NASA astronauts Dave Wolf and Chris Cassidy are replacing four of six old solar array batteries on the International Space Station on Wednesday, in a maintenance call that will last more than six hours.

The space station uses solar arrays to generate power. The arrays have been in space since 2000. The batteries are the oldest ones on the space station, according to Space.com.

It is the third of five spacewalks for the astronauts while the shuttle Endeavour is linked to the station.

To watch live stream video of the astronauts in the space click here.

International Space Station NASA Solar Space