Daylight streams through the doors of NASA's Vehicle Assembly Building as Space Shuttle Discovery begins its journey.

Daylight streams through the open doors of NASA's Vehicle Assembly Building as Space Shuttle Discovery begins its slow 4.2-mile journey via the crawlerway to Launch Pad 39B. The shuttle rests on a mobile launcher platform that sits atop a crawler-transporter. First motion was at 12:45 p.m. EDT. The rollout is an important step before launch of Discovery on mission STS-121 to the International Space Station. Discovery's launch is targeted for July 1 in a launch window that extends to July 19. During the 12-day mission, Discovery's crew will test new hardware and techniques to improve shuttle safety, as well as deliver supplies and make repairs to the station. Photo credit: NASA/Kim Shiflett