Shuttle Discovery's six veteran astronauts suited up and climbed aboard the spacecraft at launch pad 39A for a countdown dress rehearsal Friday. The test culminated with a simulated shutdown of the main engines at T-minus 4 seconds to practice emergency procedures.

Astronauts depart crew quarters for launch pad. Photo credit: NASA   Photo credit: Justin Ray/Spaceflight Now   Photo credit: Justin Ray/Spaceflight Now   Photo credit: Justin Ray/Spaceflight Now   Launch pad 39A. Photo credit: NASA   Commander Steve Lindsey. Photo credit: NASA   Mission specialist Mike Barratt. Photo credit: NASA   Pilot Eric Boe. Photo credit: NASA   Mission specialist Nicole Stott. Photo credit: NASA   Mission specialist Al Drew. Photo credit: NASA   Mission specialist Tim Kopra. Photo credit: NASA   Mission specialists Al Drew and Tim Kopra. Photo credit: NASA   Nicole Stott and Mike Barratt strapped into their seats. Photo credit: NASA   Nicole Stott and Mike Barratt strapped into their seats. Photo credit: NASA   Mike Barratt on the middeck of Discovery. Photo credit: NASA   Tim Kopra seated on the flight deck. Photo credit: NASA   Eric Boe in his seat. Photo credit: NASA   Steve Lindsey in his seat. Photo credit: NASA   Countdown stops at T-minus 4 seconds. Photo credit: Justin Ray/Spaceflight Now   Simulation ends with engine cutoff. Photo credit: Justin Ray/Spaceflight Now   Astronauts practice emergency exit. Photo credit: NASA   Crew heads to pad escape baskets. Photo credit: NASA   These baskets would take crew to pad bunker. Photo credit: NASA   Emergency training drill complete. Photo credit: NASA   Photo credit: NASA