SIRTF atop the Delta 2 at pad 17B as workers recently installed the two-halves of the rocket's nose cone. Photo: NASA

SIRTF was already sitting atop the $68 million rocket and set for the final days of launch preparations. But after multiple delaminations with the layers of carbon material that make up the nozzle of one strap-on solid rocket motor were found during pre-flight preparations, the space agency ordered a postponement. The delamination occurs when the adhesive bond between the layers is lost, NASA said.

The fourth and final of NASA's Great Observatories had been scheduled for blastoff Friday aboard the first Boeing Delta 2 Heavy rocket from Cape Canaveral Air Force Station's pad 17B. But earlier this month, senior officials in Washington put the launch on hold while additional reviews were conducted into the integrity of the Delta 2's strap-on solid-fueled boosters.

With the liftoff already delayed to the end of April, NASA was facing a May 7 deadline to get SIRTF airborne so the pad could be readied for another Delta 2 flight in June carrying the second Mars Exploration Rover. That Mars launch must fly during a tight 18-day window beginning June 25 when Earth is properly aligned to dispatch the robotic craft to the Red Planet.

On Friday, the agency announced the suspect booster -- one of nine attached to the rocket's first stage -- would have to be replaced before launch would occur. The time to make such a switch wasn't available before the May deadline, forcing SIRTF to wait until after the Mars mission.

"There simply is not enough time to remove and replace the rocket motor to support a SIRTF launch in advance of the Mars Exploration Rover-B launch window," said Karen Poniatowski, assistant associate administrator for launch services at NASA Headquarters.

NASA said the super-hot exhaust from the booster could have burned through the suspect nozzle, causing the motor to fail and dooming the launch.

Both SIRTF and that second Mars rover use the Delta 2 Heavy rocket configuration, which is distinguished by the larger solid rocket motors originally developed for the Delta 3 vehicle. The solids are 46-inches in diameter vs. the 40-inch solids normally used on Delta 2s. The motors are built by Alliant Techsystems.

Complex 17 has two launch pads but only pad 17B is designed to handle the larger solids.

NASA said the 46-inch solids earmarked to be used on the Mars launch have been cleared for flight.

Teams are now developing plans to return SIRTF to its processing facility at the Cape and disassemble the rocket from the pad.

The SIRTF rocket will be restacked on the pad following the Mars launch. The observatory should return to the seaside complex following the late-July liftoff of an Air Force GPS satellite from neighboring pad 17A.

NASA estimates the delay will add $2 million per week to the $1.19 billion mission's price tag.