Space station commander Leroy Chiao and flight engineer Salizhan Sharipov plan to stage a six-hour spacewalk early Wednesday to install experiments on the hull of the lab complex, including a European robotics system that can be controlled by radio from inside the Russian command module.

This will be the first of two spacewalks planned for the Expedition 10 crew and the 57th station EVA since assembly began in December 1998. Going into Wednesday's excursion, 39 NASA astronauts, one Canadian, one Frenchman and nine Russian cosmonauts have logged 338 hours and 17 minutes building and maintaining the international outpost.

Chiao, making his fifth spacewalk, and Sharipov, making his first, have five major objectives:

• To install a universal work platform, with base and wiring, on the hull of the command module.
  
  
• To mount the European commercial experiment Rokviss (robotic components verification on ISS), an engineering experiment to test the operation of manipulator rotary joints operated from inside the Russian command module Zvezda using a radio commanding system.
  
  
• To relocate a Japanese space exposure experiment package. In its place, the spacewalkers will install a Rokviss antenna and connect cabling between the antenna and a transciever.
  
  
• To inspect vents used by the station's Elektron oxygen generation system to make sure they are clear. The Elektron system has had problems lately and engineers are taking advantage of the spacewalk to continue troubleshooting.
  
  
• To install a Russian experiment known as Biorisk, which is designed to study the effects of the space environment on microorganisms.

The Rokviss experiment "aims at the qualification of the newest lightweight robot joint technologies as developed in (the German Aerospace Center's) lab," according to the agency's web site. "They are the basis for a new generation of ultra-light, impedance controllable ... arms which, combined with DLR's newest articulated 4-fingered hands, are the essential components for future robonaut systems."

Additional information is available here.

Chiao and Sharipov plan to open the hatch in the Pirs airlock module around 2:25 a.m. EST (0725 GMT) to begin the spacewalk. If all goes well, they will re-enter Pirs and close the hatch around 7:52 a.m. EST (1252 GMT) to end the excursion.

Since the Columbia disaster, the station has been staffed by rotating two-man crews, one less than usual. Because a third crew member isn't available to monitor the station's systems during spacewalks, precautions are taken to prevent problems.

"We don't have the third crewmember inside to respond to unexpected failures or circumstances that may happen, although unlikely, during the spacewalk," said Derek Hassmann, Expedition 10 EVA flight director. "In a typical situation when we aren't doing a spacewalk, the crew is the first line of defense for critical failures such as, for an example, loss of cabin pressure or a fire or a significant problem with a coolant loop. But of course with both crewmembers outside doing the spacewalk, we don't have the crew to help us respond.

"So one of the things we do is we close a number of internal hatches inside the spacecraft. What this does is separate the spacecraft into manageable volumes, which we can then control from the ground using ground-commanded valves. We also have the crew reconfigure the U.S.-segment cooling loops such that if we were to have a coolant leak we wouldn't lose temperature control for all of the critical U.S. avionics. Finally, we set up cameras in the spacecraft that we can use from the ground to remotely monitor the interior of the vehicle while the crew is outside."