It looks like a metallic cross and researchers joke that its purpose is to ward off space vampires. Actually, the Utah State University-built instrument to be installed Thursday on the outside of the International Space Station will play a role in protecting astronauts from a dangerous invisible force: static electricity.

The Floating Potential Measurement Unit, designed and built by USU's Space Dynamics Laboratory, will gauge electrical charges that build up on the outside of the orbiting station. It will also measure the space environment to help scientists better understand how the charges accumulate.

This isn't ordinary static cling that makes your socks crackle nastily when you take them out of the drier. It's a potentially deadly jolt.

If astronauts working in space were to touch the station where a big charge had built up, electrical arcing might hit the metal rings around their spacesuits' necks and wrists. "They might have currents passing through the space suit," said Charles Swenson, USU professor of electrical and computer engineering and the principal investigator for the project.

Such a current, he added, "might damage the spacesuit or might damage the astronaut."

Another concern is that arcing on the station's surface could hurt the anodized layer that serves as thermal protection.

NASA officials wanted an instrument that would show whether the space station's electrical discharge equipment is working properly, and that would help them understand the physics of any electrical buildup. They commissioned Swenson and his crew at the Space Dynamics Lab to design and build such a device, in a $2.5 million contract.

The outcome: the cross-shaped Floating Potential Measurement Unit. According to USU, within two years the lab developed, tested and delivered four of the units and a ground station.

Finished three years ago, the unit's installation on the station was delayed because of the Columbia disaster that grounded the remaining space shuttle fleet for many months.

The unit was stored in a bag filled with protective nitrogen, then finally launched aboard Space Shuttle Discovery on July 4. On Thursday it is to be mounted on the station's exterior, attached to a bracket originally intended to hold a camera.

"Flight engineers Jeff Williams and Thomas Reiter continue preparations for Thursday morning's space walk," says a NASA Web site. The walk is to begin at 7:55 a.m. MDT.

"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."