NASA Astronauts perform spacewalk aboard the International Space Station

HOUSTON, Texas -- Change the batteries.

It's a task that sounds simple enough, unless you're floating in outer space.

Astronauts Chris Cassidy and Robert Behnken suited up and left the confines of the International Space Station to replace aging nickel-hydrogen batteries.

The operation is expected to take as long as seven hours to complete. The batteries are part of the power system that's fed by eight solar arrays attached to the ISS.

Behnken arrived at the station on May 31 along with Doug Hurley aboard SpaceX's Crew Dragon. Cassidy is the commander of Expedition 63 and arrived in April.

Another spacewalk is planned for July 1.

The astronauts lifted off atop a Falcon 9 rocket on Saturday, becoming the first to leave U.S. soil for space in nine years.

SpaceX became the first private company to launch people into orbit, a feat achieved previously by only three governments: the U.S., Russia and China.

Welcome aboard! NASA astronauts Doug Hurley and Bob Behnken are now at their new home for the next few months.

RELATED: LIFTOFF! SpaceX Falcon 9 departs the launch pad at Kennedy Space Center

Watch the SpaceX Falcon 9, with NASA astronauts Doug Hurley and Bob Behnken in the Dragon crew capsule, lift off.

Associated Press writers Jill Colvin and Mike Schneider in Cape Canaveral, Fla., contributed to this report.