Two U.S. astronauts on the space shuttle Atlantis are on a third spacewalk for what is expected to be the most challenging and complex repair work on the Hubble Space Telescope.
Astronauts John Grunsfeld and Drew Feustel are carrying out the intricate task of repairing the Hubble survey camera. They will also conduct unprecedented work to replace an electronics board and install a sophisticated instrument, the Cosmic Origins Spectrograph, to capture images that will help explain how planets, stars and galaxies are formed.
The entire mission includes five spacewalks to repair and upgrade the 19-year-old telescope and extend its life for at least five more years.
On Friday, two other U.S. astronauts, Mike Good and Mike Massimino, completed a second spacewalk, replacing two massive battery modules.
They also worked on installing new gyroscopes that help the giant telescope point in the right direction. One of the new upgraded units would not fit properly, and the spacewalkers installed a refurbished spare unit instead.
The U.S. space agency NASA says Friday's spacewalk outside the shuttle Atlantis lasted 7 hours and 56 minutes, making it the eighth longest spacewalk in history.
On Thursday, Grunsfeld and Feustel installed a new camera that will allow Hubble to take detailed photos with a wide range of colors.
The mission to Hubble is more dangerous than other shuttle missions because the telescope is sharing an orbit filled with debris left behind by satellite collisions and rocket launches.
It also is riskier because astronauts only have supplies they are able to carry with them. In missions to the International Space Station, astronauts have enough support on the station to last up to three months.
Some information for this report was provided by AFP, AP and Reuters.