This story was updated at 10:58 a.m. EDT.
CAPE CANAVERAL, Fla. - NASA's space shuttle Discovery will have to wait one more day before joining its fuel tank and rocket boosters at Kennedy Space Center (KSC), the space agency said Thursday.
Problems with a sling designed to hoist Discovery into a vertical position inside the massive Vehicle Assembly Building (VAB) here prevented the shuttle's move from its hangar-like Orbiter Processing Facility. A sheared screw on the sling will have to be replaced and tested before NASA can press ahead with Discovery's move.
"We've stood down on operations today," NASA spokesperson Bruce Buckingham said.
Discovery is now set to move to the VAB, where its external fuel tank and twin solid rocket boosters await its arrival atop their Mobile Launch Platform, at 8:00 a.m. EDT (1200 GMT) Friday, NASA said.
Engineers hope to mate Discovery to its external tank-booster stack over the next week and roll the entire launch system out to Pad 39B here on May 19. The orbiter's rollover and rollout are key milestones for NASA, which hopes to launch Discovery on its STS-121 mission - the agency's second post-Columbia test flight - on July 1.
NASA KSC spokesperson Jessica Rye said a sheared jack screw on a spreader beam of the sling, which keeps the hoist's outer components clear of an orbiter while they are attached, led to today's delay.
"We do have a spare," Rye said, adding that it would take about three hours to swap out the jack screw.
But engineers also want to understand what caused the faulty jack screw to shear and ensure that its replacement is working properly before strapping the 100-ton Discovery orbiter in the sling and lifting it off the VAB floor.
"They've got a metallurgist coming in to evaluate the issue," Buckingham said.
The threat of thunderstorms over NASA's Florida spaceport would also have challenged a late afternoon rollover for Discovery today, he added.
Friday's planned rollover for Discovery will be the orbiter's first move since the spacecraft returned to KSC following last summer's STS-114 spaceflight.
NASA is preparing Discovery to launch seven astronauts to the International Space Station (ISS) on the STS-121 test flight. The mission has a launch window that stretches through July 19.
In addition to rollover operations, shuttle workers also moved the Italian-built Leonardo module - one of four Multi-Purpose Logistic Modules - into a payload canister Thursday as part of NASA's STS-121 preparations. Packed with new equipment, experiments and other cargo, Leonardo will launch toward the ISS inside Discovery's payload bay and carry vital supplies to the orbital laboratory's crew. Other cargo items, such as replacement ISS parts and spacewalk tools, have also been installed in the payload canister.
Buckingham told SPACE.com that current plans call for Leonardo to be delivered to Pad 39B next week, possibly before Discovery rolls out from the VAB.
Discovery launched NASA's first return to flight mission, STS-114, to the ISS last July on a 14-day mission that ended with a successful landing at Edwards Air Force Base in California's Mojave Desert. The shuttle returned to KSC on Aug. 21 atop NASA's modified 747 carrier jet.
- NASA Forgoes Fuel Tank Test for Next Shuttle Launch
- NASA's Next Shuttle to Fly Without Ice Frost Ramp Fix
- Return to Flight: NASA's Road to STS-121
- STS-114 Return to Flight Special Report