Juno in safe mode after engine malfunction

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An artist’s impression of the Juno spacecraft making one of its close passes over Jupiter. We’ll have to wait until at least December for its next chance to give us new views of the gas giant after plans to do so today were postponed. 
NASA / JPL-Caltech

On a highly anticipated close flyby of Jupiter today, NASA’s Juno spacecraft was unable to fire its rocket motor to brake into a tighter orbit around Jupiter or use its scientific instruments to take new observations. 

But neither of these mean the mission has failed.

The glitches appear to be the result of two separate events, said Scott Bolton, the mission’s principal investigator, today at a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress in Pasadena, California. 

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The engine glitch surfaced several days ago, Bolton says, when engineers at mission control noticed “a little bit of delay” in the functioning of a pair of valves pressurising the propulsion system.

Concerned they could cause a malfunction that might send the spacecraft off-course, the team decided to delay the burn until a future orbit.

Then, a few hours before the spacecraft made its closest approach to Jupiter, something caused its main computer to reboot, and go into what Bolton calls “safe mode”. 

“The spacecraft is like a smart robot,” he explains. “It evaluated the situation and then takes action.” 

In this case, it shut off its instruments, linked to Earth and waited for mission control to figure out what happened and tell it what to do next. “It did exactly what it was supposed to do,” Bolton says.

Juno is now looping outward from Jupiter while scientists and engineers try to figure out what went wrong and make plans for its next close passage, 53 days from now. 

But there’s no reason for alarm, says Juno’s project manager, Rick Nybakken, of NASA’s Jet Propulsion Laboratory in Pasadena, California: “The spacecraft is healthy and we are working our standard recovery procedure.”

Even the engine malfunction isn’t critical. Its purpose was to shift the spacecraft from a 53-day orbit to a 14-day one, thereby increasing the frequency of the close flybys in which the spacecraft skims within 5,000 kilometres of Jupiter’s cloud tops.

“The worst-case scenario is that I have to be patient and get the science more slowly,” Bolton says.

‘You’re looking at a thunderstorm half the size of the Earth and 100 kilometres tall.’

The only limiting factor is that as Jupiter orbits the sun there will come a time somewhere in the middle of 2019 when the spacecraft will pass through Jupiter’s shadow, potentially blocking solar power for long enough to pose problems. 

It’s possible, Bolton says, that by then the engineers will have come up with a way for the spacecraft to survive the power loss, but if they haven’t, they may need to risk a burn. 

“But you don’t have to make that decision until 2019,” he says. And even on a 53-day orbit, he says, by that time, the spacecraft will have made about 20 orbits – and “you can accomplish a lot of science in 20 orbits.” 

Meanwhile, there is already one round of data from the spacecraft’s close passage of Jupiter on 27 August – the first and to date only loop on which it has been able to deploy its instruments.  

One instrument, an array of microwave antennas designed to map cloud structures at depths of 350 to 400 kilometres below the cloud tops, revealed that Jupiter’s colourful surface bands extend as far down as the instruments can measure. 

“That came as a surprise to many scientists,” Bolton says. “Whatever’s making these stripes is still existing pretty far down.”

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Another surprise came from an instrument called JunoCam, which revealed giant storms over Jupiter’s poles. Similar storms have been seen at Jupiter’s easily visible lower latitudes, but not as many, Bolton says: “The poles are filled with these things, and that was a surprise.”

By luck, one storm was right at the edge of Jupiter’s day/night side, where the slanting sunlight cast it into relief, creating shadows from which the scientists could estimate how high the stormclouds rose. 

The result was startling. “You’re looking at a thunderstorm half the size of the Earth and 100 kilometres tall,” Bolton says. “It is truly a towering beast of a storm.”

Candice Hansen of the Planetary Science Institute in Tucson, Arizona, adds that JunoCam is part of a citizen-science project in which raw images are downloaded to a special website where the public is invited not only to view them, but use whatever tools they want to enhance them. 

JunoCam has a small operations team and no image-processing team, Hansen says, “so we took a leap of faith that people would actually think this is cool and would participate.” 

Already, she adds, there are some gorgeous results posted on the website. “Our leap of faith was justified.” 

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