CAPE CANAVERAL, Fla. -- Two astronauts ventured out of the International Space Station on Thursday to begin a test of how various materials survive the harsh environment of space, launching the experiment during a six-hour spacewalk, Reuters reported.

The aim is to test materials that NASA might use on future spacecraft.

Astronauts Dan Barry and Patrick Forrester, from the space shuttle Discovery, were all busy as they left the shuttle air lock and began setting up tools they would use during the day.

Housed in two suitcase-sized containers, the materials International Space Station experiment was attached to the space station's outer walls.

It was designed to test how hundreds of materials such as insulation and metal alloys resist more than a year of exposure.

Barry and Forrester also rode the shuttle's robotic arm like a cherry picker to install a coolant tank.

Thursday's was the 25th spacewalk to support assembly of the space station, a $95 billion project being built by space agencies in the United States, Russia, Japan, Canada and Europe.

Space Station Marks 1,000 Days

Meanwhile, the space station's new crew, which rode to space aboard Discovery, marked the 1,000th day of space station operations. The Russian-built Zarya module became the first module of the international science project when it was launched in late 1998.

Completion is set for 2006, when the orbiting complex will spread out over an area the size of two football fields.

The first crew took up residence late last year.

"We wanted to say a few words in tribute to all the hard work that has gone into building the station so far, the dedication of the people who built this first component as well as all the others and the success it has achieved to date," said the station's new commander, Frank Culbertson, who was joined on Zarya by his crewmates, Russians Vladimir Dezhurov and Mikhail Tyurin.

NASA reported that earlier on Thursday, at about 6 a.m. EDT (1000 GMT), the space station was able to resume steering itself as it orbits the Earth at 5 miles (eight km) per second.

That announcement came a day later than planned after a ground computer in Moscow failed to link up with a computer in the Russian segment of the station that commands large gyroscopes used to keep the station stable during flight, NASA said.

Discovery used its jets to keep the complex stable until the Russian computers could be resynchronized during a pass over Russian ground stations.