Astronauts Douglas Wheelock and Tracy Caldwell Dyson, floating in the Quest airlock module, switched their spacesuits to internal battery power at 8:27 a.m. to officially kick off a planned six-hour spacewalk. The primary goal of the excursion is to remove a shorted ammonia pump module so a replacement can be installed during a spacewalk Sunday.

This is the 149th EVA devoted to space station assembly and maintenance since construction began in 1998, the 13th so far this year, the fifth overall for Wheelock and the second for Caldwell Dyson. Going into today's outing, ISS spacewalk time stood at 929 hours and 38 minutes, or 38.7 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.

The astronauts attempted to remove the faulty pump during a spacewalk Saturday, but one of four ammonia line quick-disconnect fittings leaked, presumably because of problems with an internal valve on the outboard side of the fitting.

Today, Wheelock will make another attempt to simply close the M3 quick-disconnect fitting. If the leak persists, the astronauts plan to close quick-disconnect valves at the inboard and outboard ends of the starboard one, or S1, truss where the pump module is located to isolate the ammonia line leading to the leaking connector.

Flight controllers earlier lowered pressure in the system and after the astronauts install a vent tool to get rid of any residual ammonia trapped in the line, the M3 quick disconnect will be closed and removed. That should clear the way for disconnection of five electrical umbilicals. After loosening four bolts, the failed pump module will be removed and temporarily mounted on a powered attach fitting at the base of the station's robot arm transporter.

If all goes well, a spare pump will be installed and reconnected during a third spacewalk Sunday.

Here is an updated timeline for today's spacewalk based on the official start time (in EDT and elapsed time; best viewed with fixed-width font):

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

08:27 AM...00...00...Suits to battery power
08:32 AM...00...05...Post depress/airlock egress
08:47 AM...00...20...Setup
09:12 AM...00...45...EV-2: Close S1 outboard QD
09:27 AM...01...00...EV-1: Vent line routing
09:57 AM...01...30...EV-1: S0/S1 inboard jumper QD closed
10:12 AM...01...45...EV-2: M1 ammonia line restrained
10:27 AM...02...00...EV-2: Start venting
10:37 AM...02...10...EV-1: Close M2 ammonia line
11:07 AM...02...40...EV-1: Close vent tool
11:12 AM...02...45...EV-2: Vent tool cleanup
11:22 AM...02...55...EV-1: Close and demate M3 ammonia line
11:32 AM...03...05...EV-2: Cover and restrain M3
11:42 AM...03...15...EV-1: Break torque on failed pump module bolts
12:02 PM...03...35...EV-1: Retrieve adjustable grapple bar
12:02 PM...03...35...EV-2: Release failed PM electrical cables
12:32 PM...04...05...EV-2: Release failed PM bolts
12:47 PM...04...20...EV-1: Remove failed PM with AGB
12:57 PM...04...30...EV-2: Remove failed PM
01:32 PM...05...05...EV-1: Move failed PM to POA
01:42 PM...05...15...EV-2: Break torque on new PM; cleanup
02:02 PM...05...35...EV-1: Cleanup/airlock ingress
02:27 PM...06...00...Airlock pre-pressurization