The pieces are coming together for NASA's first space shuttle launch in more than two years as engineers prepare to roll the Discovery spacecraft out of its protective Orbiter Processing Facility (OPF).

The twin solid rocket boosters and a redesigned external fuel tank that will carry Discovery into orbit later this year stand already assembled in the vast Vehicle Assembly Building (VAB) at NASA's Kennedy Space Center (KSC) in Florida. They await delivery of Discovery, which is currently set for no earlier than next week, and NASA officials hope the entire assembly will roll out to the launch pad by the end of the month.

"Obviously, those are very important milestones, and we need to keep pushing forward," said astronaut Wendy Lawrence, a mission specialist for Discovery's STS-114 spaceflight, in a telephone interview.

NASA is currently targeting a mid-May launch for Discovery's seven-astronaut crew, with a flight window stretching from May 15 to June 3, though some officials have hinted that preparations are lagging behind, according to an MSNBC.com report Wednesday.

But orbiter ground crew officials have told SPACE.com that Discovery is very close to being ready for integration.

"The majority of the work is done [and] all the testing is completed," said Stephanie Stilson, NASA's vehicle manager for Discovery, during a telephone interview earlier this month. "Rolling over and rolling out, that's huge for the [ground] team."

Discovery's STS-114 spaceflight is expected to be NASA's first shuttle mission since the loss of seven astronauts aboard the Columbia orbiter on Feb. 1, 2003. That shuttle broke apart during reentry after sustaining critical damage to its left wing during launch. The STS-114 flight is aimed at demonstrating new safety tools and methods developed over the last two years, as well as delivering fresh cargo to the International Space Station.

Part of the work remaining for Discovery's flight includes the installation of a new digital camera to the orbiter's underside. Wiring for the camera, which replaces an older film-based system, is already in place though the actual imager will be installed once the orbiter is inside the VAB, NASA officials said.

Like its film-based predecessor, the digital camera will snap images of Discovery's external tank as it separates from the shuttle in orbit. It is expected to yield high-resolution images that will be transmitted directly to mission control from space. During previous flights, mission managers had to wait until a shuttle landed to obtain the film negatives and observe the separation event.

Stilson said that, while rolling Discovery over to the 52-story VAB for integration is a major step toward returning the shuttle fleet to flight status, the orbiter is also coming out of a maintenance period that included some 267 modifications, of which about 20 were return-to-flight related.

"It's one thing to be excited for return-to-flight," she added. "In the OPF, we have different milestones, including the first time we powered the orbiter up after having powered down for a year."

Stilson also said her team has been in close contact with the ground crews working on Atlantis, NASA's second shuttle to fly after Discovery during the STS-121 mission, in order to streamline orbiter preparations.

Atlantis shuttle officials marked their own milestone this week with the arrival of a new external tank at KSC.

Among the next tasks for Discovery's crew, however, is a March 18 check out of the shuttle's payload, said Lawrence, who will oversee the transfer of food, equipment and other cargo between the shuttle and space station in the STS-114 spaceflight.

During the check out, Lawrence and fellow STS-114 mission specialist Charlie Carmada plan to look over the Italian-built Raffaello cargo module, as well as two platforms that will be installed in Discovery's payload bay to carry replacement parts and other equipment.

"We need to look at the hardware so we can close out the bag," Lawrence said.