Earlier this week, Olivas and Stott removed an empty ammonia tank from the station. The old tank will be put into the carrier to be brought back to Earth at the end of Thursday's spacewalk.

The planned 13-day mission is designed to transport new equipment and experiments to the space station to boost its capacity as an orbiting laboratory.

The crew of six US and a Swedish astronaut brought two 'racks' for scientific experiments and a freezer to store experiments set to be sent back to Earth. One of the racks will be used to conduct research on metals, semiconductors, crystal, glass and other materials in microgravity. The other will be used to study liquids and gels in space.

The racks were transferred to the station Wednesday.