Floating in the Quest airlock module, astronauts Douglas Wheelock and Tracy Caldwell Dyson switched their spacesuits to battery power at 7:19 a.m. EDT to officially kick off a planned seven-hour spacewalk.

Credit: NASA

The primary goal of the excursion is to replace a 780-pound ammonia pump module that failed last Saturday, knocking one of the space station's two coolant loops out of action and forcing the crew to implement an extensive powerdown.

This is the fourth spacewalk for Wheelock and the first for Caldwell Dyson. For identification, Wheelock, call sign EV-1, is wearing a suit with red stripes on the legs. He will spend most of the spacewalk anchored to the end of the station's robot arm, operated by Shannon Walker in the Destiny lab module.

Caldwell Dyson, call sign EV-2, is wearing an unmarked suit and will position herself as requireds.

The International Space Station is equipped with two independent coolant loops that circulate ammonia through giant radiators to dissipate the heat generated by the lab's electronics. Both loops are needed for normal operation.

Wheelock and Caldwell Dyson already were gearing up for a planned assembly spacewalk this past Thursday. But with the failure of the loop A coolant pump, NASA managers decided to mount two repair EVAs instead.

During today's outing, Wheelock and Caldwell Dyson plan to disconnect the faulty loop A coolant pump; install a jumper box to maintain the proper pressure in the coolant loop during the repair work; remove the old pump, temporarily stowing it on a mounting fixture; and install a replacement pump, one of four aboard the space station.

If time is available, the astronauts will reconnect five power and data cables. Four ammonia lines will be connected during a second spacewalk Wednesday.

The loop A pump module is located on the forward face of the starboard one, or S1, segment of the station's main solar power truss. The loop B pump module is located on the left side of the truss in the port 1 segment.

The replacement pump is mounted on external storage platform No. 2 just ahead of the Quest airlock.

Here is an updated timeline of today's activity (in EDT and elapsed time; best viewed with fixed-width font):

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

07:19 AM...00...00...Spacesuits to battery power
07:24 AM...00...05...Post depress/airlock egress
07:49 AM...00...30...Setup
08:09 AM...00...50...EV-1: Pump module (PM) removal preps
08:19 AM...01...00...EV-2: CETA cart preps
08:59 AM...01...40...EV-1: Robot arm (SSRMS) prep and ingress
09:09 AM...01...50...EV-2: PM jumper vent
09:24 AM...02...05...Initial PM quick-disconnect ops
10:29 AM...03...10...EV-1: Retrieve adjustable grapple bar (AGB)
10:29 AM...03...10...EV-2: Prep failed PM for removal
11:14 AM...03...55...Remove failed PM
11:34 AM...04...15...EV-1: Move failed PM to POA stowage location
11:34 AM...04...15...EV-2: Spare PM preps
11:54 AM...04...35...EV-1: Spare PM preps
12:14 PM...04...55...BINGO for spare PM removal
12:44 PM...05...25...Remove spare PM
01:14 PM...05...55...Install spare PM
01:49 PM...06...30...Cleanup and ingress
02:14 PM...06...55...Pre-airlock repress
02:19 PM...07...00...Airlock repress