Tim Peake has begun the first ever spacewalk by an "official" British astronaut.
Major Peake and Nasa astronaut Tim Kopra stepped outside the International Space Station at 12:55 GMT (about 25 minutes ago. The time onboard the space station is GMT).
The crew members are scheduled to spend six-and-a-half hours on the exterior of the orbiting outpost.
Michael Foale became the first Briton to carry out a spacewalk in 1995.
He flew under a US banner with Nasa.
They will replace a faulty component on the station's exterior, which has compromised a power channel on the outpost. The failed electrical box regulates power from the solar panels.
"I think a spacewalk is absolutely the pinnacle of an astronaut's career," Major Peake told BBC Stargazing Live this week.
The European Space Agency (Esa) astronaut added: "We've put a huge amount of effort into this spacewalk. It's hugely exciting and we're ready to go."
During the six-and-a-half hour extra-vehicular activity (EVA) - the technical term for a spacewalk - the two Tims will venture to the very edge of the space station. They will travel half its length, a distance of roughly 50m - which is equivalent to an Olympic-sized swimming pool.
Peake and Kopra will move along the exterior using their arms, but will be attached to the space station's external structure via a steel cord, or tether.
The ISS is currently over the Pacific, to the north east of Papua New Guinea.
Tim Peake begins historic spacewalk
By Paul Rincon Science editor, BBC News website
BBC News
15 January 2016
Tim Peake has begun the first ever spacewalk by an "official" British astronaut.
Major Peake and Nasa astronaut Tim Kopra stepped outside the International Space Station at 12:55 GMT.
The crew members are scheduled to spend six-and-a-half hours on the exterior of the orbiting outpost.
Michael Foale became the first Briton to carry out a spacewalk in 1995.
He flew under a US banner with Nasa.
They will replace a faulty component on the station's exterior, which has compromised a power channel on the outpost. The failed electrical box regulates power from the solar panels.
"I think a spacewalk is absolutely the pinnacle of an astronaut's career," Major Peake told BBC Stargazing Live this week.
The European Space Agency (Esa) astronaut added: "We've put a huge amount of effort into this spacewalk. It's hugely exciting and we're ready to go."
During the six-and-a-half hour extra-vehicular activity (EVA) - the technical term for a spacewalk - the two Tims will venture to the very edge of the space station. They will travel half its length, a distance of roughly 50m - which is equivalent to an Olympic-sized swimming pool.
Peake and Kopra will move along the exterior using their arms, but will be attached to the space station's external structure via a steel cord, or tether.
Colonel Kopra will head out of the Quest airlock first; proceeding to the worksite with a toolbox, where he will anchor a foot restraint as an additional safety measure. The US astronaut will then give a "Go" signal and Major Peake will follow his colleague, carrying the replacement electrical box.
"I'll have a few moments where I'll just be hanging beneath the airlock and I can have a look around... look down on planet Earth and see what that feels like," said Major Peake.
Luca Parmitano, an Italian Esa astronaut who conducted two spacewalks in 2013,
told BBC Five Live: "It is unimaginable... there are no words to describe the feeling of seeing our planet from above through a visor - that's all that separates us from the view, a thin visor of plexiglass."
"When language evolved, it described what we saw around us. Being on the space station, doing an EVA, walking outside is not something we have experienced [as a species]."
The spacewalkers spent two hours "pre-breathing" pure oxygen in the airlock
Both astronauts have trained in a large indoor water tank called the Neutral Buoyancy Laboratory at Houston's Johnson Space Center.
Nasa astronaut
Michael López-Alegría, who holds the US record for cumulative spacewalking time, said the two Tims were well prepared, but added that there were key differences between underwater training and a real EVA.
"One is that translation [moving from one point to another] is quite a bit different without the viscosity of the water," he told BBC News.
"This is especially true when [Major Peake] will be transporting relatively large masses (like the SSU) relatively long distances. This fact will change his centre of gravity and will affect the results of the inputs he makes with his hands and arms during translation."
But he explained: "This kind of thing is 'all in a day's work' for a spacewalker. I'm very confident that the two Tims will complete their tasks with great skill, efficiently and, most importantly, safely."
The failed electrical component - known as a Sequential Shunt Unit (SSU) - is relatively straightforward to swap out: it involves undoing just one bolt.
But the ISS takes 90 minutes to orbit the Earth, so the astronauts experience 45 minutes of daylight followed by 45 minutes of total darkness.
Peake and Kopra can only work on the SSU in darkness, because in daylight, there could be a current running through the box.
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They have been given a safe window of 31 minutes to work on changing the SSU.
"We have to be very careful when we go out to the worksite, because there's nothing protecting us from the high voltages generated by the solar panel," said Major Peake.
"We need to change that box out and then hopefully the fresh one will work correctly and we can continue with some more tasks."
After changing the SSU, Major Peake will carry the failed box back to the starting point.
The astronauts will also deploy cables for new docking ports and reinstall a valve that was removed for relocation of the station's Leonardo module last year.
Michael Foale, who was born in Louth, Lincolnshire, became the first Briton to make a spacewalk on 9 February 1995, during a US space shuttle mission.
Michael Foale (L) became the first Briton to carry out a spacewalk on a US shuttle mission in 1995. However, he was a Nasa astronaut and not an official British one
Born to a British father and American mother, Mr Foale has dual British and US citizenship. He was selected under Nasa's astronaut programme and therefore "flew" as an American.
Traditionally, the British government has not funded human spaceflight, leading a generation of budding astronauts to look to the US space agency as their only route to orbit.
But the UK changed its policy after Mr Peake was selected in 2009 as a European Space Agency astronaut. Thus, Mr Peake is the first person since Helen Sharman in 1991 to wear the Union flag in space.
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Tim Peake begins historic spacewalk - BBC News
