CAPE CANAVERAL, Fla. -- NASA has halted plans to fly its HESSI solar probe aboard an Orbital Sciences Pegasus rocket until investigators determine what likely caused the botched X-43A launch earlier this month.

Orbital's L-1011 carrier aircraft at Cape Canaveral with Pegasus rocket and HESSI attached to its belly. Photo: NASA-KSC

Facing a Saturday deadline to get HESSI launched before internal batteries on the rocket expired, the space agency had hoped the $85 million mission designed to observe and study solar flares could begin Friday.

HESSI will be carried aloft by an air-launched Pegasus dropped from the belly of Orbital Sciences' L-1011 "Stargazer" aircraft 39,000 feet above the Atlantic Ocean about 60 miles east of the Florida coastline.

But officials decided on Tuesday to put the launch on hold and return the Pegasus rocket to its home base of Vandenberg Air Force Base in California. The cross-country ferry flight is scheduled for Thursday.

Once back at Vandenberg, the rocket's flight termination system batteries will be replaced and the HESSI spacecraft will be serviced.

When the launch might be rescheduled remains very uncertain. NASA has five other unmanned rocket launches scheduled over the next two months, so fitting HESSI back into the line up will be tough, officials say.

And since it isn't clear when the X-43A failure board will complete its inquiry, NASA is reluctant to establish a new HESSI target date.

NASA was poised to clear the Pegasus rocket for flight this week after engineers studying data from the X-43A mission concluded it was safe to launch HESSI. Based upon the data the engineers had seen so far, it appeared that the HESSI launcher was exonerated.

But senior managers were worried that investigators might uncover a problem as their study continues.

"They just felt like they wanted the investigation to get along a little further," NASA spokesman George Diller said. "The bottom line is the prudent thing to do is wait."

Image taken from chase plane shows the Pegasus rocket's out-of-control track during failed X-43A launch. Photo: NASA-DFRC

HESSI will ride a full three-stage Pegasus XL vehicle. For the X-43A test, only the winged first stage of the Pegasus rocket was used. The stage also featured modifications to its thermal protection, plus a new guidance system and repackaged avionics.

Ever since the June 2 X-43A failure engineers have been looking for evidence that shows the mishap was caused by something specific to the combined Pegasus/X-43A vehicle, or anything else that would not doom the HESSI mission.

The root cause of the X-43A launch failure still hasn't been found, NASA says. Pieces were seen breaking off the Pegasus vehicle as veered out of control eight seconds into the ill-fated flight. Safety personnel triggered explosives on the booster to destroy it.

Lost in the failure was the first of three X-43 aircraft as part of NASA's $185 million Hyper X program aimed at testing a supersonic combustion engine, called the scramjet.

NASA says the investigation team continues to meet at Dryden Flight Research Center at Edwards, California, where the X-43A mission originated.

Robert Hughes, chairman of the investigation board, says the team at Dryden expects to join other team members already at Orbital Sciences' facility in Chandler, Arizona, by June 24. That is where the Pegasus used with the X-43A was built.

The investigation board has released the NASA B-52B mother ship -- which carried the Pegasus/X-43A vehicle and then released it for the air-launch over the Pacific -- as well as Dryden's control rooms for other duties, Hughes said. These assets had been isolated since the mishap to permit the board to study them in detail.

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

The HESSI mission is already running nearly a year behind schedule. Set for launch last July, the satellite was significantly damaged in ground vibration 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.