For NASA's Juno spacecraft, all had been going well since its July 4th insertion into orbit around Jupiter—as well as things can go when radiation is slowly eating away at a spacecraft, that is. That ended when mission managers tried to send a command to the robotic probe on Thursday.

According to a NASA news release, two helium check valves that play an important role in the firing of the spacecraft's main engine did not operate properly during the command sequence. "The valves should have opened in a few seconds, but it took several minutes," Rick Nybakken, Juno project manager at NASA's Jet Propulsion Laboratory, said. "We need to better understand this issue before moving forward with a burn of the main engine."

NASA had intended to fire the spacecraft's Leros 1b engine, its primary source of thrust, next Wednesday. The goal was to bring Juno into a shorter orbital period around the gas giant, from 53.4 to 14 days. The optimal time for such a "period reduction maneuver" is when the spacecraft is closest to the planet, so Juno's next opportunity for this engine burn will not come until Dec. 11. This was to be the final burn of the Leros 1b engine, which fired perfectly on July 4 to put Juno into a precise orbit around Jupiter. Future maneuvers can be conducted by smaller onboard thrusters.

Mission scientists emphasized that the longer orbital period would not affect the quality of science that Juno can collect as it flies close to the planet's poles. However, if the issue cannot be resolved, the spacecraft will not be able to make as many flybys as scientists hoped due to expected degradation of the spacecraft and its scientific instruments as it flies through Jupiter's harsh radiation environment. NASA hoped Juno would make 36 orbits during the next 20 months.