Space station astronauts Jeff Williams and Thomas Reiter are gearing up for a planned six-and-a-half-hour spacewalk today to ready the lab complex for upcoming shuttle assembly flights. The excursion is scheduled to begin at 9:55 a.m. EDT.

This will be the 69th spacewalk devoted to space station construction and maintenance since assembly began in 1998. It is the third for Williams, who performed a station assembly spacewalk in 2000 and a more recent outing in June, and the third for Reiter, who performed two EVAs aboard the Russian Mir space station in 1995 and 1996.

The call sign for Williams, whose spacesuit features red stripes, is EV-1. Reiter, in an unmarked white suit, will be EV-2. NASA television coverage is scheduled to begin at 9 a.m.

The goals of today's excursion, staged from the U.S. Quest airlock module, include installing a sensor to measure the electrical field around the station; to replace a motor controller and a computer needed to operate cooling system radiators; and to mount two space exposure materials science sample packages on the station's hull.

Williams and Reiter also plan to replace a short section of cooling system plumbing, attach mechanical devices to ensure cooling system quick-disconnect fittings seat properly and test an infrared camera. In addition, Williams plans to remove a faulty Global Positioning System antenna and install a new floodlight on the truss to assist future spacewalkers.

"We're now ready to execute one of the most challenging sets of flights in the history of the space station program," said deputy program manager Kirk Shireman. "Between now and a year from now, we'll have four Progress (resupply) flights, two Soyuz (crew ferry) flights, four more shuttle flights, we'll add a port and starboard solar array, a total of four truss segments between now and then.

"We'll have rechannelized our central power system and we'll have activated the central cooling system. That's not to mention a whole series of eight (spacewalks) we'll do in between those shuttle flights, three Russian and five U.S."

Today's spacewalk, the first of those five U.S. expedition EVAs, is "very important to put us in position to conduct all those flights," Shireman said.

It is also the first carried out with a full complement of three station crew members since downsizing in the wake of the 2003 Columbia disaster. Reiter, a European Space Agency astronaut, joined Williams and station commander Pavel Vinogradov last month after a ferry flight aboard the shuttle Discovery.

"We are really excited to have three crew members back on board space station," said Dana Weigel, lead flight director for the spacewalk. "This is significant help for all of our EVA operations. Now we have an extra body to help the crew get in and out of the suits as well as someone inside in case we have work we need them to do during the EVA."

Most of the work in today's spacewalk will help ready the station for assembly work planned two shuttle flights from now, in December, when the station's main cooling system will be activated for the first time.

"That system has two independent cooling loops, loop A, which is on the S1 (starboard) truss, and loop B, which is on the P1 (port) truss," Weigel said. "Along with that activation, we'll be filling both systems with ammonia and before we do that, we want to make sure the system is functioning properly. So for this EVA, we're going to do some work on loop A."

Each cooling loop features three accordian-like radiator wings mounted on a beam that can rotate to maximize heat rejection and to prevent the ammonia inside from freezing. Only the central radiator in each loop is currently deployed.

Devices called rotary joint motor controllers, or RJMCs, are responsible for operating the motors used to rotate the radiator beams.

"There are two of those RJMCs on each of our cooling loops and one of the ones on loop A failed about a year and a half ago," Weigel said. "We removed it during Expedition 12 back in December and brought it home on this last (shuttle) mission. In this EVA, we'll just be doing an installation."

While Williams is installing the RJMC, Reiter will remove and replace the starboard thermal radiator multiplexer/demultiplexer, a computer that controls the valves used to regulate the flow of ammonia through the radiators and also to properly position the big panels. The MDM failed about two years ago.

Williams and Reiter also will replace a so-called "shunt jumper," a short section of coolant piping that currently serves as a temporary bypass for the cooling system of the new node module that will be plumbed in next year.

After the shunt jumper is installed, Reiter will test the infrared camera, part of ongoing work to develop better ways of detecting shuttle heat shield damage, while Williams works to install the floodlight and remove GPS antenna No. 2.

Here is a timeline of today's activities (in EDT):

TIME            EVENT
___________________________________________________________

09:55 AM        Airlock egress and tool setup (30 minutes)
10:25 AM        FPMU (electric field sensor) installation (01:50)
12:15 PM        Astronauts move back to the Quest airlock module (00:30)
12:45 PM        Installation of materials science packages (00:30)
01:15 PM        Williams installs the S1 RJMC (00:40)
01:15 PM        Reiter installs S1 MDM (00:45)
01:55 PM        Williams installs S0 starboard shunt jumper (00:50)
02:00 PM        Reiter inspects rotary beam QD connectors (00:30)
02:30 PM        Reiter modifies port shunt jumper quick-disconnects (00:35)
02:45 PM        Williams installs floodlight (00:30)
03:05 PM        Reiter tests the IR camera (00:50)
03:15 PM        Williams removes GPS antenna No. 2 (00:40)
03:55 PM        Tool cleanup and airlock ingress (00:30)
04:25 PM        Spacewalk ends