Juno at Jupiter
An illustration of NASA's Juno spacecraft in orbit around Jupiter. Credit: NASA/JPL-Caltech

WASHINGTON — NASA’s Juno Jupiter orbiter, already dealing with a thruster problem that postponed a key maneuver, went into a safe mode hours before a close approach to the giant planet Oct. 19.

At an Oct. 19 press conference during a planetary science meeting in Pasadena, California, project officials said that Juno went into safe mode at 1:47 a.m. Eastern, more than 13 hours before the spacecraft made its closest approach to Jupiter, about 5,000 kilometers above its cloud tops, in its elliptical orbit around the planet.

The spacecraft “detected a condition that was not expected,” Scott Bolton, principal investigator for the Juno mission, said. “It did exactly what it was supposed to do when it detects a condition that is not expected.” That includes shutting down unneeded subsystems and awaiting instructions from controllers.

That safe mode means that Juno did not collect any data during the flyby. “The spacecraft safe mode condition eliminated the science, but everything’s okay,” he said.

Bolton said it was too soon to determine what triggered the safe mode. “It did happen pretty far away from Jupiter, so my instinct is that it may not have been tied to the intense radiation belts we’re so fearful of,” he said. “That doesn’t mean there isn’t something else in Jupiter’s environment that may have caused it.”

“We were still quite a ways from the planet’s more intense radiation belts and magnetic fields,” Rick Nybakken, Juno project manager at the Jet Propulsion Laboratory, said in a statement. “The spacecraft is healthy and we are working our standard recovery procedure.”

The mission had originally planned not to collect science on the close approach. Instead, those plans called for Juno to fire its main engine to reduce the period of its orbit from the current 53.4 days to 14 days, allowing the main science phase of the mission to begin. However, NASA announced Oct. 14 that it had postponed the maneuver because of an issue with the thruster.

Bolton said that valves that are part of the pressurization system for the engine opened slower than expected during preparations for the maneuver last week. “They behaved with a little bit of a delay, which meant maybe the valves were a little sticky,” he said. “We didn’t want to take any chances that some adverse type of condition would cause the burn to go in a way that we didn’t expect.”

The next opportunity to perform the maneuver will come on Juno’s next close approach to Jupiter on Dec. 11. However, Bolton said the mission would take its time to understand the problem before deciding how to proceed. “We’re no rush to make any of these changes,” he said.

He left open the possibility of keeping Juno in its current 53-day orbit, noting that the mission’s science is done primarily during each close approach to the planet. “We can obtain all of the science goals of Juno even if we stay in a 53-day orbit,” he said. “We were changing to 14 days primarily because we wanted the science faster.” Radiation would not be an issue for an extended mission, he added, because the spacecraft is exposed to intense radiation only during each close approach to Jupiter.

Keeping Juno in its current orbit, though, would stretch out the length of the mission. One issue Bolton raised is that Juno is currently in an orbit that keeps it illuminated by the sun at all times. By early 2019, he said, the orbit geometry would have shifted so that the Jupiter would eclipse the sun for several hours of each orbit. “If we were never going to change out of the 53-day orbit, we would have to go investigate how to get past an eclipse,” he said.

Juno is the second of NASA’s New Frontiers medium-class planetary science missions, after the New Horizons Pluto flyby mission. Launched in 2011, Juno entered orbit around Jupiter July 4. The Oct. 19 flyby marked the end of its second orbit around the planet since arrival.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...