Tim Peake will become the first Briton to walk in space later when he ventures outside the International Space Station (ISS) to help fix a broken power unit.
The spacewalk - referred to as an 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.
They have been getting ready for weeks, but also practised for months on Earth.
Major Peake arrived at the ISS on 15 December and will stay for six months.
On Tuesday he posted a picture on Twitter with the caption: "Final suit check prior to Friday's EVA - feels just great!"
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.
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 ESA warned: "A spacewalk is a test of patience and alertness."
The spacewalk 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 not sustained any damage.
The pair will be able to drink water from pouches, but will not be able to go to the toilet.
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.
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