Major Peake is set to become the first Briton to walk in space on a six-and-a-half-hour mission to repair a broken power unit.
Tim Peake will become the first Briton to walk in space later when he ventures outside the International Space Station (ISS) to fix a broken power unit.
The spacewalk – known as Extra-Vehicular Activity (EVA) – will last almost six-and-a-half hours and is scheduled to begin at 12.55pm UK time.
Major Peake will carry out the mission with NASA colleague Colonel Tim Kopra, who will be taking part in his third spacewalk.
The pair will work in 45-minute blocks of daylight, then complete darkness, as the ISS orbits Earth every 90 minutes.
The European Space Agency (ESA) said every detail of the spacewalk has been “choreographed minutely”.
Major Peake will start breathing pure oxygen two hours prior to the mission, because the pressure inside their suits is lower than that of the ISS.
The astronauts will enter an airlock before opening its hatch and venturing outside.
They will remain tethered to Space Station supports, although their suits will have built-in jet packs that can be used if they accidentally drift away.
Colonel Kopra will lead, heading to the solar units that need to be repaired.
Major Peake will follow with the replacement equipment once he is given the green light.
The repairs should be completed in under three hours, and then ground control will perform checks.
In the second half of the spacewalk, the pair will lay cables for new docking ports and reinstall a valve that was removed last year.
If they are ahead of schedule, the astronauts will be given “bonus tasks”, including laying another cable and cutting some unnecessary power caps.
The spacewalk will be tiring for the pair as they fight against the pressurised suits.
NASA astronaut Leroy Chiao, who carried out six spacewalks, told Sky News: “It’s very physically demanding.
“You’re working inside a pressure suit. Every time you move your arm or even your fingers it takes a bit of energy so you quickly build up a workload.
“You are really working against that pressure, you’ve got limited visibility, but you’ve practiced lots so you know how to move and you know how to get around.”
Sunrises can also be “blinding for moments” from space, according to the ESA, and the pair will need to check on each other frequently and make sure their suits have not sustained any damage.
Once they get back, their ISS colleagues will help with a 25-minute clean-up and further checks.
Then they will be able to take off their suits and adjust to the pressure back in the station.
Soon after arriving at the ISS in December Tim Peake said he was hoping to be assigned a spacewalk.
“I watched a sunset and a sunrise at different times. Looking out of the space station is incredible,” he said.
“To think you might be out there on a spacewalk when that happens is going to be the most incredible sensation ever.”
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