Two U.S. astronauts are on a second spacewalk as the shuttle Atlantis crew continues an ambitious and dangerous mission to service the Hubble Space Telescope.

Mike Good and Mike Massimino are on a six-and-a-half-hour spacewalk to work on installing six new gyroscopes, replacing three that are broken.  The U.S. space agency NASA has made the task a priority since the gyroscopes help the giant telescope point in the right direction.  The astronauts will also replace two massive battery modules.

On Thursday, crewmates John Grunsfeld and Drew Feustel installed a new camera that will allow Hubble to take detailed photos with a wide range of colors.  During that spacewalk that lasted more than seven hours, they also replaced a broken computer and installed a device to which other vehicles can attach themselves.

The crew is scheduled to carry out five spacewalks to extend Hubble's ability to operate at least five more years.  Hubble has been in orbit for 19 years and this is the final mission to repair and upgrade it.

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.

NASA says the space shuttle Endeavour is on standby at the Kennedy Space Station in the southeastern state of Florida in case the Atlantis crew has to be rescued.

Some information for this report was provided by AFP and AP.