NASA’s Space Shuttle Discovery was rolled back from its launch pad to the Vehicle Assembly Building at Kennedy Space Center in Florida May 26 so engineers can swap out the orbiter’s external fuel tank as a safety precaution.

Discovery’s seven-astronaut crew had planned to launch spaceward May 22 for a 12-day flight that would not only restore NASA’s independent ability to loft humans into orbit, but also test new safety measures and resupply the international space station (ISS). However, concerns over the potential danger of ice debris from Discovery’s external tank led mission managers to call for a swap, delaying the launch to no earlier than July 13.

Discovery’s delay extended the amount of time the space station’s two-person Expedition 11 crew must wait for a shuttle resupply mission. NASA space shuttles have not visited the ISS since December 2002.

“We’d rather see the space shuttles up here sooner rather than later,” NASA astronaut John Phillips, flight engineer for Expedition 11, said during a NASA TV video downlink with CBS News. “I was certainly disappointed.”

But astronauts generally agree that paying strict attention to safety and risk management — not schedules — should be the prime concern for NASA’s space shuttle program.

“I know that people are disappointed when the shuttle launch delays ,” veteran astronaut Eileen Collins, commander of Discovery’s STS-114 mission, said during a press conference earlier in May . “But we want to make sure we understand everything that we’re studying and that we don’t rush into things.”

NASA’s three remaining space shuttles have been grounded since Feb. 1, 2003, when the Columbia orbiter broke apart during re-entry, killing all seven astronauts onboard. Wing damage sustained at launch by external tank foam debris was later found to be the accident’s cause. Engineers have since modified the fuel tank for all future shuttle flights.

Technicians in the Vehicle Assembly Building are now working to separate Discovery from its propulsion stack and attach it to another stack whose external fuel tank is equipped with a new heater to minimize the buildup of potentially harmful ice . The rollback process was itself delayed slightly to allow shuttle engineers time to check Discovery’s landing gear for cracks.

According to a May 26 NASA press release, the new fuel tank is expected to be attached by June 7.

Discovery’s launch date has slipped much more than the nearly two months between May 22 and July 13. After the initial shuttle grounding, several launch dates were announced, then missed as engineers and mission managers revamped Discovery and its Atlantis sister ship for safer flight, modified the external tanks and developed new tools and methods for in-orbit repair demonstrations.

Meanwhile, the two Expedition 11 crew members aboard the ISS are eagerly awaiting Discovery’s launch, if not just for the company of another seven humans at the station.

Phillips and Expedition 11 commander Sergei Krikalev have worked feverishly to prepare for Discovery since arriving at the space station April 17 aboard a Russian Soyuz capsule.

On May 25, they rehearsed the photographic inspection procedures they will perform as Discovery flips around during docking to expose its thermal tile-covered underbelly to the ISS crew. Krikalev and Phillips also have practiced airlock operations to prepare for the three spacewalks planned while Discovery is docked at the space station, as well as packed up trash, used equipment and other unneeded items that will return to Earth with the shuttle.

Krikalev pointed out that while Discovery’s slow progress toward launch may be disheartening to NASA personnel , there are global effects as well.

“Not only are people in Houston and Florida disappointed, but also NASA’s international partners,” Krikalev, a veteran Russian cosmonaut, said during the video downlink. “The problem is integrated, and every delay has a chain effect.”

Without regular shuttle flights, space station crews have depended on Russia’s Soyuz and Progress spacecraft to deliver new astronauts and cargo to the ISS. Discovery’s delay also has stretched the timeline needed to complete construction of the ISS .

“We have three big electrical trusses, a Japanese lab, a European lab and other components all waiting to launch,” Phillips said . “In order to complete the station, we have to get the shuttle flying.”