The Pegasus XL rocket -- with HESSI enclosed in its nose -- prepares to roll out of Orbital Sciences' hangar at Vandenberg for attachment to L-1011 carrier jet and ferry flight to the Cape. Photo: UCB/NASA

NASA has postponed this week's planned flight of the Pegasus rocket carrying a Sun-studying probe while investigators determine what went wrong during Saturday's X-43A launch that used a similar booster.

The High Energy Solar Spectroscopic Imager, or HESSI satellite, was scheduled to rocket into space on Thursday aboard an air-launched Orbital Sciences Pegasus XL booster.

But officials decided on Monday to delay the launch at least five days to no sooner than June 12, giving engineers more time to figure out why the Pegasus stage carrying the X-43A flew out of control off the coast of California and if the problem could affect the HESSI rocket. Read more on X-43A mishap.

"It will depend on how fast we learn information from the West Coast investigation," NASA spokesman George Diller said.

In order to launch on June 12, NASA says HESSI's Pegasus rocket must be exonerated by Friday so pre-flight preparations can resume.

An investigation board is expected to be announced on Tuesday or Wednesday to begin sifting through the data, records and information gathered by the interim board since Saturday's failure.

The X-43A aircraft was being propelled to 95,000 feet in altitude by a modified first stage of a Pegasus where it would have been released to test an experimental scramjet.

The rocket booster used in the X-43 launch was fundamentally the same first stage of the Pegasus that will carry HESSI, with the exception of different thermal protection, a new guidance system and repackaged avionics.

The Pegasus launching HESSI will also have two additional stages and a protective fairing to shroud the satellite during the ascent through the atmosphere.

The Orbital Sciences L-1011 jet, with the fully assembled Pegasus and HESSI vehicle mounted to its belly, arrived at Cape Canaveral Air Force Station in Florida on Saturday at about 7:55 p.m. EDT, nearly three hours after the X-43A failure.

Artist's concept of HESSI satellite orbiting Earth. Photo: NASA

The "Stargazer" aircraft was on the cross-country trek from the Pegasus home base of Vandenberg Air Force Base in California to the Cape when the accident happened.

Over the past two days workers have completed some routine tests and the mission dress rehearsal to prepare for HESSI's eventual launch. "Stargazer" remains parked at the air station's Skid Strip.

The $85 million HESSI mission is already running 11 months behind schedule. Set for launch last July, the satellite was significantly damaged in ground testing and had to be repaired. Then part of the Pegasus' stage separation system had to be redesigned, delaying the launch from this spring.

HESSI is destined to orbit 373 miles above the planet to observe the Sun and take color X-ray images of solar flares. Scientists hope the two-to-three year mission will yield clues about what triggers solar flares, which are the most intense explosions in our solar system.