NASA has announced that it will conduct an additional spacewalk on the Space Shuttle Discovery’s upcoming mission to test a heat-shield repair technique. The shuttle is scheduled to launch on October 23, 2007.

The technique, developed after the 2003 Columbia disaster, requires using a device similar to a caulk gun: astronauts squirt a rubberlike material into any cracks or holes in damaged thermal tiles. The material serves to restore the shuttle’s ability to protect itself from the extreme heat encountered during reentry to the earth’s atmosphere.

Normally, the tiles are covered with a black, glass surface that rejects heat during reentry. When this outer coating is damaged and the white silica of the tiles is exposed, it leaves them vulnerable to melting. NASA opted against using the repair method on the Space Shuttle Endeavour last month after debris from the external fuel tank came off during launch and struck thermal tiles located on the underside of the shuttle. The strike caused a hole about the size of a baseball in a pair of tiles and stripped off the thermal protective coating of several others. Although NASA decided not to risk an emergency spacewalk to repair the damage, the scare has prompted engineers to include the test as part of Discovery’s mission.

How the repair procedure will work in space is still unknown. “Any technology prior to being approved for spaceflight has to undergo rigorous design and testing so that the components can withstand operations in space,” says Joe Lavelle, a senior engineer at NASA Ames Research Center. But no one knows exactly how the technology is going to perform until it can be tested in space, he says.

To implement the test repair, astronauts (with tiles in hand) will be attached to the end of the Orbiter Boom Sensor System, a 50-foot-long stick developed specifically for inspecting the hard-to-see areas of the shuttle for possible damage after liftoff. The boom was instrumental in detecting the damage to Endeavour’s tiles. It was introduced on the Space Shuttle Discovery’s “Return to Flight” mission. (See “NASA’s New Cameras.”)

Once the astronauts are correctly positioned, they will prepare the damaged areas of the tiles using a brush coated with a sticky gel that removes any loose particles or debris. Next, the astronauts will apply an emittance wash, or “black paint,” which is a mix of silicon-carbide particles and a silicon-based rubber commonly used in industrial applications.

The wash applicator is similar to a shoe-polish applicator: it’s a cylinder with a foam tip for dabbing on the wash. The wash will serve as a primer for the second material to be applied–the rubberlike goop–but it can be a stand-alone repair technique for minimally damaged tiles.

The rubberlike material can take a whole lot of heat before the material starts to degrade. To increase its ability to insulate, silica fillers with high thermal properties are added. The combined material is applied to the damaged site using a 52-pound cylinder tank and a pistol grip gun connected by a hose. The silicon-based rubber and silica fillers are stored separately inside the dispenser until a cartridge of carbon dioxide is popped, pressurizing the dispenser and sending the materials flowing through a static mixer located in the gun. Once the cracks and holes in the tiles have been filled with the material, they will be placed in a sealed container and transported back to Earth in the shuttle’s cargo bay and tested in simulated environments at NASA.

Kevin Wells, a NASA engineer and the project’s leader, says that in-orbit testing is what’s needed for NASA to be confident in the technique and in the astronauts’ ability to perform the repair. “We want to make sure the mixing works and that we have addressed all the device’s failure modes,” he says. “We are intent and have expended a great deal of effort to make sure that we can make hardware that will increase the safety of the shuttle system so we can bring those onboard home safely.”