Photos: Atlas 5 rocket aims for International Space Station

Two astronauts floated outside the International Space Station Sunday for the first in a series of spacewalks to upgrade the lab’s aging solar power system, assembling a support fixture that eventually will hold a new solar blanket.

After four years of brainstorming, custom tool development and training, two astronauts plan to venture outside the International Space Station Friday for the first of four spacewalks to repair a $2 billion cosmic ray detector. The excursions are considered the most challenging since work to service the Hubble Space Telescope.

Gusty winds associated with a subtropical low pressure system prevented United Launch Alliance from sending an Atlas 5 rocket into orbit Saturday from Cape Canaveral with the U.S. Air Force’s clandestine X-37B spaceplane. ULA plans to try again Sunday, and a SpaceX Falcon 9 rocket launch from a nearby pad has been delayed to Tuesday.