Astronauts cleared for third coolant system repair EVA

Astronauts Douglas Wheelock and Tracy Caldwell Dyson are gearing up for a third spacewalk Monday to install a replacement ammonia pump that should allow flight controllers to reactivate a critical space station coolant loop that broke down two weeks ago.

Wheelock and Caldwell Dyson planned to spend the night in the Quest airlock at a reduced pressure of 10.2 pounds per square inch to help purge nitrogen from their bloodstreams. They plan to switch their spacesuits to battery power around 6:55 a.m. EDT to officially kick off a planned six-hour spacewalk.

This will be the 150th EVA devoted to space station assembly and maintenance since construction began in 1998, the 14th so far this year, the sixth overall for Wheelock and the third for Caldwell Dyson. Going into today's outing, ISS spacewalk time stood at 937 hours and four minutes, or 39 days.

For identification, Wheelock, call sign EV-1, will be wearing a suit with red stripes around the legs and use helmet cam No. 19. Caldwell Dyson, EV-2, will wear an unmarked suit and use helmet cam No. 20.

Coolant loop A, one of two thermal control systems used to dissipate heat generated by the space station's electronics, failed when its ammonia pump shorted out July 31. The lab's six-member crew was forced to implement a widespread powerdown, relying on loop B to cool life support systems, communications gear and other critical gear.

Wheelock and Caldwell Dyson originally planned two spacewalks, one to remove the old pump and install a spare and a second to complete the required ammonia line connections. But during their first excursion Aug. 7, one of four ammonia lines leaked, preventing the faulty pump's removal.

Before a second spacewalk Aug. 11, flight controllers lowered pressure in the line to stop the leak and Wheelock, applying a bit of elbow grease, was able to disconnect the balky M3 connector. The astronauts then unbolted the failed pump and Wheelock, anchored to the end of the station's robot arm, moved it to an attachment fitting at the base of the arm's mobile transporter.

During their third spacewalk Monday, Wheelock and Caldwell Dyson plan to remove a spare pump module from external stowage platform No. 2, install it in the starboard one, or S1, truss segment and reconnect the ammonia lines and five electrical cables. The spare pump, launched to the station in 2006, is the oldest of four stored aboard the space station as a hedge against coolant system failures.

If time is available, the astronauts will install an electrical cable between the Quest airlock and the Unity module that is required before attachment of a cargo module during a shuttle flight in November.

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

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

06:55 AM...00...00...Suits to battery power
07:00 AM...00...05...Post depress/airlock egress
07:15 AM...00...20...EV-1: Tether swap; ingress robot arm (SSRMS); loosen four
.....................bolts on spare pump module (PM); release three bolts
07:15 AM...00...20...EV-2: Assist EV-1; remove caps
08:15 AM...01...20...EV-1: Release final bolt; remove spare PM from external
.....................stowage platform No. 2 (ESP-2)
08:15 AM...01...20...EV-2: Assist EV-1; cleanup ESP-2
08:45 AM...01...50...EV-1: PM install in S1 truss; engage four bolts
08:45 AM...01...50...EV-2: Mate electrical cables
09:45 AM...02...50...EV-1: Mate PM quick disconnects (QDs); open valves;
.....................install SPDs
09:45 AM...02...50...EV-2: Assist with QDs
11:10 AM...04...15...EV-1: Retrieve vent tool, PM stowage bags; move to ESP-2
11:10 AM...04...15...EV-2: Cleanup S1 tethers; couple CETA carts
11:40 AM...04...45...EV-1: Remove SSRMS foot restraint; install on ESP-2
12:10 PM...05...15...EV-1: Install J612 extension cable between Quest and Unity
.....................for cargo module attachment during November shuttle flight
12:10 PM...05...15...EV-2: J612 extension cable install
12:55 PM...06...00...Cleanup/airlock ingress
01:20 PM...06...25...Airlock pre-pressurization

Flight controllers plan to electrically activate the new pump while the spacewalk is underway to make sure it is working properly. But it will take several days to reactivate coolant loop A and bring up dormant electrical systems.

A fourth spacewalk may be required later to move the failed ammonia pump to ESP-2. But no decisions have been made about when the pump might be moved.