Nasa has ordered astronauts to carry out urgent repairs at the International Space Station.
Two astronauts will venture out on a spacewalk to replace a data relay box that broke down over the weekend.
The job falls to the two Americans on board - commander Peggy Whitson and Jack Fischer.
The failed unit is one of two that control the station's radiators and solar panels, among other critical systems.
Nasa said everything is still safe in orbit because one relay box is still working. But officials want the broken one quickly replaced, in case the good one also goes down.
Astronauts performed a similar spacewalk in 2014.
Tuesday morning's spacewalk should last around two hours. Whitson and Fischer last went spacewalking just over a week ago.