY | HI-DEF
VIDEO: THE SUN RISES ON ATLANTIS DURING ROLLOUT PLAY | HI-DEF
VIDEO: ATLANTIS EMERGES FROM ASSEMBLY BUILDING PLAY | HI-DEF
VIDEO: ORBITER VERTICAL FOR MATING TO FUEL TANK PLAY | HI-DEF
VIDEO: ATLANTIS MOVES BACK TO ASSEMBLY BUILDING PLAY | HI-DEF

VIDEO: ATLANTIS LEAVES LAUNCH PAD 39A IN OCT. PLAY
VIDEO: TIMELAPSE OF THE ROLLBACK GETTING UNDERWAY PLAY

VIDEO: ASTRONAUTS REHEARSE QUICK SHUTTLE EXIT PLAY
VIDEO: CREW BOARDS ATLANTIS FOR MOCK COUNTDOWN PLAY
VIDEO: ASTRONAUTS DON SUITS FOR PRACTICE COUNTDOWN PLAY
VIDEO: CREW'S TRAINING ON ESCAPE BASKETS AND BUNKER PLAY
VIDEO: ASTRONAUTS CHAT WITH PRESS AT LAUNCH PAD PLAY
VIDEO: CREW TEST DRIVES EMERGENCY PAD ESCAPE VEHICLE PLAY
VIDEO: COMMANDER AND PILOT PRACTICE LANDING APPROACHES PLAY
VIDEO: CREW ARRIVES AT CAPE FOR PRACTICE COUNTDOWN PLAY
VIDEO: COMMENTS FROM COMMANDER AFTER ARRIVING PLAY

VIDEO: AERIAL VIEWS OF ATLANTIS AND ENDEAVOUR PLAY | HI-DEF

VIDEO: PRE-FLIGHT BRIEFING: THE STS-125 MISSION PLAY
VIDEO: PRE-FLIGHT BRIEFING: THE FIVE SPACEWALKS PLAY
VIDEO: PRE-FLIGHT BRIEFING: THE SEVEN ASTRONAUTS PLAY
VIDEO: PRE-FLIGHT BRIEFING: NASA LEADERSHIP PLAY
VIDEO: PRE-FLIGHT BRIEFING: SHUTTLE PROGRAM BOSS PLAY
VIDEO: PRE-FLIGHT BRIEFING: HUBBLE OVERVIEW PLAY
VIDEO: PRE-FLIGHT BRIEFING: HUBBLE SCIENCE PLAY

VIDEO: SHUTTLE ARRIVES ATOP PAD 39A PLAY
VIDEO: ATLANTIS EMERGES FROM VEHICLE ASSEMBLY BUILDING PLAY
VIDEO: TIME-LAPSE MOVIE OF ATLANTIS' ROLLOUT TO LAUNCH PAD PLAY

VIDEO: ATLANTIS ROLLS FROM HANGAR TO VAB PLAY
VIDEO: ATLANTIS LIFTED INTO PLACE FOR ATTACHMENT PLAY
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