Two Russian cosmonauts are conducting a 6.5-hour spacewalk in order to carry out maintenance work to the outside of the International Space Station.
The purpose of the maintenance work is to move a hand-operated cargo crane from the Pirs docking compartment to the Zarya module in preparation for a new Russian laboratory module.
This video explains the procedures the cosmonauts will perform:
Cosmonauts Gennady Padalka and Yuri Malenchenko will also install a set of debris shields and "launch" a small spherical satellite as part of a Russian science experiment.
If time permits, they will also retrieve two science experiments that expose various materials to the conditions of open Space, and install some support struts on a ladder.
Both cosmonauts have plenty of experience having completed 12 previous spacewalks between them.
Their preparations for the spacewalk began about two weeks ago and included extensive testing of their equipment and space suits, and health checks.
They will be wearing Russian-made Orlan space suits for the mission.
In a video released by NASA, the two cosmonauts can be seen making final alterations to their space suits.
Today's mission is being run from the Mission Control Centre at Korolyov just outside the Russian capital Moscow.
It is also being monitored from NASA's ISS flight control room in Houston, Texas.It is the 135th spacewalk to be conducted from the ISS.