Flight engineer Timothy Kopra and European Space Agency astronaut Timothy Peake will take a spacewalk from the International Space Station (ISS) on Friday starting at 7:55 a.m. EDT (GMT-5). The spacewalk is likely to be of six and a half hours, with the aim of replacing a failed solar array voltage regulator due to which one of the eight power sources of the ISS, named channel 1B, has stopped working. In addition, certain ongoing tasks for installing route power and data cables will also be completed during this spacewalk.

The 200-pound voltage regulator is called a sequential shunt unit (SSU). Each solar power wing of the ISS is equipped with eight such regulators that provide relay for the regulation of 160-volt DC current to downstream components. Subsequently, current moves down to the 124 volts of electricity utilized by the equipment in the ISS. The excessive electricity is transferred back to the solar panels for dissipation as heat.

The failure of the SSU occurred in November 2015 and the flight controllers connected all the systems that operated on channel 1B to other channels, minimizing the impact. "From a station perspective, we could live in this state for quite a while, but the reality is if we were to have an additional failure in another channel, we'd probably find ourselves a little more strapped,” said Kenny Todd, the ISS Operations and Integration Manager at the Johnson Space Center in Houston.

U.S. EVA-35 mission will involve Kopra supporting a suit with red stripes and will be called EV-1, while Peake will wear an unmarked suit and helmetcam 20, and will be called EV-2. The replacement of the regulator is expected to take 15 minutes and will be carried out during the 31-minute orbital darkness period of the ISS. This will ensure that the astronauts are protected from any possible shocks due to solar energy generation.