Astronauts John Grunsfeld and Andrew Feustel installed new equipment Saturday on the orbiting Hubble Space Telescope to help scientists study the origins of the universe.
US space agency NASA said that the third of the current shuttle mission's five scheduled spacewalks was completed in six and a half hours.
Grunsfeld and Feustel installed a new instrument known as the Cosmic Origins Spectrograph, designed to break up light into its components. The device will allow astronomers to study the large-scale structure and origins of the universe, including how galaxies, stars and planets formed and how elements developed.
In two previous spacewalks from the shuttle Atlantis, astronauts installed a new camera on the ageing telescope and replaced a computer and the gyroscopes that keep it aligned.
Scientists say the repairs and upgrades, which US space agency NASA hopes will extend Hubble's functioning life span until at least 2014, will continue to provide clues about the origin and nature of the universe.
Since its launch in 1990, Hubble has helped scientists to place the age of the universe at 13.7 billion years, learn that black holes are at the centre of most galaxies, monitor planetary formation and discover that the universe is expanding at an ever-faster pace.