Britain’s first astronaut Tim Peake will perform his first spacewalk this week

The discovery of water in the helmet of a U.S. astronaut brought an early end to a spacewalk Friday at the International Space Station, NASA said.

British astronaut Tim Peake is today set for his biggest challenge yet – his first space walk.

The astronaut has assisted in a similar walk while onboard the ISS – but remained inside the craft.

Kopra, who was making his third spacewalk, and Peake had replaced a failed voltage regulator in the station’s power system shortly after leaving the station’s airlock at around 8 a.m. EST/1300 GMT.

Flying the flag: Tim Peake tries his spacesuit on for size. The two Tims will first replace Sequential Shunt Unit 1B, which helps regulates power from the ISS’s solar array.

Celebrity well-wishers included Sir Paul McCartney, who added: “We’re all watching, no pressure!”

The spacewalk will be tiring for the pair as they fight against the pressurised suits. They will be able to drink water from pouches, but will not have a toilet break.

While we all crave a little of bit of sun, too much exposure to sunlight can be unsafe for the astronauts and the pair will need to check on each other frequently and make sure their suits have not been damaged.

Peake said he had spent months on Earth training for the mission, including virtual-reality sessions to prepare “for the worst-case scenario of becoming detached from the space station”.

To start the mission they were put into an airlock, closing the interior door tight behind them before opening the second door to exit the station.

In addition, Peake is keeping track of his skin condition, as part of the Skin-B experiment, and any signs of headaches during his time in space.

He arrived at the ISS on December 15 and will stay for six months.

He told the BBC Radio 4 Today programme: “When you float out, it is a remarkable feeling. It’s hugely exciting and we’re ready to go”. Watch live via NASA TV from 11:30 GMT (12:30 CET) and follow @esaoperations on Twitter for updates.

Other Britons in space, including Michael Foale in 1995, have had USA or dual citizenship, or were on privately funded or sponsored trips.