MELBOURNE, Fla. (Reuters) - NASA has delayed by a week the earliest possible launch date for the first space shuttle mission since the 2003 Columbia disaster, to allow more time to review safety assessments and design changes, program managers said on Wednesday.

The U.S. space agency had been planning to launch the shuttle Discovery on May 15, the first date in a launch window that runs to June 3. NASA has now marked May 22 as its new target date but additional delays are possible, shuttle manager Bill Parsons and deputy manager Wayne Hale told reporters in a teleconference.

"We are aiming for our launch date based on our best knowledge," Hale said. "But we are not schedule-driven and we will not launch until we are ready to fly."

Program managers and engineers completed a two-day design review on Wednesday, certifying that 19 of 20 major modifications to the shuttle were ready for flight, Parsons said.

Still pending approval is a new 50-foot sensor-laden extension to the shuttle's robot arm that will be used to inspect the orbiter's belly and wings for damage during flight.

Being able to inspect the shuttle in space was among the 15 recommendations made by the panel that investigated the Columbia disaster. The shuttle was destroyed and seven astronauts killed when the ship disintegrated as it reentered Earth's atmosphere for landing.

Neither flight controllers nor the crew knew that Columbia's left wing had been seriously damaged during launch 16 days earlier by a piece of foam insulation that fell off the external fuel tank. Superheated gases blasted into the wing hole as Columbia returned, melting the structure.

NASA has until June 3 to launch Discovery before its first launch period closes, primarily due to new restrictions that force it to launch and discard the fuel tank -- jettisoned as the shuttle nears orbit -- only during daylight.

The requirement is part of NASA's new safety plan to spot any damages to the shuttle once it leaves the ground.

Discovery's prime mission is to test new safety upgrades made after the Columbia accident and deliver supplies and equipment to the International Space Station.

The next launch opportunity occurs in mid-July, when NASA hopes to fly shuttle Atlantis on a follow-up mission.

"We will feel very successful if we're able to get both these launches off during the first two windows," Hale said.