Four International Space Station crew members will embark on two spacewalks to install new equipment and replace defective installations, the US space agency NASA said Tuesday.

Russia's Gennady Padalka and Yury Malenchenko will carry out a six-hour spacewalk on August 20, the lead flight director for the current ISS mission, Dina Contella, told reporters from the Johnson Space Center in Houston, Texas.

The cosmonauts, who have racked up a total of 12 spacewalks between them, will install a shield to protect one of the ISS modules from micro-meteorites and space debris. They will also reposition a telescopic crane.

On August 30, American Sunita Williams and Japan's Akihiko Hoshide will leave the station for about six and a half hours to replace a faulty camera and cables.

They will also install the equipment needed for the upcoming arrival of a Russian laboratory module at the ISS.

This will be Williams' fifth spacewalk and Hoshide's first.

The main challenge of the two missions will be to realize the tasks within the allotted time, said lead US spacewalk officer Kieth Johnson.

"We want to make sure we don't let the crew run long on a task," Johnson said. "We've timelined it so we can stop at various points to get them inside."

In the first spacewalk, the most difficult task for the cosmonauts will be moving the telescopic crane, Contella said.

In addition to the spacewalks, the current ISS team is preparing for the arrival of four cargo vessels -- two from Russia, one from Europe and one from Japan.