Tim Peake is due to make the first British spacewalk, carrying out maintenance on the ISS, since Michael Foale (flying under US citizenship) in 1995. He will be working alongside NASA's Tim Kopra to replace some faulty equipment and install cabling and parts elsewhere.
A technically challenging EVA for a few reasons, not least the time limit, but hopefully fairly straightforward - both astronauts have undergone extensive training and as always, safety is the paramount concern.
EVAs are a semi-regular part of life on the ISS (usually between 5 and 10 per year) as parts fail and must be repaired and replaced, and preparatory work is done for expanding the station and adding or moving new components and modules. They usually last for at least 5 hours, sometimes as long as 8.
Major Peake and Nasa astronaut Tim Kopra are scheduled to step outside the International Space Station at 12:55 GMT. The six-and-a-half hour extra-vehicular activity (EVA) will see the two Tims replace a failed electrical box which regulates power from the solar panels. The failed electrical component - known as a Sequential Shunt Unit (SSU) - is relatively straightforward to swap out: it involves undoing just one bolt. But 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.