Astronauts Douglas Wheelock and Tracy Caldwell Dyson made final preparations Friday for a planned spacewalk Saturday to replace a faulty ammonia pump module in the International Space Station's external cooling system.

Credit: NASA

Wheelock and Caldwell Dyson planned to shut themselves in the Quest airlock module around 4 p.m., lowering the pressure to 10.2 pounds per square inch overnight to help purge nitrogen from their bloodstreams. Crew sleep was scheduled to begin at 5:30 p.m. with wakeup at 2 a.m. Saturday.

"I just want to let everyone know we have unanimous concurrence on a 'go' for EVA tomorrow," Kathy Bolt radioed the crew from mission control at the Johnson Space Center in Houston.

"And you've got a 'go' from the crew," Wheelock replied. "Thanks, we're very excited, Kathy, that's great news. We're ready to perform."

The space station features two ammonia coolant loops to dissipate the heat generated by the lab's electronics. A short circuit knocked the loop A ammonia pump module out of action Saturday, forcing the crew to implement an extensive powerdown. NASA managers quickly decided to defer a previously planned spacewalk and to implement a pump replacement EVA instead.

Wheelock, making his fourth spacewalk, and Caldwell Dyson, making her first, trained for a pump replacement in September 2009. Engineers and astronauts at the Johnson Space Center have spent the past week refining procedures and fine-tuning the timeline.

For identification, Wheelock, call sign EV-1, will be wearing a spacesuit with red stripes while Caldwell Dyson, EV-2, will be wearing an unmarked suit. Wheelock will spend most of his time anchored to the end of the station's robot arm, operated by Shannon Walker inside the Destiny laboratory module. Caldwell Dyson will free float, using foot restraints as required.

The station's other three crew members - Russian commander Alexander Skvortsov and cosmonauts Mikhail Kornienko and Fyodor Yurchikhin - are not directly involved in the repair work.

To replace the pump module, Wheelock and Caldwell Dyson will have to disconnect five power and data lines, three 1.5-inch ammonia lines and one half-inch coolant line. Two of those lines must be quickly connected to a "jumper box" to prevent pressure extremes in the ammonia supply as the station moves into and out of Earth's shadow.

Once disconnected, the old pump will be unbolted, pulled out and moved to a powered payload attachment fitting at the base of the robot arm's mobile transporter. The astronauts then will move to external stowage platform No. 2, just in front of the Quest airlock module, to retrieve a replacement pump.

The primary goal of the spacewalk is to get the new pump module bolted in place. If time is available, Wheelock and Caldwell Dyson will reconnect the power and data lines. The ammonia lines will be connected during a spacewalk Wednesday.

Here is an updated timeline for Saturday's spacewalk (in EDT and elapsed time; best viewed with fixed-width font):

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

02:00 AM.............Crew wakeup
02:35 AM.............Hygiene break
03:45 AM.............Spacewalk preparations resume
05:15 AM.............Spacesuit purge begins
06:00 AM.............NASA television coverage begins
06:20 AM.............Airlock depressurization begins
06:55 AM...00...00...Spacesuits to battery power (official start time)
07:00 AM...00...05...Post depress/airlock egress
07:30 AM...00...35...Tools/tether setup
07:50 AM...00...55...EV-1: Pump module (PM) removal preps
08:00 AM...01...05...EV-2: CETA cart preps
08:40 AM...01...45...EV-1: Robot arm (SSRMS) prep and ingress
08:50 AM...01...55...EV-2: PM jumper vent
09:05 AM...02...10...Initial PM quick-disconnect ops
10:10 AM...03...15...EV-1: Retrieve adjustable grapple bar (AGB)
10:10 AM...03...15...EV-2: Prep failed PM for removal
10:55 AM...04...00...Remove failed PM
11:15 AM...04...20...EV-1: Failed PM to temporary stowage fixture
11:15 AM...04...20...EV-2: Spare PM preps
11:35 AM...04...40...EV-1: Spare PM preps
11:50 AM...04...55...Deadline for beginning spare PM preps/removal
12:25 PM...05...30...Remove spare PM from external storage platform 2
12:55 PM...06...00...Install spare PM in S1 truss
01:30 PM...06...35...Cleanup and ingress
01:55 PM...07...00...Pre-airlock repress
02:00 PM...07...05...Airlock repress
03:30 PM.............Post-spacewalk news briefing on NTV (approximate)

The spacewalk's duration will be driven in part by a requirement to make sure the astronauts have enough power and air at the end of the excursion for a lengthy "bake out" procedure if their suits get contaminated by leaking ammonia. The idea is to make sure no ammonia inadvertently makes it into the station's pressurized crew modules.

Because of that, multiple "breakout" points have been built into the spacewalk timeline that will leave the coolant system in a safe configuration if the astronauts run into problems and end up deferring some tasks to the second EVA.