An ambitious plan to revive a forsaken NASA satellite and return it to Earth orbit ended in disappointment Wednesday when engineers conceded that a failure in the spacecraft's propulsion system made it impossible to fire thrusters and alter the probe's flight path.
"There's only so much you can do before you have to say, 'It's dead, Jim,' " said Keith Cowing, a former NASA astrobiologist and a spokesman for the privately-run ISEE-3 Reboot Project, which operates out of a converted McDonald's restaurant at NASA's Ames Research Center in Mountain View, Calif.
Unable to nudge the aging probe from its orbit around the sun, project engineers say they have switched the spacecraft to "science mode" and will collect data from it for as long as they can -- perhaps a couple of months.
NASA launched the International Sun-Earth Explorer 3 in 1978 to study space weather and retired it 17 years ago. As the spacecraft neared Earth on a pre-arranged flyby set for Aug. 10, Cowing and others won permission to take control of the craft, the first time NASA has ever handed over one of its assets to a private group.
In just three months, the team of former NASA employees, engineers and self-described space cowboys managed to raise $160,000 on crowd-funding websites; wake the probe from an electronic slumber; communicate with it using the massive radio telescope in Arecibo, Puerto Rico; and fire several of its engines.
But the thrusters stopped working Tuesday, just as the team prepared to maneuver the spacecraft into a new orbital path.
On Wednesday, members of the ISEE-3 Reboot Project spent two hours attempting to diagnose the problem by "jiggling" fuel valves on and off. When the thrusters still failed to work, the disappointed engineers concluded that the satellite's fuel system had lost critical pressure.
"We have exhaustively tested the propulsion system with no good results," project member Dennis Wingo, the chief executive of Skycorp Inc., reported on Twitter.
The satellite, like many others, uses hydrazine fuel. The highly toxic substance is stored in eight tanks and kept under pressure by nitrogen gas. When ground controllers instruct the satellite to open a fuel valve, the gas is supposed to push the hydrazine through fuel lines and into a catalyst. When it reaches the catalyst, the hydrazine should break down and produce an invisible puff of hot gas that propels the spacecraft.
Reboot engineers calculated that it would take about 432 puffs of gas to redirect the spacecraft into Earth orbit. But without fuel pressure, the thrusters will not fire.
Cowing said it was possible there was a minute leak that allowed the gas to escape slowly -- maybe even a molecule at a time -- over decades. "There may still be some stuff in there; it's just not enough," he said.
The thruster firings that did occur were probably the result of residual gas pressure in the fuel lines, he said.
Since ground controllers were able to fire the thrusters initially, the satellite's flight path has been slightly changed, and there's a small chance that it may be on a collision course with the moon.
A lunar collision, they noted, would provide scientists with more data than they would collect in a normal flyby.
Reboot project engineers hope to determine the spacecraft's exact location and new trajectory on Friday, when they will have access to NASA's Deep Space Network, its powerful telecommunications system.
The propulsion system failure surprised one of the chief architects of the satellite's orbital path, Bob Farquhar. The former NASA engineer helped send the satellite into space 36 years ago and charted a complex course that allowed it to become the first spacecraft ever to pass through a comet's tail.
Farquhar was among the engineers working on the reboot project, and although he was highly skeptical they would pull it off, he said he never anticipated an equipment malfunction.
"I always thought the spacecraft would work OK," Farquhar said. "I can't understand why all the nitrogen would be gone."
The 82-year-old had lobbied NASA for years to reactivate the satellite. He often said he had a supernatural connection to the spacecraft -- a belief he developed when he suffered a heart attack at almost the same time ISEE-3 lost function of its only battery.
"I feel deflated," Farquhar joked Wednesday. "I think I'm losing pressure in my lungs."
Despite the failure, Farquhar said he planned to mark the spacecraft's flyby next month.
"I'll know where it is in the sky and at what time, and I'm just going to look up and wave goodbye to my old friend," he said.