15/01/2016 02:45 GMT | Updated 14/01/2017 05:12 GMT

British Astronaut Tim Peake Set For Spacewalk To Repair ISS Power Unit

British astronaut Tim Peake will become the first Briton to walk in space today when he ventures out of the International Space Station (ISS) to help repair a broken power unit.

The spacewalk - referred to as an Extra-Vehicular Activity (EVA) - will be the first for Major Peake, 43, and the third for his Nasa colleague Colonel Tim Kopra, 52.

It will last almost six-and-a-half hours, and was scheduled to begin at 12.55 GMT.

The two will work in 45-minute blocks of daylight, then complete darkness, as the station orbits Earth every one-and-a-half hours.

They have been preparing for weeks but also practised for months on Earth, and on Tuesday Major Peake posted a photo on Twitter with the message: "Final suit fit check prior to Friday's EVA - feels just great!"

In an overview of the spacewalk, the European Space Agency (ESA) said every detail was "choreographed minutely".

Major Peake will begin to breathe pure oxygen two hours prior, because the pressure inside their suits is lower than that of the ISS.

The pair will enter an airlock before opening its hatch and heading outside.

Like rock climbers, the astronauts must always be tethered to Space Station supports.

Colonel Kopra will lead, heading to the solar units that need to be repaired. Once given the green light, Major Peake will follow with the replacement equipment.

They should finish the repairs in under three hours, and at that point ground control will perform some checks.

During the second half of the EVA, the astronauts will lay cables for new docking ports and reinstall a valve that was removed last year.

If they are ahead of schedule, the pair will be assigned "bonus tasks", including laying another cable and cutting some unnecessary power caps.

The ESA warned: "A spacewalk is a test of patience and alertness."

The six hours will be tiring for the pair as they fight against the pressurised suits. Sunrises can also be "blinding for moments" from space, according to the ESA, and they will need to check on each other frequently and make sure their suits have no damage.

They will be able to drink water from pouches, but will not have a toilet break.

Once they return, their colleagues inside the station will help with a 25-minute clean-up and further checks. Only then will they be able to get out of their suits and adjust to the pressure back in the station.

There have been 191 spacewalks for maintenance at the space station, including a similar walk completed in 2014 by Nasa astronaut Reid Wiseman, who will help guide the current EVA from mission control in Houston.

Major Peake, a former Army Air Corps and helicopter test pilot, is the first Briton aboard the ISS and the first fully British professional astronaut employed by a space agency.

Previous "Brits in space" have either had US or dual citizenship or been on privately funded or sponsored trips.

He arrived at the ISS on December 15 and will stay for six months.