Russian cosmonauts Oleg Artemyev and Sergei Prokopyev have spent nearly eight hours on a spacewalk to install apparatus on the International Space Station designed to track the migration patterns of animals on Earth.
The spacewalk lasted seven hours and 46 minutes, more than an hour longer than expected.
NASA spokesman Rob Navias said the cosmonauts were in "great shape" after completing all their tasks.
"Everything went very well - by the book - no issues during the course of today's excursion," Navias said.
The apparatus that the cosmonauts installed is a German-Russian collaborative experiment known as Icarus.
It will use an antenna and global positioning system hardware to study the migration patterns of animals tagged with receivers.
"The experiment may provide data about how animals move from one location to another, how animal population density shifts over time, and how diseases spread," NASA said in a statement.
In addition to the Icarus experiment, the cosmonauts deployed four small satellites, including one that will assess how satellites group themselves in orbit and one that will measure the density of space, Navias said on NASA television.
The spacewalk is the first for Prokopyev since he arrived at the station in June for a half-year mission. It is the third for Artemyev.
