Prime Minister David Cameron has wished British astronaut Tim Peake good luck ahead of his historic spacewalk.
Major Peake, 43, will become the first Briton to walk in space when he ventures out of the International Space Station (ISS) to help repair a broken power unit from 12.55 GMT on Friday.
Mr Cameron took to Twitter hours before the spacewalk, posting: "Good luck to @astro_timpeake on today's #spacewalk"
He added: "The country will be watching you make history #ScienceIsGREAT"
Major Peake said "nothing can fully prepare (us) for the feeling of being outside of a spacecraft in the vacuum of space" as he completed the final preparations
The spacewalk - referred to as an Extra-Vehicular Activity (EVA) - will be the first for Major Peake and the third for his Nasa colleague, Colonel Tim Kopra, 52.
It will last almost six-and-a-half hours.
Major Peake, writing on his blog, said he felt "exhilarated" by the prospect of walking in space, but said: "I have no time to dwell on these emotions.
"The six hours and 30 minutes we will work on the Space Station's hull are meticulously planned and Tim and I need to execute each step methodically.
"Spacewalks, like many critical operations, operate on the buddy-buddy system.
"Tim and I will constantly be checking each other and relying on each other for assistance if something should go wrong."
The pair will work in 45-minute blocks of daylight, then complete darkness, as the station orbits Earth every one-and-a-half hours.
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 beforehand, 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.
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.
Major Peake arrived at the ISS on December 15 and will stay for six months.
British-born US astronaut Nicholas Patrick, a spacewalk veteran, said Major Peake should find time to enjoy the "majesty of the view" during his EVA.
Dr Patrick said: "When you float out, it is a remarkable feeling. You are used to floating - by this point Tim has been in space for a month, he will know exactly what floating is like - what he won't be used to is being outside the space station with a fabulous view and, perhaps more importantly, with a very difficult set of time-critical tasks ahead of him.
"My bet is that, like me, when he goes out he won't be looking down initially, he will be looking left and right, finding his way around and getting ready for the tasks ahead. But it's a great thing to go out, it's really a wonderful experience."
He told the BBC Radio 4 Today programme: "Take a look around, look down and enjoy it, would be my advice to Tim. It's the astronaut's equivalent of stopping to smell the roses."
The biggest challenge for Major Peake on the spacewalk will be figuring out how to move around in space and orienting himself, according to Nottingham University associate professor and anti-gravity expert Dr Nathaniel Szewczyk.
He said: "The physical task of just moving around will be more difficult than people realise."