Juno Jupiter NASA
Artist's depiction of NASA's Juno spacecraft at Jupiter, with solar arrays and main antenna facing the sun and Earth. Credit: NASA/JPL-Caltech

KENNEDY SPACE CENTER, Fla. — NASA announced Feb. 17 that it will keep its Juno spacecraft in a longer orbit around Jupiter than originally planned because of concerns with the spacecraft’s main engine.

Juno arrived at Jupiter in July and entered an initial 53-day orbit around the giant planet. The spacecraft was originally scheduled to fire its main engine in October to reduce its orbital period to 14 days, its planned primary science orbit. NASA, though, delayed that maneuver several days in advance after spacecraft engineers found a problem with a slow-opening helium valve in the propulsion system.

“The decision to forego the burn is the right thing to do — preserving a valuable asset so that Juno can continue its exciting journey of discovery,” Thomas Zurbuchen, NASA associate administrator for science, said in a statement about the decision to keep Juno in its current orbit.

An investigation concluded the risk of a problem occuring during that maneuver was not worth any benefit gained by putting Juno into that lower orbit. “In the end, there’s always a non-zero risk when you use the main engine,” Rick Nybakken, Juno project manager at NASA’s Jet Propulsion Laboratory, said in a phone interview. “We ultimately decided that the risk associated with doing that was not worth taking.”

The problem, Nybakken said, was not with the engine itself but with two helium check valves that are part of a system used to pressurize the engine’s propellants. Those valves opened more slowly than planned during preparations for the October maneuver that was ultimately cancelled.

The concern, he said, is that the valves would also open slowly during the burn itself. The engine usually runs in what’s called “regulated” mode, where helium is used to keep propellant tanks at a constant pressure during a burn to provide greater predictability and reliability. “Once we determined that the valves didn’t work, it took that off the table,” he said.

Engineers examined running the main engine in an alternative “blowdown” mode that doesn’t require the valves to open, but allows the pressure in the system to decrease during the burn. The risk of some kind of engine problem while operating in blowdown mode led to the decision not to operate the engine at all.

Nybakken added there was no commonality between the issue with Juno’s propulsion system and recent failures involving orbit-raising engines on two geostationary satellites. NASA officials previously said they were looking into any links, but Nybakken said those other failures involved the engines themselves, while the Juno problem was with check valves outside of the engine.

Keeping Juno in the longer orbit does not necessarily reduce the amount of science that the mission will do, but it will take longer since it primarily collects data during each close approach to the planet. Juno is currently funded through July 2018, which will give it 12 science orbits. Original plans called for ending the mission in February 2018 after completing three times as many orbits.

NASA, in a statement about the decision not to change the spacecraft’s orbit, said that the Juno mission will be eligible to request an extended mission to collect more data in the next senior review of planetary science missions in 2018.

Nybakken said there are no technical issues associated with extending Juno’s mission beyond 2018. He said the spacecraft’s radiation exposure will be slightly lower than on the original mission since the spacecraft will spend less time in the vicinity of the planet’s radiation belts. A review of other spacecraft subsystems turned up no issues that could restrict the mission’s lifetime.

“This mission is, in many ways, a lower risk one than the baseline mission because of the lower radiation levels,” he said.

One issue the mission will have to deal with in an extended mission is that the changing geometry of the spacecraft’s orbit, as Jupiter orbits the sun, will put Juno into brief moments of eclipse starting in late 2019. That could cause problems for the solar-powered spacecraft, which is always in sunlight on its current orbit.

Nybakken said it should be possible to avoid the eclipses by adjusting the orbit with smaller thrusters on the spacecraft. “With specifically timed and designed maneuvers, we can change the inclination of the orbit to avoid any eclipses,” he said.

The extended mission also offers the opportunity for additional science by studying the extended magnetosphere of Jupiter that Juno can probe on its longer orbits. “The science is in many ways a lot better” in the extended orbit, he said.

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...