An artist's conception of the HETE 2 spacecraft. Photo: MIT

Risk. NASA says there is too much risk with one of its scientific research satellites and officials have halted the mission's launch to conduct more thorough testing.

The $8.5 million High Energy Transient Explorer 2, designed to seek out gamma ray bursts, was slated for launch next week aboard an Orbital Sciences Pegasus rocket.

But technicians detached HETE 2 from the Pegasus rocket on Wednesday inside a hangar at Vandenberg Air Force Base, California.

The craft soon will be shipped back to the Massachusetts Institute of Technology, which built, tested and will operate HETE 2 for NASA under a cooperative venture.

The satellite and Pegasus were undergoing final preparations when senior NASA officials stopped the launch following routine readiness reviews this month.

"Upper management people got together and decided it would probably be prudent to do more testing and give us some more time to get our primary ground stations up," said Gus Comeyne, the HETE 2 mission manager at NASA's Goddard Space Flight Center.

NASA's decision comes in the wake of two disastrous Mars missions and other recent satellite troubles, including the abort engine firing by an asteroid probe and an electrical problem that ultimately ended the WIRE mission days after launch.

Concerns about HETE 2 arose among NASA officials because the satellite had spent only a week in thermal vacuum testing, which is done to simulate the harsh environment in space. Additionally, one of the satellite's soft X-ray cameras and an electronics board were replaced and follow-up vibration testing was not performed to mimic the launch into orbit.

"Rather then rush into this, why don't we bring the spacecraft back and do some additional testing and build the confidence to ensure mission success," Comeyne said of NASA's decision.

"We are trying to make sure we have a quality spacecraft and have enough time on it that we are confident everything will work well."

Also of concern is the network of ground stations used to relay information and commands between controllers and the satellite. At the time of review meeting earlier this month, only one of the three primary ground stations were ready to support HETE 2.

The station in Kwajalein Atoll in the Republic of the Marshall Islands was prepared, but stations in Cayenne, French Guiana and Singapore weren't ready.

In Cayenne, the station wasn't expected to be operational until just days before launch, reducing the amount of time to perform testing. Equipment upgrades in Singapore made that station unprepared.

NASA was worried with only one operating station, controllers could lose the satellite if an onboard problem occurred. Such was the case with another NASA-sponsored satellite, called Terriers, last year. That craft suffered an attitude control problem and was lost a day after launch because of limited ground station coverage.

The HETE 2 spacecraft during construction. Photo: MIT

If the upcoming testing goes well and no problems are uncovered, NASA could reschedule HETE's launch for May.

The space agency says the launch opportunity should be available in May with the Pegasus fleet and the Kwajalein Missile Range where the mission will be staged.

Besides the $8.5 million spent on the new satellite, NASA has budgeted $15 million for the Pegasus launch and $3 million for mission operations. The space agency will pay for the extra testing. How much the delay will cost is not yet known.

HETE 2 is a replacement satellite built after its predecessor was lost in a launch failure three years ago. NASA opted to fund a reflight using spare hardware from the first satellite.

Once in space, HETE 2 will detect gamma ray bursts and relay information about their locations to ground-based observers within seconds. Quick alerts should allow detailed observations of the bursts' initial phases, scientists say.

Researchers hope HETE's efforts will lead to clues about the origin and nature of the bursts.

The 273-pound craft will operate for at least 18 months in an orbit around Earth's equator. It carries three science instruments including a set of wide-field gamma ray spectrometers, a wide-field X-ray monitor and a set of soft X-ray cameras.