(AP) -- Astronauts are getting ready for what could be the most complex spacewalking of their mission to upgrade the Hubble Space Telescope.
Astronauts John Grunsfeld and Drew Feustel (FOYstahl) will venture out of the shuttle Atlantis Saturday to work on Hubble's survey camera.
They were given an extra hour to sleep because Friday's spacewalks were so long and hard.
But scientists have said Saturday's work could be some of the most intricate of the mission. The astronauts will be attempting to fix a scientific instrument while in orbit. Before, they've just swapped one instrument for another while spacewalking.
The effort involves new tools and dozens of pieces that could fly around and do damage to Hubble.
©2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Explore further: Image: Simulating space for JWST's four infrared instruments