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.

The twin solid rocket boosters and a redesigned external fuel tank that will carry Discovery into orbit later this year have already been assembled in the vast Vehicle Assembly Building at NASA’s Kennedy Space Center (KSC) in Florida. They await delivery of Discovery, which is currently set for no earlier than next week.

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,” astronaut Wendy Lawrence, a mission specialist for Discovery’s spaceflight, said 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 report Wednesday.

But orbiter ground crew officials said 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 flight will be NASA’s first shuttle mission since the loss of seven astronauts aboard the Columbia orbiter Feb. 1, 2003. Columbia broke apart during re-entry after sustaining critical damage to its left wing during launch. The flight of Discovery, which NASA designated STS-114, 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 Vehicle Assembly Building, 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 Vehicle Assembly Building for integration is a major step toward returning the shuttle fleet to flight status, the orbiter also is 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 [Orbiter Processing Facility], 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, which is scheduled to be the next shuttle to fly after Discovery, 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.