NASA managers Tuesday approved plans for a second spacewalk Wednesday to deal with an unexpected ammonia leak that blocked removal of a faulty coolant system pump aboard the International Space Station during a spacewalk Saturday. If the plan works, the old pump will be removed and a new pump installed and hooked up during a third spacewalk Sunday.

Credit: NASA

Astronauts Douglas Wheelock and Tracy Caldwell Dyson planned to spend the night in the station's Quest airlock module at a reduced pressure of 10.2 pounds per square inch to help purge nitrogen from their bloodstreams before the planned 6:55 a.m. EDT start of the six-hour spacewalk.

NASA managers decided late Tuesday, however, to delay the start of the EVA by one hour, letting the astronauts get a bit more sleep and giving engineers more time to fine-tune robotics procedures. Under the revised timeline, Wheelock and Caldwell Dyson will switch their spacesuits to battery power at 7:55 a.m. to officially kick off the excursion.

For identification, Wheelock, call sign EV-1, will be wearing a suit with red stripes around the legs. Caldwell Dyson, EV-2, will wear an unmarked suit. This will be the fifth spacewalk for Wheelock, the second for Caldwell Dyson, the 13th station EVA so far this year and the 149th since assembly began in 1998.

The ammonia pump in coolant loop A failed July 31, leaving the space station with just one coolant system to dissipate the heat generated by the lab's electronics. The loop A failure forced the astronauts to implement a widespread powerdown to prevent critical equipment from overheating.

Four spare pump modules are on board and NASA planners quickly developed plans for two spacewalks to remove the old pump and install a replacement. But those plans went awry Saturday when the spacewalkers ran into major problems disconnecting one of four ammonia lines.

After struggling to close a quick-release fitting on the M3 line, the astronauts reported a significant leak. The quick-disconnect was reopened and the spacewalkers headed back to the airlock for routine decontamination procedures.

Engineers suspect a problem with one of two internal valves on the outboard side of the pump module.

For the second spacewalk, Wheelock and Caldwell Dyson first will close quick-disconnect valves on both ends of the starboard one, or S1, truss segment where the pump module is mounted. With those valves closed, the ammonia line leading to the leaking connector will be isolated.

Flight controllers planned to reduce pressure in the line before the spacewalk began, clearing the way for Wheelock and Caldwell Dyson to install a vent tool to release any residual ammonia trapped in the pump and its plumbing.

Then, if all goes well, the now-evacuated connector can be disconnected and moved out of the way. After unplugging five electrical cables, Wheelock and Caldwell Dyson will loosen four bolts and remove the old pump module, temporarily mounting it on a stowage fixture at the base of the station's robot arm transporter.

The replacement pump module would be installed and reconnected during a third spacewalk Sunday. The quick-disconnect valves that were closed to isolate the leaking connector would be reopened at that point and flight controllers would be free to re-activate coolant loop A.

Flight controllers are considering an alternative scenario for Wednesday's spacewalk that includes an initial attempt to simply close the M3 quick disconnect in hopes that whatever caused the leak Saturday is no longer present. If no leak is observed, Wheelock could detach the M3 ammonia line straight away and press ahead with the pump's removal.

If the leak reappears, the astronaut would reopen the quick-disconnect and press ahead with work to isolate that section of the line. It is not yet clear whether flight controllers will, in fact, clear Wheelock to spend the time needed to reach the M3 valve as opposed to simply isolating the line right away as currently planned.

Here is an updated timeline of major events (in EDT and elapsed time; best viewed with fixed-width font):

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

03:00 AM.............Crew wakeup
03:35 AM.............Crew hygiene break
04:45 AM.............EVA preps continue
06:15 AM.............Spacesuit purge begins
07:20 AM.............Airlock depressurization begins
07:55 AM...00...00...Suits to battery power
08:00 AM...00...05...Post depress/airlock egress
08:15 AM...00...20...Setup
08:40 AM...00...45...EV-2: Close S1 outboard quick-disconnect
08:55 AM...01...00...EV-1: Vent line routing
09:25 AM...01...30...EV-1: Inboard S0/S1 jumper QD closed
09:40 AM...01...45...EV-2: M1 ammonia line restrained
09:55 AM...02...00...EV-2: Start venting
10:05 AM...02...10...EV-1: Close M2 ammonia line
10:35 AM...02...40...EV-1: Close vent tool
10:40 AM...02...45...EV-2: Vent tool cleanup
10:50 AM...02...55...EV-1: Close and demate M3 ammonia line
11:00 AM...03...05...EV-2: Cover and restrain M3
11:10 AM...03...15...EV-1: Break torque on failed pump module bolts
11:30 AM...03...35...EV-1: Retrieve adjustable grapple bar
11:30 AM...03...35...EV-2: Release failed PM electrical cables
12:00 AM...04...05...EV-2: Release failed PM bolts
12:15 AM...04...20...EV-1: Remove failed PM with AGB
12:25 AM...04...30...EV-2: Remove failed PM
01:00 PM...05...05...EV-1: Move failed PM to stowage fixture
01:10 PM...05...15...EV-2: Break torque on new PM; cleanup
01:30 PM...05...35...EV-1: Cleanup/airlock ingress
01:55 PM...06...00...Airlock pre-pressurization