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A space experiment in fundamental physics that has been 40 years in the making is once again facing possible cancellation because of problems discovered during a recent ground test of the spacecraft.
Program officials said the problem is easily fixed and that Gravity Probe B -- a spacecraft first proposed in 1962 to test Einsteins General Theory of Relativity -- should be ready to launch by November.
But NASAs space science chief, Ed Weiler, has once again raised the prospect of canceling the program, which has cost the U.S. space agency about $500 million to date.
Delays are nothing new for Gravity Probe B, a Stanford University-led project being built with the help of Lockheed Martin Missiles and Space, Sunnyvale, Calif. Two previous hardware problems have caused the missions price tag to grow by more than $100 million and miss its planned 2000 launch.
Program officials said the latest problem pales in comparison to the earlier problems. The latest glitch revealed during thermal vacuum testing in December that there were three blown fuses on the spacecraft. The Gravity Probe B team believes those fuses failed before testing began. A fourth blown fuse, program officials said, was likely caused by a faulty power supply that has since been identified and replaced.
Weiler acknowledged that Gravity Probe Bs most recent technical problems appear to be minor compared to its past problems. While he is not so worried about replacing fuses or paying for what might only be a four-month delay, Weiler said he is concerned about the unknowns that seem to keep hitting us year after year after year.
This is a program that has been within a year of launch every year since 1999, Weiler said.
Weiler, who threatened in 1999 to cancel Gravity Probe B after a critical component of the spacecraft failed a key performance test, said he has commissioned two independent reviews of the program that are to be completed by mid-April. He said those reviews would include recommendations about whether the program should be allowed to continue.
In addition to examining the cost, schedule, and viability of Gravity Probe B in light of the latest setback, the review panel will also be asked to measure the scientific value of the mission against the cost of forgoing other space science activities to pay for the new overruns. Each month the mission is delayed could cost NASA $4 million to $5 million, Weiler said.
Our policy here is to try to solve problems within themes, so it would be weighed against science in (the Structure and Evolution of the Universe theme), Weiler said. Its some pretty heavyweight science that would be threatened by these overruns.
Weiler said he has not instructed the Gravity Probe B team at Stanford and Lockheed Martin to stop work or slow their efforts while the review is under way. We are carrying on as if we will continue, he said.
Weiler also said he would expect Gravity Probe B to complete a new thermal vacuum test before he would clear the spacecraft for launch.
Bradford Parkinson, a Stanford University professor and associate program manger for Gravity Probe B, said the team expects to be ready to conduct a new thermal vacuum test by mid-May. The test, which is designed to expose the finished spacecraft to the same thermal extremes it would encounter on orbit, would wrap up in early June, he said.
Parkinson said that before the December test was cut short due to the blown fuses, the spacecraft was performing well. In particular, the test gave the team confidence that it had successfully remedied earlier problems with the spacecrafts four precision gyroscopes and a large helium-filled, thermos-like designed to keep them super-cooled.
Parkinson said the team also feels it has the cause of the fuse problem well in hand. One of the fuses blew due to a faulty power supply that is being replaced with a new unit, he said. The other three fuses, he said, were most likely blown before the test due to miscommunication between shifts of technicians that had been working around the clock to keep the project on schedule.
Parkinson said that Lockheed Martin technicians rewiring Gravity Probe Bs heating unit did not insulate three circuits before clocking out for the day. If the next shift powered on the unit with the wires still unsheathed and Parkinson believes they did the fuses would have blown. That there was a problem with those three fuses, he said, was not discovered until the spacecraft was already in the thermal vacuum chamber undergoing what would have been one of its final tests before launch.
Lockheed Martin officials said they are still trying to understand the precise cause of the fuse failures.
Hugh Dougherty, Lockheed Martins Gravity Probe B program manager, said in a written statement that corrective actions are in the works. He said the two components affected by the fuse failures -- a power distribution unit and attitude control electronics box -- have been returned to Gilbert, Ariz.-based Spectrum Astro, a subcontractor on the project, where they are being repaired and re-tested.
Before Gravity Probe Bs latest setback, NASA expected to have spent a total of $570 million on the project by the end of this year.
NASA has funded the project since 1964, although the first two decades of the effort was confined to studies and technology development. The complicated spacecraft is designed to test Einsteins theory that Earth -- or any large body -- warps time and space as it travels through space. To test the theory, Gravity Probe B, which is equipped with four precisely polished gyroscopes, would be trained on a reference star. If Einsteins theory is correct, the axis of each gyroscope should drift slightly from the reference star.
Parkinson, who has been with the project off and on since 1982, said he would hate to see NASA cancel the project so close to launch, I frankly think this four- to five-month slip is aggravating, but having worked on it for 20 years and Stanford has worked on it for 41 years it just seems like we should go ahead and complete what we started.
Parkinson said that 93 percent of the money required to do the experiment has already been spent. Even the Delta 2 rocket that would launch Gravity Probe B into polar orbit from Vandenberg Air Force Base in California is largely paid for, he said.
He said pulling the plug on the project now would save NASA $20 million to $30 million at best.
Once on orbit and fully operational, Gravity Probe B would need at least seven to eight months to conduct its experiment, Parkinson said.
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