A critical moment in NASA'S Juno mission has been postponed while engineers at the Jet Propulsion Laboratory attempt to sort out a mysterious engine issue. If the problem is not resolved quickly, it could reduce the amount of high-quality data the Jupiter-orbiting probe is able to collect during its scientific mission.
On October 19th, at its point of closest approach to Jupiter (called periapsis), the Juno spacecraft was scheduled to perform its final main engine burn, a "period reduction maneuver" that would narrow its orbit from 53.4 days to 2 weeks. Once in its "science orbit," the spacecraft's main data collection phase will commence.
But on Friday, the space agency decided to delay the burn due to an unexpected issue with a pair of helium valves that are part of the engine's fuel pressurization system. As Juno project manager Rick Nybakken said in a news release, these valves "did not operate as expected during a command sequence that was initiated [Thursday]."
"The valves should have opened in a few seconds, but it took several minutes," Nybakken continued. "We need to better understand this issue before moving forward with a burn of the main engine."
http://gizmodo.com/something-went-wrong-with-the-juno-spacecrafts-engine-1787873807
[Source]: NASA
(Score: 2, Informative) by khallow on Tuesday October 18 2016, @03:00PM
Why helium? Because it’s non-combustible? If something is strong enough to puncture a pressure vessel, it’s going to blow up anyway. I would think oxygen or hydrogen would be more useful gas.
Non-reactive (no hydrogen embrittlement or oxygen corrosion of metals, for example) and low molecular weight. The latter means you get a significantly higher propellant velocity and thus, better delta-v for the mass of propellant expended.