A pair of American astronauts began a spacewalk on Monday to move a stalled rail car outside the International Space Station, NASA said.
The outing by Scott Kelly and Tim Kopra began at12:45 GMT, the US space agency said.
During the three-hour spacewalk, the two men will attempt to lift the brake on the mobile transporter rail car, which carries the robotic arm from one location to another on the outside of the orbiting lab.
The brake is believed to have become stuck unexpectedly last week, and mission control in Houston has been unable to fix the problem robotically.
The astronauts plan to move the rail car a few inches and latch it in place, so it will not interfere with the arrival of the Russian Progress supply ship on Wednesday.