Yahoo Finance Britain's Tim Peake Takes Spacewalk
Tim Peake said it was a "privilege" when he became the first Briton to walk in space after venturing outside the International Space Station (ISS) to fix a broken power unit.
The spacewalk, which began just after 12.55pm GMT, will last almost six-and-a-half hours.
As he prepared to exit the air lock, one of Major Peake's colleagues, Commander Scott Kelly, remarked it was "really cool" to see the Union flag outside.
Mr Kelly said: "The Union Jack has explored all over the world, now it's exploring space."
Major Peake replied: "It's great to be wearing it. A privilege, a proud moment."
His wife, Rebecca Peake, left a short message for her husband as he left the ISS.
"@astro--timpeake thank you for taking our boys with you into the vacuum of space," she wrote on Twitter.
Back on earth, well-wishers also took to Twitter to express amazement and wish the astronauts good luck.
Sir Paul McCartney wrote: "We're all watching, no pressure! Wishing you a happy stroll outdoors in the universe."
Earlier, Prime Minister David Cameron posted: "Good luck to @astro--timpeake on today's #spacewalk. The country will be watching you make history #ScienceIsGREAT"
Musicians Pixie Lott, The Who, Peter Gabriel and the Pet Shop Boys also sent messages of support.
Major Peake, 43, is carrying out the mission with NASA colleague Colonel Tim Kopra, who is now on 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".
The two men will remain tethered to Space Station supports, although their suits will have built-in jet packs in case they accidentally drift away.
Colonel Kopra is taking the lead, heading first to the solar units that need repairing.
Major Peake, from Chichester in Sussex, 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 finish ahead of schedule, they will be given "bonus tasks", including laying another cable and cutting some unnecessary power caps.
The spacewalk will be tiring for the two men 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 regularly and make sure their suits have not been damaged.
