LIVE: ISS Expedition 63 astronauts perform spacewalk

NASA astronauts Chris Cassidy and Bob Behnken perform a spacewalk outside the International Space Station (ISS) on Tuesday, July 21.

Cassidy arrived at the ISS in early April as part of the Soyuz MS-16 space expedition. He's the current head of ISS Expedition 63.

Behken recently joined the ISS crew aboard the first space flight launched from US soil since 2011, a joint effort of NASA and SpaceX.

This spacewalk, which is expected to last about seven hours, will be the final one to upgrade the power system of the ISS, an effort which has lasted 3,5 years. Credit : NASA TV