Live in Orbit: Second spacewalk under way

Update, 6:17 p.m.: Olivas and Fuglesang have switched their spacesuits to internal battery power, officially beginning today's spacewalk at 6:12 p.m., nearly an hour behind schedule.

Update, 5:50 p.m.: The Quest airlock is being depressurized to vacuum level. Spacesuits will be checked twice before the airlock hatch is opened and the spacewalk begins.

Two astronauts are getting ready to step outside the International Space Station for the second of three spacewalks during shuttle Discovery's 13-day mission to the outpost.

The planned six-hour and 30-minute effort by mission specialists Danny Olivas and Christer Fuglesang, both veterans of three spacewalks, is dedicated to completing replacement of coolant tanks on the station's central truss.

Scheduled to start at 5:19 p.m., the spacewalk is running about 30 minutes behind schedule because of minor spacesuit snags.

Fuglesang had to replace his communications cap because of excessive air flow over its microphones. Olivas' chin strap came loose and his helmet had to be removed to snap it back into place, which forced him to restart a process of breathing pure oxygen.

Fellow Discovery mission specialists Tim Kopra and Jose Hernandez have been helping the spacewalkers get suited.

Olivas and Fuglesang - a Swedish physicist representing the European Space Agency - plan to resume work that Olivas and station flight engineer Nicole Stott began Tuesday.

They removed a depleted set of two ammonia tanks and stowed the large box holding them on the end of the station's 58-foot robotic arm (left).

Today, Olivas and Fuglesang will retrieve a fresh set from a carrier in Discovery's payload bay and install it on the Port 1 truss segment, then fasten the old ones in the vacated spot on the carrier.

The new tank assembly weighed about 1,700 pounds at liftoff, including 600 pounds of ammonia that helps cool the station's electrical systems.

With about a third of its ammonia left, the old assembly weighs closer to 1,300 pounds.

Although the objects are being handled in weightlessness, their large size and mass make them difficult to handle, and they could still cause damage if banged into something.

There's also potential for toxic ammonia or nitrogen to leak from lines that must be hooked up between the assembly and the truss, though there was no problem during Tuesday's activity.

The spacewalk will be the 132nd supporting space station assembly and maintenance over more than a decade of work.

You can watch the events live here by clicking on the NASA TV still image at right to launch a viewer.

Check out a full schedule of upcoming mission highlights in this NASA TV schedule.

This Flight Day 7 Execute Package shows more detailed timelines for each crew member and notes sent by Mission Control.