By Irene Mona Klotz
at the Kennedy Space Center, in Florida
The shuttle Discovery arrived at its seaside launch pad on Wednesday for the US space agency's second attempt to return its orbiter fleet to flight.
The journey to the pad is always a slow one
It is bolted to what Nasa believes is the safest fuel tank ever made.
Discovery reached the top of Launch Pad 39B shortly after noon Eastern Time, with five days contingency time to make the targeted 13 July launch date.
Managers scrubbed Discovery's first launch opportunity in May to replace the fuel tank.
Their decision to send the orbiter back to the Vehicle Assembly Building followed heated discussions about whether potential ice build-ups on the outside of a fully fuelled tank could pose a hazard during launch.
Discovery's tank, originally earmarked for Atlantis' mission on the second post-Columbia flight, sports an additional heater near the top of the tank to thwart ice formations in a particularly vulnerable region around a flexible liquid oxygen feedline bellows.
Managers are not sure yet if the upgrade will be enough. Engineers are scheduled to present their final debris risk-assessments on 24 June.
Debris impacts on the shuttle have been a prime concern since the 1 February, 2003, Columbia accident.
The shuttle was damaged during launch by a piece of foam insulation falling off the external fuel tank.
Columbia was destroyed and its seven astronauts killed 16 days later when the orbiter attempted to fly through the atmosphere for landing.
Nasa quickly began working to repair the tank, replacing some insulation with heaters and improving foam application procedures so that the insulation would stay properly affixed during launch.
The foam helps keep the shuttle's cryogenic propellants at the proper temperature and also helps prevent ice from building up on the outside of the tank. Like foam, ice could fall from the tank during lift-off and strike the orbiter.
Recognizing the hazards of ice impacts, however, lagged behind Nasa's assessments and remedies for foam debris.
Discovery is bolted to the tank originally intended for Atlantis
"We should have probably have started the ice debris (analysis) earlier than we did," Joseph Cuzzupoli, a member of the task force overseeing Nasa's return-to-flight efforts, said during a press conference last week.
"Foam is understood now and ice isn't, and we started maybe too late. But right now we're catching up," he added. Meanwhile, the Kennedy Space Center launch team is moving ahead with flight preparations. The shuttle left the assembly building at 0218 Eastern Time on Wednesday for its second trek to the launch pad.
Moving at an average of less than a half-mile per hour (0.2m/s), the 4.2-mile (6.7km) journey finally came to an end after noon at the top of Pad 39B.
The massive crawler transporter, which hauled the shuttle to the launch complex, had to make four pit stops, primarily to make sure bearing temperatures were not getting too high under the heavy load.
With an eye on the Florida summertime storm clouds gathering in the distance, workers prepared to rotate a protective service structure back around the shuttle.
The equipment Discovery will carry into space arrived at the launch pad on Monday and is scheduled to be transferred into the cargo hold on Friday, said Kennedy Space Center spokeswoman Tracy Young.
The gear includes a new gyroscope for the space station, part of a fuel-free system to keep the outpost properly positioned in orbit, and test stands for spacewalking astronauts to practise shuttle heat-shield repair techniques, which Nasa, with its new external fuel tank, hopes never to have to use.