Nov. 23 (Bloomberg) -- Two U.S. astronauts completed the final spacewalk of their 11-day shuttle mission today by installing a testing platform outside the International Space Station that may determine how future crafts are made.

The Materials International Space Station Experiment will allow NASA scientists to see how substances and coatings react to direct sunlight, radiation, atomic oxygen and extreme temperatures. The agency can use the results when planning spacecraft, NASA said on its Web site.

Robert Satcher Jr. and Randy Bresnik from the shuttle Atlantis spent more than 5 1/2 hours in the vacuum of space. They also installed an oxygen-filled gas tank that will be used to pressurize and depressurize an airlock for future spacewalks, NASA said.

The start of the spacewalk was delayed about an hour until a loose valve on Satcher’s drink bag could be reattached. During the spacewalk, a small object seemed to float away from Satcher’s work area, and Mission Control is trying to figure out what it was, according to NASA.

Today’s spacewalk was the last of three for Atlantis’s mission. The astronauts began work at about 8:24 a.m. New York time and finished officially at 2:06 p.m.

Atlantis launched from Cape Canaveral, Florida, on Nov. 16 and is scheduled to return to Earth on Nov. 27. The aim of the mission is to stock the orbiter with spare parts for air conditioning, communications, electrical, plumbing and robotics systems before the shuttle program’s retirement next year.

Five more shuttle missions are scheduled before the National Aeronautics and Space Administration retires the program in September 2010 to make way for the next generation of manned U.S. spacecraft. Americans will fly on Russian vessels to the space station during the five years until the shuttle’s successor is built.

To contact the reporter on this story: Ryan Flinn in San Francisco at rflinn@bloomberg.net

Last Updated: November 23, 2009 14:36 EST