Tim Peake has begun the first ever spacewalk by an “official” British astronaut.
Major Peake stepped outside the International Space Station’s Quest airlock just after 13:00 GMT.
He and Nasa astronaut Tim Kopra 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.
Major Peake and Colonel Kopra 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 EVA – the technical term for a spacewalk – the two Tims ventured to the very edge of the space station. They had to travel half its length, a distance of roughly 50m – which is equivalent to an Olympic-sized swimming pool.
Peake and Kopra moved along the exterior using their arms, but are attached to the space station’s external structure via steel cords, or tethers.
Colonel Kopra exited the Quest airlock first, followed by his British colleague a few minutes later. Kopra then proceeded to the worksite with a toolbox, where anchored a foot restraint as an additional safety measure. The US astronaut then gave a “Go” signal for Major Peake to follow the Nasa astronaut, carrying the replacement electrical box.
While he was waiting for Col Kopra’s signal, Tim Peake was able to get accustomed to being outside for the first time, including looking down on planet Earth.
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].”
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 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.
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.
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.