NASA managers, assessing timelines for two planned spacewalks to replace a faulty ammonia coolant pump aboard the International Space Station, decided Thursday to delay the first of two repair spacewalks, from Friday to Saturday, to give planners more time to fine-tune the required procedures.

The second spacewalk, which had been tentatively targeted for Monday, will slip an additional two days to Wednesday. Both spacewalks will still begin around 6:55 a.m. EDT.

There are no major issues or "show stoppers," officials said, but spacewalk planners needed more time to work out the details, including what to do in a variety of contingency situations.

"The Team 4 group has decided we will move the EVA to Saturday," Mike Jensen radioed the astronauts from mission control at the Johnson Space Center. "Expect for the three of you working the EVA stuff, we'll have probably a half-duty day tomorrow focussed on EVA prep."

"Understand all, Mike, thank you for the words," astronaut Douglas Wheelock replied from the station. "And thanks for the hard work. We know people are still working in the evenings and through the night and we really appreciate it. Thanks."

Wheelock, call sign EV-1, and Tracy Caldwell Dyson, EV-2, will carry out both spacewalks, assisted by Shannon Walker, operating the station's robot arm from inside the Destiny laboratory module. The station's three Russian crew members - commander Alexander Skvortsov, Mikhail Kornienko and Fyodor Yurchikhin - will not be directly invovled in the repair work.

The 780-pound ammonia pump module failed Saturday, knocking one of the space station's two coolant loops out of action and forcing flight controllers to implement a major powerdown to keep critical equipment from overheating.

The faulty loop A pump module is mounted in the starboard one - S1 - segment of the station's main power truss. An identical module is mounted on the port one, or P1, segment. Four spare pump modules are mounted on various external storage platforms.

The preliminary plan calls for Wheelock and Caldwell Dyson to remove the faulty S1 pump module during the initial spacewalk Saturday, disconnecting a half-dozen electrical cables and four ammonia lines. The first two ammonia lines to be disconnected - M3 and M4 - will be connected to a "jumper box" that will prevent the loop A coolant system from over pressurizing during temperature swings in orbit.

With the jumper box in place, the old pump will be removed and temporarily mounted on a nearby fixture. Caldwell Dyson, the designated 'free floater," and Wheelock, anchored to the end of the station's robot arm, then will retrieve the replacement pump from external storage platform No. 2 just in front of the Quest airlock and bolt it into place on the S1 truss. The astronauts plan to wrap up the excursion by reconnecting electrical and data lines.

During the second spacewalk Wednesday, Wheelock and Caldwell Dyson will reconnect the ammonia lines to finish the repair.

The ammonia lines must be disconnected and reinstalled in a specific order and the astronauts must guard against any leaks that might contaminate their spacesuits. Flight planners are fine tuning procedures to deal with any troublesome quick-disconnect fittings that might crop up.

Delaying the spacewalks to Saturday and Wednesday "will allow for an additional day of time for the teams, who have been working twenty four hours around the clock, to prepare these procedures, make sure all of the safety blocks are checked in terms of the disconnect and reconnect of the cables and quick disconnects associated with these ammonia coolant lines," said NASA spokesman Kyle Herring.

"All of that is coming together, the teams just need a little more time to refine all of that," he said. "So the plan now is for the two crew members to begin their preparations ... tomorrow afternoon rather than today."

Here is the latest timeline for the first spacewalk Saturday (in EDT and elapsed time):

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

06:55 AM...00...00...Spacesuits to battery power
07:00 AM...00...05...Post depress/airlock egress
07:25 AM...00...30...Tools/tether setup
07:45 AM...00...50...EV-1: Pump module (PM) removal preps
07:55 AM...01...00...EV-2: CETA cart preps
08:35 AM...01...40...EV-1: Robot arm (SSRMS) prep and ingress
08:45 AM...01...50...EV-2: PM jumper vent
09:00 AM...02...05...Initial PM quick-disconnect ops
10:05 AM...03...10...EV-1: Retrieve adjustable grapple bar (AGB)
10:05 AM...03...10...EV-2: Prep failed PM for removal
10:50 AM...03...55...Remove failed PM
11:10 AM...04...15...EV-1: Move failed PM to POA fixture
11:10 AM...04...15...EV-2: Spare PM preps
11:30 AM...04...35...EV-1: Spare PM preps
11:50 AM...04...55...Deadline for beginning spare PM removal
12:20 PM...05...25...Remove spare PM from external storage platform No. 2
12:50 PM...05...55...Install spare PM in S1 truss
01:25 PM...06...30...Cleanup and ingress
01:50 PM...06...55...Pre-airlock repress
01:55 PM...07...00...Airlock repress