"Their main job is to install a device that measures electrical charging around the truss structure and solar arrays. After completing other duties, they should wrap up their space walk at 4:15 p.m. (2:15 p.m. MST). Commander Pavel Vinogradov will assist his two crewmates from inside the International Space Station."

Swenson, whose research area is instrumentation for space science, said he is excited that the equipment will be installed at last. "There's a lot of interesting data that should come off this instrument," he said.

"I imagine we'll be looking at data sometime within the next few days."

Around 30 people were involved in the project at the Space Dynamics Laboratory, he estimated.

The device is a cross with sensors at the ends of the arms, about 4 feet across and 4 or 5 feet tall. "The little arms fold down and it all folds into a nice package."

During the space walk, Williams and Reiter will pull off the instrument's covers and lock the arms into place, then install it on the outside of the International Space Station.

As components are added to the station, the electrical charging problem could become more severe.

Its root cause is that, contrary to the name, the station really doesn't fly through airless space; it's orbiting in the ionosphere, the wispy outer edges of the atmosphere.

The ionosphere is a thin gas that begins about 60 or 70 miles above Earth and extends 10 times that distance, he said. The gas molecules absorb sunlight and in the process electrons are knocked free. The ionosphere contains regular gas, ions and electrons.

"The space station is flying through this charged gas and these charges build up on the surface of the International Space Station," he said.

The connections between the station's solar cells collect charges. Eventually, the station itself could become charged.

NASA engineers built in devices called plasma contactors to bleed off the charges. Swenson said agency officials "basically came to us and said we have concern whether that equipment is working correctly."

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E-mail: bau@desnews.com