Nasa spacewalk: How to watch live stream of astronauts' 7-hour ISS mission

If successful, Bob Behnken and Chris Cassidy will complete a mission first started in January 2017

Nasa astronauts Bob Behnken and Chris Cassidy will take part in a 7-hour spacewalk today outside the International Space Station (ISS) in an effort to complete a 3.5-year mission to upgrade the station's power system.

The 231st spacewalk at the ISS is scheduled to begin at approximately 12.35pm BST and will be live streamed on the US space agency's official YouTube channel.

It is the second in a pair of spacewalks by the US astronauts to replace the last remaining nickel-hydrogen batteries with more modern lithium-ion batteries.

The mission to change out the ageing batteries first began in January 2017, and so far 11 spacewalks have been performed.

Once the power system upgrade is successfully completed, the astronauts will move on to other assembly and maintenance tasks on the outside of the orbiting laboratory.

"The astronauts will shift gears and remove two lifting fixtures used for ground processing of the station's solar arrays prior to their launch," Nasa explained on its website.

"They'll also begin preparing the Tranquillity module for the installation of a commercial airlock provided by NanoRacks and scheduled to arrive on a SpaceX cargo flight later this year. The airlock will be used to deploy commercial and government-sponsored experiments into space."

Live streams of spacewalks typically attract millions of viewers, with people from around the world tuning in to watch the progress of the astronauts.

To distinguish between the two astronauts, Mr Behnken will wear a spacesuit bearing red stripes, while Mr Cassidy will wear one with no stripes.

Mr Behnken was one of the astronauts aboard the first crewed flight of the SpaceX Crew Dragon mission on 30 May, which heralded a "new era in human space flight", according to Nasa.

It was the first time since 2011 that astronauts launched into low-Earth orbit from American soil, and the first time in history using a commercially built and operated spacecraft.