CAPE CANAVERAL, Fla. — The Atlantis astronauts are out spacewalking again, but this time they will attempt an unprecedented repair job at the Hubble Space Telescope.

During Saturday's spacewalk, John Grunsfeld and Andrew Feustel will try to fix a burned-out camera. That's never been done before at Hubble. They also will give the 19-year-old observatory a new science instrument designed to detect faint light from faraway quasars.

NASA considers the camera repairs extremely intricate and the toughest yet. Another broken science instrument needs to be fixed on Sunday. New tools were designed for the job.

This is the third spacewalk for the crew on the shuttle Atlantis. The first two ended up being harder than expected.